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ABSTRACT
.A COMPARISON OF THE ANATOMY
AND NATURAL HISTORY OF
COLUBOTELSON THOMSONI NICHOLLS,
A SOUTH TEMPERATE, FRESH-WATER ISOPOD AND

ASELLUS COMMUNIS SAY,
A NORTH TEMPERATE, FRESH-WATER ISOPOD.

by Joseph George Engemann

ISOpOdS of the family Phreatoicidae are commonly
found inhabiting the detritus of shallow fresh-waters
in Tasmania. Isopods of the family Asellidae are found
in similar habitats in north temperate latitudes. The
families are in separate sub-orders that have very dis-
tinct characteristics. There are no published life-
histories of iSOpOds in the Phreatoicidae.

I Colubotelson thomsoni Nicholls, an isopod in the
family Phreatoicidae, was studied in its natural environ-
ment, temporary pools on MOunt Wellington, Tasmania, and
Specimens were collected for further study of life pheno-
mena and preservation for anatomical and histological
study. Asellus communis Say, an iSOpod in the family
Asellidae, was also studied in its natural enwironment,

a vernal pond in East Lansing, Michigan, and specimens

Engemann

'were collected for further study. Life cycles are
determined by analysis of graphed length and date of
collection data correlated with observations on the iSOpod's
reproductive cycles. A comparison of the anatomy and
natural history of the two Species is then made.

Gross external anatomical differences between the two
are due largely to (a) orientation of the pleural portion
of the exoskeleton.which gives Q, thomsoni a laterally
compressed appearance in contrast to the dorso-ventrally
compressed appearance of A, communis, (b) reduction of
the number of external abdominal segments and speciali-
zation of abdominal appendages in A, communis, and (c)

a nearly uniform width of body regions of g, thomsoni in

 

contrast to the anterior tapering of A, communis.

 

A close correspondence is present in cephalic and
thoracic appendages of the two Species.

.A similar internal anatomy is noted with some
differences including (a) a typhlosole present only in
Q, thomsoni, and (b) three pair of hepato-pancreatic
glands in.§, thomsoni, but only two pair in A, communis.

Embryos of A, communis develop a pair of dorsal
thoracic appendages, with a histological resemblance to

the adult pleOpod, which disappears after they hatch.

Engemann

Serial sections of g, thomsoni embryos have only a bulge
in the position of the dorsal appendage of A, communis
embryos.

Q, thomsoni has only one reproductive period per
year. Males may be paired with females for several
months. Eggs are carried approximately six months before
they hatch. Size data is interpreted to indicate two
years are necessary to reach sexual maturity.

In contrast to the preceding, A, communis may have
two or more reproductive periods per year. Pairing is of
shorter duration and development requires approximately
two weeks. Growth and deve10pment into maturity may
occur in three months according to interpretation of size
distribution data.

Major environmental differences observed between the
two environments were higher summer temperatures, a
greater abundance and diversity of other invertebrates,
and a greater annual increment of detritus-forming leaf-
litter in the vernal pond in Michigan. The lamellar
structure of the vernal pond detritus caused by the
abundance of leaves from deciduous trees, is considered
to have been of possible significance in selecting the

structural modification evolved in A, communis. The above

 

Engemann

environmental differences, as well as a greater variety of
aquatic vertebrates known from.waters in Michigan which

are capable of preying on aquatic isopods, are considered
significant in the deve10pment of differences in life cycles,
number of young produced, and growth rates.

Both Species were able to survive the summer dry
periods by remaining inactive in the moist bottom.mud.
Specimens collected in such a condition became active in
less than a minute after water was added.

The taxonomic position of the two Species is discussed.
Also discussed is the possible role the differences in

length of life cycle might have on speciation and evolution.

A COMPARISON OF THE ANATOMY
AND NATURAL HISTORY OF
COLUBOTELSON THOMSONI NICHOLLS,
A SOUTH TEMPERATE, FRESH-WATER ISOPOD AND
ASELLUS COMMUNIS SAY,

A NORTH TEMPERATE, FRESH-WATER ISOPOD.

by

Joseph George Engemann

A THESIS

Submitted to
'Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Zoology

1963

3;?0279

“6/24/1424

ACKNOWLEDGMENTS

Many thanks are due Dr. T. W. Rorter for his constant
patience and encouragement during the prolonged production
of this thesis. His help is greatly appreciated.

Mrs. Bernadette Henderson was a source of encourage-
ment and help that will always be remembered.

Appreciation is hereby expressed to Dr. P. Clark and
Dr. M. Hensley for their constructive criticisms and care-
ful reading of the thesis. Use of the facilities of
Michigan State university, both on campus and at the
W: K. Kellogg Gull Lake Biological Station were an impor-
tant aid in the research work.

Thanks are extended the University of Tasmania for the
laboratory facilities and transportation provided during
the Australian phase of research. Particular thanks are
due Professor V. V. Hickman and Dr. E. R. Guiler for their
aid and guidance when in Tasmania.

The United States Government made much of this possible
with a Fulbright grant for study in Australia.

Friends and associates at the above mentioned places
as well as at'western Michigan University were a source of
ideas, help, and encouragement that will always be thank-

fully remembered.

ii

TABLE OF CONTENTS

Introduction.
Review of the Literature.
Methods .
Results .
Anatomy of g, thomsoni and A, communis .
External Anatomy .
Locomotion of Q, thomsoni.
Locomotion of A, communis.

Internal Anatomy .

Natural History of Colubotelson thomsoni .

Size Distribution.
Laboratory Rearing .
Reproductive Cycle .
Incubation .
Ecdysis.

Life Cycles of Related Species .
General Ecology of g, thomsoni .
Natural History of Asellus communis.

Size Distribution.
Pairing and Egg Deposition .

Incubation and Growth.

iii

11
15
15
15
46
51
52
70
7O
70
77
78
79
80
81
94
94
94

97

Number of Broods.

Brood Size.

Sex Ratio .

General Ecology of A, communis.
Taxonomical Discussion .
Discussion .

Anatomical Comparison .

Life Cycles .

Sexual Differences.
Comparison of the Pools .
Faunal‘Differences.
Evolutionary Considerations .

Temporal Reproductive Isolation and
Speciation .

Marine Relatives.
Summary.

Literature Cited .

iv

98
101
101
102
111
114
114
124
126
128
130

133

134
135
137

139

LIST OF TABLES

Table

1

Reproductive cycle of Q, thomsoni
on Mt. Wellington, Tasmania.

Temperature of pool, Mt. Wellington
and remarks about collection of

g. thoms oni

List of the taxa of invertebrates,
other than g, thomsoni, having examples
observed in the sample pool onth.

Wellington , Tasmania .

Size distribution of A, communis

in collections from.Baker'flbodlot pool .

Data of collections containing

A, communis females over 6% mm. long .

Temperature of Baker‘WOOdlot pond and
temperature and precipitation records

taken at the Horticulture Farm, East
Lansing.

List of taxa represented in the
observed biota of Baker‘WOodlot
roadside pond, other than.A, communis.

Tabular comparison of g, thomsoni and

A, communis; differences .

86

90

96

100

104

109

117'

LIST OF FIGURES

Figure

10
11

12

13

14

15

Diagram of apparatus used to provide
a constant flow of water for several
groups of isolated isopods, or other
bottom dwelling organisms.

Tip of antennule, 13 mm, male 9, thomsoni.

Tip of antennule, 9 mm. female Asellus
communis .

Labrum of 9 mm. female A, communis .

Labrum.of 13 mm. male 9, thomsoni.

Left mandible of 13 mm, male g, thomsoni .

Section through mandibles of female
Q. thoms oni .

Labium of 13 mm, male g, thomsoni.
Labium.of 9 mm, female A, communis .

Maxillula of 13 mm, male 9, thomsoni .

First maxilla of 9 mm. female A, communis.

Section through mouthparts of Q, thomsoni
showing coupling Spines of maxillipeds .

Coupling spines of maxilliped of 11% mm.
male A. communis .

 

Gnathopod carpus, propod and dactyl

of male Asellus communis .

Gnathopod carpus, propod and dactyl
of female A. communis.

vi

14

l9

19
23
23
24

24
26
26
27

27

31

31

32

32

Figure

16

17

18

19

20

21

22
23
24
25

26

27

28

29
30
31

32

Gnathopod carpus, propod and dactyl
of male 9, thomsoni.

Gnathopod carpus, propod and dactyl
of female g, thomsoni.

Carpus, propod and dactyl, fourth leg,
11% mm. male A. communis .

Carpus, propod and dactyl, fourth leg,
9 mm. female A. communis .

Carpus, propod and dactyl, fourth leg,
13 mm. male Q. thomsoni.

Carpus, propod and dactyl, fourth leg,
10 mm. female 9. thomsoni.

Third leg of 13 mm, male 9, thomsoni .
Basis of seventh leg of male 9, thomsoni .

Third leg of 1135 mm. male A. communis.

Fifth pleopod of 10 mm. female g, thomsoni .

Endopodite of second pleopod of 13 mm.
male g. thomsoni . . . .

Second pleopod of 11% mm, male A, communis .

Spines of margin of telsonic pleura of
13 mm. male 9. thomsoni.

v
Uropod of 10 mm. female 9, thomsoni.
Uropod of 9 mm. female A, communis .

Plane of movement of legs of g, thomsoni .

Plane of movement of legs of A, communis .

vii

32

34

34

34

34
35
36
40
41

42
42

44
45
45
47
48

Figure

33

34

35

36

37

38

39

40

41

42

43
44

45
46

Ganglion of Q, thomsoni.

Cross-section of abdominal region

of female g, thomsoni showing hepato-
pancreatic glands, abdominal cecae of
intestine, heart and typhlosole.

Junction of intestine and rectum, cross-
section of male 9, thomsoni.

 

Cross-section of female 9, thomsoni
through oviduct and genital opening.

 

Tangential section of gut wall of
g, thomsoni showing regular arrangement
of cells .

 

Section through ostium.in heart of
female g. thomsoni .

Section through arteries leaving heart
of female 9, thomsoni. . . .

Drawing of an embryo of g. thomsoni

(longitudinal section) .

Drawing of an embryo of g, thomsoni
(cross-section).

Drawing of an embryo of M. capensis
(longitudinal section) . . . . . .

Dorsal appendage of A, communis.

Lateral view drawing of adult, non-gravid
female 9, thomsoni .

Size distribution of g, thomsoni .

Size distribution of'Metaphreatoicus
magistri . . . . . .

viii

54

55

57

59

6O

60

65

66

67

68

69

72

74

Figure

47 Photograph of specimens used for size
distribution in figure 45.

48 S. E. Tasmania .

49 'MOunt‘Wellington collecting site with
pool full

50 ‘MOunt wellington collecting site with
dried pool bottom.

50a Mount wellington collecting site Showing
dried pool bottom.

50b Mbunt wellington collecting Site as
viewed from.Battery Point, Hobart.

51 Map of vernal pond at edge of Baker
Ebodlot. .

52 Diagram of Isopod Relationships.

75

82

84

84

85

85

107

113

INTRODUCTION

Fresh-water isopods are one of the most conspicuous
and abundant groups of macroscopic crustaceans inhabiting
the detritus in the shallows of many of the world's fresh
waters. ‘Most of the isopods are in the Sub-order Asell-
oidea, but in the Southern Hemisphere species from the
sub-order Phreatoicoidea are more common in many areas
(Green, 1961). This thesis is a report of a study and
comparison of a Species from.each of the sub-orders, one
from.the Southern Hemisphere, the other from a somewhat
similar habitat in the Northern Hemisphere.

Colubotelson thomsoni Nicholls is an isopod of the
sub-order Phreatoicoidea, family Phreatoicidae. It is
restricted in distribution to fresh water on Mount welling-
ton, Tasmania, from altitudes of a few hundred feet to
over 4,000 feet. Thus the Specimens in this study were
from the type ldbality (Latitude 3., approx. 42° 54';
Longitude E., approx. 147° 17').

The sub-order Phreatoicoidea is a group of isoPOds
with a remarkably similar facies (Nicholls, 1943). At
present, members of the sub-order are found only in New

Zealand, Australia, India and South Africa. Most species

-2-

occur in Australia and they are found most abundantly

and concentrated in the Australian island State, Tasmania.
With the exception of one species in India, the distri-
bution of the sub-order is restricted to the Southern

Hemisphere. The specimens of g. thomsoni used in this

 

study came from a population found in temporary pools of
the North West Bay River on a sub-alpine flat, about 4,000
feet above sea level, near the summit of Mount wellington.
Asellus communis Say is an isopod of the sub-order
Aselloidea (Asellota), family Asellidae. It occurs in
shallow aquatic environments in the Northeastern United
States. The specimens used in this study came from.a
temporary roadside pool in Baker WOodlot on the campus of
‘Michigan State University, East Lansing, Michigan (Lati-
tude N. approx. 42° 45'; Longitude W. approx. 840 30'),

The original description of A, communis utilized

 

general characteristics so that many aquatic isopods might
be misidentified as this species. 'More precise analysis
of morphological features has resulted in the description

of many Species within the family Asellidae. A, militaris

 

Hay is similar but distinguishable from type locality
Specimens of A, communis. Richardson (1905) and van Name

(1936) synonymize A, militaris with A, communis. Records

 

-3-

of the genus Asellus can be found in many areas around
the North Temperate zone (Van Name, 1936; Racovitza, 1920;
Nicholls, 1929; Mackin and Hubricht, 1938).

A fossil of a phreatoicid exists from.the Triassic
beds of Australia (Nicholls, 1943, p. 111). Although
Australia has been separated from Asia since the Creta-
ceous, some transportation of phreatoicids on the feet
of wading birds may have occurred. Aselloids and phrea-
toicids are probably more closely related to each other
than to any otherisopods. The close relationship is
indicated by their Similarity of anatomy and the presence
of a pair of unique dorsal buds in the embryos of each
(Barnard, 1927).

This study is a comparison of g, thomsoni and A,

 

communis. Comparisons of anatomical structures are a

very significant phase of evolutionary and phylogenetic
studies. Snodgrass (1956, 1951, 1938) has worked extensively
on comparative arthrOpod anatomy. Sanders (1957) has

used comparative studies of crustacean appendages in his
recent study of crustacean phylogeny. Many comparisons

are either narrow in scope, to establish the nearest taxon

to the taxon in question, or broad in scope, to establish

systems of phylogeny. This study takes an alternative

-4-

approach in its comparisons. .The relationship of the
animals studied is accepted as being rather remote within
the same order (Chilton, 1894). The similarities and
differences are used as a gauge of the relative rate of
the evolutionary process on various aspects of the natural
history of the two species. Since selective pressures of
the environment are an important factor in most evolu-
tionary change, the analysis of the life history and
anatomy may aid in evaluating the more significant dif-

ferences in the environment of the two species.

REVIEW OF THE LITERATURE

Colubotelson thomsoni was named by Nicholls (1944)
at the time he erected the genus Colubotelson. Prior to
Nicholls' monograph (1943, 1944) of the Phreatoicoidea,
Q, thomsoni was usually identified as the Species PAEAAf
toicus australis (Thomson, 1893; Smith, 1909; Barnard,
1914 and 1927; Sheppard, 1927) or Phreatoicus tasmaniae
(Thomson, 1894a). Nicholls (1943, 1944) removed all the
Australian species from the genus Phreatoicus Chilton
because they had considerable differences from.the geno-
type which was a rather specialized subterranean form.
The Australian species have been placed by Nicholls (1943,
1944) in several genera.

There is no information in the literature on the life

cycle of Q, thomsoni.

 

Smith (1909) has a figure of a uropod of the Mount
wellington species in his figures of Phreatoicus australis.
Nicholls (1944) has a description of the external anatomy
as well as some illustrations of parts. He reported that
it is morphologically intermediate between 9, joygeri and
'Metaphreatoicus magistri. Barnard (1927) stated that

specimens from Mt. Wellington (g, thomsoni) that he

examined did not support Smith's (1909) statement that they

-5-

possess a typhlosole.

Ecological notes about Q, thomsoni are limited to
those made by Thomson (1894b), Smith (1909), and Nicholls
(1944).

The only major references not in the literature cited
in Nicholls (1943, 1944) monograph containing information
about the Phreatoicoidea are Hutchinson (1928), Barnard
(1940), Nicholls (1946), Chopra (1947), Ch0pra and Tiwari
(1950), Guiler (1952), Tiwari (1952), Guiler (l956),and
Glaessner (1957). None contain significant new informatinn
about g, thomsoni.

There are several very important works on the
Phreatoicoidea. Chilton's (1894) paper contains a detailed
anatomy of Phreatoicus typicus and a comparison of the
then known phreatoicids with the Asellidae. Sheppard
(1927) published a taxonomic monograph on the Phreatoicidae.
Nicholls (1943, 1944) published a taxonomic monograph on
the Phreatoicoidea. Barnard's (1927) paper includes a
fairly comprehensive study on the life cycle and anatomy
of the South African phreatoicids of the family AmphiSOpidae.

The literature on asellids is much more extensive than
the literature on phreatoicids. The genotype, Asellus

aguaticus (Linnaeus, 1761) Geoffroy St. Hilaire, 1746, is

-7-
abundant and widely distributed in Europe. Bovallius
(1886) has twenty-seven literature citations for the
Species in his paper on the Asellidae. Since then papers
on A, aguaticus have appeared more frequently as various
aspects of its biology are explored. One worker, A. E.
Needham (1937, 23 £23-) has over a dozen papers describing
normal and experimental observations on A, aquaticus.

The Species of Asellus here reported is Asellus
communis Say, 1818, and is the first asellid described
from North America. Say (1818) described A, communis as
follows:

"1. A, communis.* Body Oblong oval,
furnished with short rigid hairs; interior
antennae equal to the peduncle of the
exterior ones; caudal appendices, peduncle
depressed.

Inhabits small streams of fresh water, under
stones. -

cabinet of the Academy.

Body oblong-oval, a little narrowed before,
segments transverse, subequal, indistinctly
emarginate on the edges each side, hairy;
hairs very short on the disk, longer on the
edges and feet; third and fourth segments
linear, the anterior ones a little curved
forwards and the posterior ones backwards;
head narrower than the first segment, and
not longer; superior antennae extending to
the base of the tail; inferior antennae
equal to the peduncle of the superior ones;
eyes obovate, oblique, prominent, black; tail
as broad as the segments of the body, trans-
verse-suborbicular, equal to the last two
segments conjunctly, depressed, and a little
prominent between the appendices; appendices

 

 

-3-

as long as the tail, laciniae subequal, ped-
uncle dilated; anterior feet hardly longer,
monodactyle, unarmed; thumb as long as the
hand; hand oval; carpus triangular; remaining
feet gradually longer to the hind pair, which
is the longest, first and second joints sub-
oval, gibbous above, third joint triangular,
extended over the base of the succeeding one
and tipped with long hairs, fourth and fifth
linear; tarsi half as long as the preceding
joint, simple acute.

Length one fourth of an inch, breadth less
than one tenth.

A very common species in our fresh waters,
particularly in rivulets under stones. It is
frequently introduced with the Schuylkill
water into Philadelphia. The female may be
distinguished from the male by a valvular
pectoral follicle in which the young are
protected. In one of these I counted twenty-
eight young ones."

 

 

Subsequent paper containing information about A, 22m:
EEEEE have been mostly redescriptions, distributional
records and short notes on natural history (Milne-Edwards,
1840; Harger, 1874; Paulmier, 1905; Richardson, 1905;
Fowler, 1912; Kunkel, 1918; Racovitza, 1920, and 1925;
Johansen, 1920; Stammer, 1932; van Name, 1936). A number
of non-taxonomic papers exist. MeMurrich (1895) has

figured the early embryology of A, communis and his obser-

 

vations were similar to Rathke's (1834) more extensive
observations on A, aquaticus. Forbes (1878) reported the

occurrence of A, communis in the stomach contents of Illinois

-9-

fisheS. Richardsofis (1904) work on natural history of
isopods includes Asellus in genera discussed.

Allee (1912, SE A23.) has probably done more work with
A,communis than any other man. His work was ecological,
physiological, and behavioral.

Further observations on A, communis have been pub-
lished by Banta (1910), Shelford (1913), Hatchett (1947),
Kenk (1949), and Ellis (1961). Extensive reports on the
biology of species closely related to A, communis are those
by Sars (1867, 1899), Kaulbersz (1913), Marcus (1930), and
Ellis (1961). waterman (1961) has cited many papers on
the physiology of isopods.

Comparisons of anatomy of asellids and phreatoicids
have been made by Chilton (1894), Thomson (1893), Barnard
(1927), Sheppard (1927) and Nicholls (1943). Chilton
(1894) noted the presence of coupling spines on the maxilli-
peds, the propodus of the gnathopod, male reproductive
organs and spermatozoa were similar in Phreatoicus spp.
and Asellus spp. He also noted a general similarity of
cephalic and thoracic appendages while general body shape
and pleon were markedly different in the two genera.
Barnard (1927) noted the Similarity of Metaphreatoicus spp.

and Asellus spp. in the fourth male periqxfl, gastric mill,

-10-

brood-pouch, presence of dorsal appendage in the embryo,
and Season the young are released. Sheppard (1927) noted.
the difference in the Spine-row of Asellus spp. and phrea-
toicids, while there was a similarity of the coxal lobe of
the maxilliped of ovigerous females and the condition of
the coxopodites of the thoracic appendages of the two
groups. Nicholls (1943) noted the differences between
Asellus spp. and phreatoicids in the "subocular" area and
relative development of mouthparts. Green (1961) has
pointed out that the two groups are Northern and Southern
Hemisphere counterparts. There are no publications with-

direct comparisons of Q, thomsoni and A, communis.

 

That isopods are worthy of study can.be inferred from
reading Hynes (1960). He considers Asellus spp. an impor-
tant member of the "pollution fauna" found on the surface
of mud of "depositing" substrata where the isopods feed
on dead leaves and bacterial growth. Asellus spp. are so
important that he designates an Asellus Zone or an Asellus/

Cladophora Zone in the recovering region of an organically

polluted river.

METHODS

Qualitative sampling involved removing specimens from
their environment with forceps, pipette, strainer, or other
suitable means. An oversize pipette with a % ounce bulb
and no constriction in the end of the pipette was convenient
for handling living Specimens. Submerged objects were exam-
ined and Specimens removed. Detritus and litter were con-
centrated by means of a common kitchen strainer with a dia-
meter of about five inches and a mesh with about fifteen
Openings per inch, and then placed in a white 10 x 14 inch
enamel pan for sorting. A small amount of water was added
and the material was sorted. Specimens were removed as soon
as they were located. Material was inSpected individually
in the pan. Then it was concentrated in clumps and left un-
disturbed for a short period. Eventually the animals would
leave the clumps of debris and could be readily detected on
the contrasting pan.

A Special effort was made to secure every Specimen in
samples taken for use in determining pOpulation size class
frequencies. The same pool was collected in repeatedly and
in as nearly the same location as possible as a means of
examining the same population for seasonal differences. A
piece of fine brass screening with fourty-two Openings per

-11-

-12-

inch was occasionally used to check against the efficiency
of the kitchen strainer. It was found that the kitchen
strainer collected Specimens of the smallest sizes found on

the brass screen.

Material was usually taken to the laboratory in pint
jars for a more thorough qualitative inspection. Mest Speci-
mens were preserved while at the collecting site in 3" x 3/4"
shell vials containing 70% ethyl alcohol. Other specimens
selected for histological preparations were killed and fixed
in either Bouin's Fluid or acetic-alcohol fixatives before

preservation in 70% alcohol.

Macroscopic invertebrates from the area were collected
and preserved in 70% ethyl alcohol. Observations on the ac-
tivities of the macroscopic invertebrates were made in the

field as the Opportunity arose.

Temperature of the water, several inches below the
water surface, was taken with an immersion type Fahrenheit

thermometer.

The length of preserved specimens was measured from.the
anterior end of the head capsule to the posterior end of the
telson. Specimens were straightened and shortened if neces-
sary to eliminate any unpigmented intersegmental region.

ISOpods were maintained in the laboratory in various

-13-
glass containers. 'Water was kept at a low level, less than
two inches deep, so that sufficient oxygen would be available..
Greater depth was allowed in a series of rectangular glass
tanks through which a continuous flow of water was siphoned
(as diagrammed in fig. 1). For food, the Tasmanian iSOpods
were provided with moss and liverworts. Michigan isopods were
fed maple, elm, ash, beech, or oak leaves, which were par-
tially decayed when taken from their natural habitat. Stack-
ing finger bowls or covered containers proved more satisfac-
tory than uncovered containers for growing A, communis. The
rate of evaporation was reduced and formation of surface

scum.was inhibited in stacked, or covered, containers.

Appendages mounted in glycerine-jelly or CMC-lO were
much more easily seen than ones mounted in balsam, which has
a refractive index too similar to that of the appendages.

Crystals frequently form in unringed slides made with ONE-

10. Serial sections were prepared by the paraffin technique
for observation of internal anatomy. Serial sections were
routinely stained with hematoxylin and eosin. Photomicro-
graphs were taken using Kodak Tri-X film and an A0 Spencer
phase contrast microsc0pe with a trinocular body and a 35 mm.
camera with a focal plane Shutter. Outline drawings were

prepared free-hand, by tracing photographs, and by the aid

of a camera lucida.

-14-

q

“mamom Op uozv .3oauam>o .o “coneam .m “mane:m noun: 3

.mEmHmeso mcHHHOSO souuoo nmnpo no .mUOQOmH OmpmHOmH no museum Hmampmm
pow pmumz Mo Boa“ pcmumcoo m mea>opn ow new: mapmnwamm mo ampmmda .H .mdm

 

RESULTS

Anatomy of Q. thomsoni and A. communis.
EXTERNAL ANATOMY
Length

Nicholls (1944) states the Size of g, thomsoni as
12.5 mm. This is about the average length for males in
my August 1957 collections. The longest male was 15 mm.,
the longest female 12 mm. After they reach 7 or 8 mm. the
sexes can be differentiated. The smallest Specimens col-
1ected were 2.5 mm. Length is approximately 7 times width
in adult males and about 5 to 7 times width in females.

The length of individuals of A, communis used in this
study was greater than the quarter inch of Say's (1818)
specimens. The maximum length for males was 13% mm., for
females 10 mm, Most specimens over five millimeters long
can be sexed. The smallest specimens collected were one
millimeter long. The breadth of the organism is about
one-fourth the length.

QEQEEfSECtiOHal EEEEE.

Cross-sections Show the adults of both Species of
isopods are, more or less, circular in the intersegmental
region.

The pleura and sterna of g, thomsoni project lateroven-
trally in regions where the legs are attached. The arrangement

-15-

-l6-
of the appendages and the downward elongation of the pleura
of the abdominal segments results in a laterally compressed
appearance.

The dorso-ventrally flattened appearance of A, communis

 

is caused by the lateral extension of the terga and epimera
to the attachment of the legs.
External Sexual Differences.

Both species have several sexual differences in common.
The gnathopods of the males are more robust than those of
the female. The males have a slightly modified fourth walk-
ing leg. The pene through which the vas deferens opens ex-
tends medio-ventrally from the coxa of the seventh male
perioPOd. Both males have modified second pleopods (figs.
26 and 27). Females of both have oostegites extending med-
ially from the coxa of the first four pairs of legs. The
oostegites of gravid females are so enlarged that they over-
lap to form.a marsupium, 'Males of both have a larger maxi-
mum.size.

The female of A, communis, esPecially the gravid female,
is proportionally wider in the anterior thoracic region than
the male. The A, communis female has only four pairs of

pleopods.
The Head.

 

The head of E, thomsoni has a rounded capsular appearance

 

-17-

dorsally and the mouthparts extend ventrally occupying as
much Space as the head proper. The anterior margin of the
head is smoothly excavate. The head is as wide as the rest
of the organism,

The head of A, communis is excavate anteriorly, wider
than long and rounded laterally. There is a postero-lateral
process which is not prominent unless the head is rotated
slightly. The head is the narrowest region.

Imam-
The eyes are flush with the surface of the head. Each

eye of g, thomsoni has approximately 20 ocelli, and is lo-

cated near the anterior of the head in a lateral position.

There are approximately 12 ocelli in the eyes of A, communis

 

and they are located midway along the lateral margins of
the head.
The Antennules (First Antennae).

The antennule of g, thomsoni is slightly shorter than
the peduncle of the antenna. It is immediately dorsal and
medial to the antenna. If the first three articles (seg-
‘ments) are considered peduncle, it then has a five articled
flagellum, The segments are of about equal length but are
variable in different individuals. The Terminal segment

is always much smaller. In addition to typical hair-like
setae, the distal articles contain a number of Special-

ized setae which have a terminal half blunt, slightly

-13-

enlarged and cylinder-like (fig. 2). These correspond to
the "olfactory cylinders" of Chilton (1894, p. 188, and
P1. XVI, fig. 2) on Neophreatoicus assimilis.

The antennule of A, communis is also Slightly shorter
than the peduncle of the antenna. It is immediately dor-
sal to the antenna. The basal segment is about as wide as
long. The second segment is of smaller diameter but it
is the longest segment of the antennule. The third is of
still smaller diameter and is second in length. There
are eleven segments in the flagellum which are of the same
diameter and shorter than the three most proximal segments
which form.the peduncle. The first three segments have a
few long setae, the rest a few short setae. Some of the
setae on the distal segments are blunt with the distal
part forming a cylinder distinct from the basal part
(fig. 3). Perhaps these correspond to the olfactory tubes
(Bovallius, 1886) of the European asellids. They are very
similar, in construction, to the specialized setae on the
antennulegof Q. thomsoni.

The Antennae (Second Antennae).

The antennae, in both species, are attached immediately

in front of the eyes. Both have antennae with five-

segmented peduncles. The flagellum is twice as long as

-19-

  
    

Fig. 2. 'Tip of antennule, 13 mm. male 9, thomsoni.
0, "olfactory setae." x 180.

Fig. 3. 1p of antennule, 9 mm. female Asellus
communis. O, "olfactory setae)‘ x 180.

-20-

the peduncle. The segments of the peduncle and the flag-
ellum.have setae at their distal ends. Setae of the
peduncle are longest and their length eQuals the diameter
of the peduncle. Segments of the flagellum.are Shorter
than wide at the proximal end and grade unto ones that are?
three (9, thomsoni) or four (A,communis) times longer than
wide distally. The change in relative dimensions is due
to a distal reduction in diameter of the flagellum, while
the actual segment length increases only slightly.

The first two segments of the antennal peduncle of
Q, thomsoni are each shorter than'wide,,and as long together
as the third or fourth. The fifth segment is longest but
shorter than the third and fourth together. The flagellum
has from 18 to 23 articles. Antenna length is much less
than the combined length of head and thorax.

The first three segments of the antennal peduncle of
A, communis are each shorter than wide. The two distal
peduncular segments are each equal to the length of a body
segment, the most distal being slightly the longer. The
f1age11um.has approximately 60 articles. Antenna length

is equal to the combined length of head and thorax.

-21-

masts.
Thg.Labrum,

The 1abrum (figs. 4 and 5) of both species is shaped
like an inverted dome. The ventral margin is densely
fringed with short, stout setae. The asymmetry of the
labrum of g, thomsoni observed by Nicholls (1944) is
apparently due to the posture of the setae.

The Mandibles.

Both species have mandibles with a definite asymmetry.
The right mandible lacks a lacinia mObilis. The left man-
dible has a lacinia mobilis between the incisor process
and the Spine-row (fig. 6). The lacinia mobilis has three
teeth (or less in A, communis). The palps are three seg-
mented. The two distal segments have on their distal half
a series of pectinate setae. The hairs forming the pecti-
nation are only on the terminal end of the setae. The
setae of the terminal segment form.a dense curved brush.
The palp projects anteriOrly beneath the peduncles of the
antennae.

g, thomsoni has a double row of spines in the spine-

 

row directed toward the mplar process when the mandibles
are closed. The arrangement assists the inward movement

of food and prevents its escape from.the molar process

-22-

(fig. 7). The molar process has a roughly square grinding
surface with twenty striated rows on its surface.

The spine-row of A, communis is composed of plumose

 

setae, rather than spines as in g, thomsoni. The molar
process is somewhat circular and has over thirty striations
visible on it. The setae on the two terminal segments of
the palp somewhat oppose each other leaving a sub-circular
opening between them,

TAA'Labium.

The labium of g, thomsoni is deeply cleft. The sides
of the cleft diverge at almost a right angle to each other
(fig. 8). Halfway along the inner margin of the side of
the cleft is an indentation which separates the margin
into an inner part, forming the notch of the V and bearing
very tiny setae and an outer half bearing a dense brush of
larger setae which are directed medially.

The labium of A, communis is deeply cleft (fig. 9).
Distally there is a dense fringe of setae which are proe:
gressively Shorter in the cleft in contrast to the abrupt
transition from large setae to small setae along the cleft
of g, thomsoni.

The‘Maxillae.
The paired first maxillae (maxillula of Nicholls) of

‘Q. thomsoni have two rami (fig. 10). The medial ramus is

 

-23-

   

1 ,
Fig. 4. Labrum of 9 mm. female A, communis.
x 180.

 

 

Fig. 6. Left mandible of 13 mm. male 9. thomsoni.
I, incisor; S, spine row; M, molar process; and
L, lacinia mnbilis. x 80.

 

Fig. 7. Section through mandibles of female g,
thomsoni showing opposing action of right and left
incisive and molar processes with inwardly directed
Spine row. I, incisor; S, spine row; M, molar pro-
cess; and L, lacinia mobilis. x 80.

-25-

shorter, narrower and bears four stout plumose setae temmi-
nally. The lateral ramus bears about a dozen tooth-like
terminal setae, one of which is usually a plumose seto-
spine, and two sub-terminal plumose setae. Both curve so
that their ends are directed somewhat medially.

The second maxilla of Q. thomsoni is more leaf-like

 

with three lobes. The middle lobe is frequently difficult
to distinguish. The medial margin of the medial lobe bears
a double row of setae. Th; setae of one row are plumose
to some extent. At the distal end are several stouter
setae which are pectinate along their medial margin. Be-
tween the right and left maxillae is a small curved median
process of unknown function.

The first maxilla of A, communis has two rami (fig. 11).
The medial ramus bears five stout plumose setae. The
lateral ramus bears over a dozen pectinate or forked Spines
in which most of the teeth are near the terminal end and
are blunt. The spines are in three rows. The second maxilla
is three lobed and more leaf-like. The inner ramus is the
widest and bears two terminal rows of setae, one row is of
simple setae, one is of pectinate setae on which the teeth
point laterally. The outer two lobes, or rami, bear long

setae which curve medially and on the medial surface of the

‘ -26-

 
  

Fig. 8. Labium of 13 mm. male 9, thomsoni. x 80.

Fig. 9. Labium of 9 mm. female A. communis. x 80.

-27-

    

Fig. 10. Maxillula of 13 mm. male g, thomsoni.
M, medial ramus; L, lateral ramus. x 180.

 

 

Fig. 11. First maxilla of 9 mm. female A.

communis. M, medial ramus; L, lateral ramus.
x 360.

-23-

distal end they bear many short uniform teeth.
The Maxillipeds.

The maxillipeds of g, thomsoni are the most pos-
tero-ventral mouth parts with palps that extend to the
area near the incisive processes of the mandibles. The
epipodites of the maxillipeds are broad and plate-like,
arising from.the coxae and covering the basal portions
of the maxillae. The palp on the latero-distal margin
of the basis possesses a large plumose setae which
Nicholls (1943, p. 16) states probably represents the
point where the exopodite once was carried. The basis
also gives rise to a plate projecting antero-dorsally and
provided with several coupling hooks (fig. 12) on the
medial side. The plate of the basis has a simple setae
on its proximal dorsal margin grading into pectinate
setae terminally. The coupling hooks are apparently de-
rived from this series of setae. The four remaining
segments of the palp are progressively smaller and provi-
ded with simple setae which are more abundant along the
medial margin. Projecting posteriorly from the coxal
segment in the female is a lobe provided with several
long setae.

The maxillipeds are considered the first thoracic

-29-

appendages. Slightly in advance of the posterior margin
of the head of g, thomsoni is the cervical groove which

represents the fusion of the maxilliped segment with the
primary head (Nicholls, 1943).

The maxillipeds of A, communis are also the most

 

postero-ventral of the mouth parts. They possess plate-
1ike epipodites which cdver the bases of the maxillae
laterally. There is a five-segmented palp, as in Q.
thomsoni, which extends as far as the mandibles. Along
the midline is an anterior extension of the basis with a
dorsally extending Side. This process is provided with
plumose setae. Five coupling spines (fig. 13) extend
medially with their distal end toothed. A lObe of the
coxal segment of the female extends posteriorly and is
tipped with plumose setae. The setae of the palp and
epipodite are simple.

walking Aggg (Legs or Pereiopods).

The seven pairs of walking legs and their component
segments are of similar construction in both species.
Egg 2: Q. thomsoni. .

g, thomsoni has two pairs of legs at the posterior

of the thorax which are longer than the preceding pairs.

The first leg (gnathopod) is sub-chelate, i.e. the dactyl

-30-

is folded down upon a raised palmar area on the propod
(figs. 16 and 17). The palmar area is provided with approx-
imately nine stout denticulate Spines. The dactyl possesses
very fine spines on the surface opposing the palm. In the
adult male the gnathopod is more robust. The fourth leg

of the male is also modified in possessing one or two

stout spines on the propod and a dactyl which is distinctly
more curved (fig. 20), than those of the other legs.

Each leg has the coxal segment fused with the bedy
segment, but less definitely so in the posterior legs. A
leg is illustrated by fig. 22. The basal segment is the
largest and longest segment. A keel, bearing a few long
setae on its dorsal edge (the side opposite that to which
the dactyl is directed), is W611 developed on the basal
segment of the posterior three pairs of legs. The keel,
Which contains no appreciable musculature (fig. 23), is
directed forward in the first four pair of legs and back-
ward in the last three pair. The ischial segment is
shorter than the basos to which it is attached. There is
a slight keel on this segment also.

There are stout setae which arise from the ventral
distal portion of the basos and extend outward near the

ischios. The ischios has median and terminal tufts of the

 

Fig. 12. Section through mouthparts of Q.
thom$oni showing coupling spines of maxillipeds.
C, coupling Spine; M, maxillary gland. x 80.

    

Fig. 13. Coupling spines (c) of maxilliped
of 1135 mm. male A. communis. x 360.

-32-

 

14

Fig. 14. Gnathopod carpus, prOpod and dactyl
of male Asellus communis. x 58.

 

Fig. 15. Gnathopod carpus, propod and dactyl
of female A. communis. x 58.

 

 

Fig. 16. Gnathopod carpus, propod and dactyl
of male Q, thomsoni. x 58.

Fig. 17. Gnathopod carpus, propod and dactyl
of female Q, thomsoni. x 58.

-33-

same type of setae. Both tufts are oriented in the same
direction on the same side of the segment. Somewhat
similar distal tufts are present also on the meros, carpus,
and prOpod. In general the tufts are most highly devel-
oped on the Side Opposite the direction of flexion of the
joint by which they are located.

The meros arches dorsally to extend a process over
its distal joint. This arch serves as the origin of muscle
fibers which converge to their insertion on the carpus.

The carpus and propod tend to be straight cylindrical
segments. However, in the gnathopod the carpus is trian-
gular. The gnathopodal propod is enlarged and possesses
a palmar process as mentioned before. Close examination
of the dactyl shows a few subterminal setae around the
claw-like end which may be just a stout terminal Spine.

The leg flexes dorsally at the joint between basos
and ischios as well as at the one between ischios and meros.
The meros and carpus are usually held in line. The flex-
ure is ventral at the junctions of carpus and propod, and
propod and dactyl.

Tn the female the oostegites extend medially from the
coxa of the first four pair of legs. The oostegites of

gravid females are so enlarged that three or four overlap

-34-

;

Fig. 18. Carpus, prOpod and dactyl,
1135 mm. male A. communis. x 58.
Fig. 19. Carpus, prOpod and dactyl,
9 mm. female A, communis. x 58.
Fig. 20. Carpus, prOpod and dactyl,
13 mm. male Q, thomsoni. x 58.

Fig. 21. Carpus, propod and dactyl,
10 mm. female Q. thomsoni. x 58.

fourth leg,
fourth leg,
fourth leg,

fourth leg,

-35-

 

Fig. 22. Third leg of 13 mm. male Q, thomsoni. x 58.

 

-36-

 

Fig. 23. Basis of seventh leg of male
Q. thomsoni. Note how musculature does
not extend into keel.

-37-

at most locations along the ventral midline of the marsu-
pium.

The pene through which the vas deferens opens extends
medic-ventrally from the coxa of the seventh male periopod.

Legs 9; A. communis.

 

The legs are successively longer toward the posterior.
In the male the fourth leg is as short as the first (gnatho-
pod) however. The legs are similar except the first (sub-
chelate gnathopod) of both sexes (figs. 14 and 15) and
the male's fourth leg (fig. 18). The male gnathopod has
two processes (fig. 14) and a row of setae on the palm.

The larger process arises in the middle of the palm and
slants toward the articulation of the dactyl. The small
process is a low hump between the other process and the
dactyl. The basis is the largest segment of a leg and
the first movable one. The basis of the leg is only
slightly keeled on the dorsal surface, the musculature
sometimes extending into the dorsal expansion.

The ischium resembles the basis but is smaller. The
merus is shorter than the other segments but has a long
arching process which is tipped with long, stout spines
extending over the carpus. The carpus is of smaller

diameter than the preceding segments but of approximately

;. 3:45.329: smirk). ..
0 5d: aadd rajrcni 5
i3 :1 doidw masoouq
.euqrso adj rsvo an

. ”tbsp-seq 9d: anti: ‘1

-33-

the same length as the ischium, The prOpod is as long as
the basis but of smaller diameter. The dactyl is less than
half the length of the propod except on the gnathopod. There,
the dactyl on the palmar surface has short stiff setae giv-
ing a toothed appearance to the dactyl (fig. 14). The other
legs have fewer, but longer, setae on the correSponding sur-
face of the dactyl. There are a few setae on the dorsal and
ventral ridges of the segments and on the distal ends. A
leg is illustrated in fig. 24.
Th5,Pleopods.

The pleopods of Q, thomsoni are biramous (fig. 25).
The exopodite is fringed with setae most of which are plumose.
The exopodite is two lobed except on the first pleopod. The
endopodite is posterior to the exopodite and resembles the
basal lobe of the ex0podite but lacks setae. On the second
male pleOpod is a penial stylet of no known function (fig.
26). It is a curved cylindrical Structure along the medial
side of the endopodite. The penial stylet is provided with
about four terminal setae plus some proximal ones on the
medial side. There are numerous muscles inserting on the
lateral Side of the stylet's articulation with the base of
the pleOpod. The stylet could easily reach the opening of

the vas deferens if directed toward it. Since a penial

-39-

stylet does not occur in the female it is probably reason-
able to assume that it is used in Sperm transfer. The pos-
terior three pair of pleOpods have epipodites developed
into small lamellae which are also fringed with setae.

The pleopods of A, communis are primarily respiratory
in function. ‘MOdification has occurred on the third pleo-
pod which has the eXOpodite develOped as a protective cover
with a distinctive suture or fold angled across its middle.
The second pleopod of the male is not primarily respiratory
but has the end0podite modified for Sperm transfer (van
Name, 1936, p. 19). In the male the first pleopod is two-
segmented and very small with a few small setae fringing its
tip. The second pleopod of the male has a fringe of long se-
tae on the exopodite, whereas the endopodite is small and
curved (fig. 27), being only about two-thirds the length of
the exopodite.

The female lacks one pair of pleopods. Needham (1938,
1941) states that the first pair are lacking in females of

A. aquaticus and A, meridianus. The only pair present anterior

 

 

to the modified cover-like pair in A, communis females is the
pair of small plates fringed with setae. The exopodites

of the remaining pleOpods have at least a few setae on

Fig.

24.

-40-

 

 

 

Third leg of 11% mm. male A, communis.

x 58.

-41-

 

Fig. 25. Fifth pleopod of 10 mm. female Q, thomsoni.
A, proximal end; B, distal end. x 50.

 

Fig. 26. Endopodite of second pleopod of 13 mm.
male Q. thomsoni. M, muscle fibers; P, penial
stylet. x 50.

     

Fig. 27. Second pleopod of 11% mm. male A,
communis. E, endopodite. x 80.

-43-
their lateral proximal margin. The third has the most plus
a row proximal to the angling fold. In addition to the
setae the fourth pleopod's ex0podite has a fringe of fine
hairs along the proximal lateral portion.

Telson and UroEods.

The ventral marginsof the telsonic pleura of Q, thom-

 

£221 bear a series of stiff setae which have short terminal
branches (fig. 28). The telsonic apex is small, upturned,
and bears four stout setae and several finer ones. Lateral
to this is a lobe bearing a stout seta. The uropods are
inserted along the latero-ventral margin. Each uropod has
a large basal segment and two rami. Between the rami is a
small process with several terminal Spines (fig. 29). The
rami are lateral and medial in arrangement but their great-
est thickness is in the dorso-ventral plane. There are two
tufts of setae located ventrally on the basal segment, and
several dorsal Spines. Although not much movement was
observed in the uropods during laboratory observations on
locomotion they are well supplied with muscles.

A, communis does not have a distinct telson on the

 

abdomen. The ur0pods are inserted along the posterior edge
of the abdomen with a Space between about equal to the
length of a thoracic segment. They have a flattened appea-

rance, eSpecially in the male. The endopodite is

-44-

 

Fig. 28. Spines of margin of telsonic pleura
of 13 mm. male Q, thomsoni. x 110.

-45-

 

Fig-29. Uropod of 10 mm. female Q, thomsoni
showing small, spinous process (P) between
the rami (R). x 80.

 

Fig. 30. Ur0pod of 9 mm. female A, communis.
x 80.

-46-

longer than the exopodite (fig. 30). Each has a tuft of
apical setae. Individual setae are spaced along the mar-
gins. The exopodite and endopodite are lateral and medial
in position. The majority of their setae are also lateral
and medial in position.

Locomotion o_f Q. thomsoni.

Normal locomotion is by walking or crawling and
includes a variety of possibilities. Each leg has an
anterior-posterior plane of normal movement. They can also
be moved laterally. The gnathopod is most median in posi-
tion. Subsequent legs move in a plane lateral to the
immediately preceding leg until the sixth leg is reached.
Leg six is meSial in position to leg five, and leg seven
is mesial to leg six. Thus despite close points of
attachment legs can move rather freely. For movement over
a glass surface by captive animals the first, second, third
and seventh legs have the most effect. Legs four and Six
may undergo only token movement, rarely contacting the
surface or they may be used with seeming efficiency. Leg
five is essentially stationary and occupies a position
nearly hOrizontal, often flexing dorsally if the animal
is tipped on its Side and thus may be used in righting

itself. Leg three and sometimes leg two may be less useful

-47-

 

 

 

 

 

Fig. 31. Plane of movement of legs of Q, thomsoni.

-49-

in some females with brood pouches; they are then some-
times arched SO that only legs one and seven are in contact
‘with.the substrate. The plane of movement is indicated in
.fig. 31.

Legs one through four are usually well synchronized.
The beginning of movement seems to be initiated by the
first or second leg. When under motion leg four reaches
forward and pulls backward Slightly in advance of leg three
and so on until the motion reaches the front. Legs six and
seven commonly move at a slower rate than legs one through
four. The right and left corresponding legs are usually
in the same phase of movement or nearly so in legs one
through four, while legs of pairs Six and seven may move
alternately or in phase.

Main points of bending the legs are at the distal
end of the coxa and at the distal end of the basis. In
the gnathropod the dactyl is also extended and brought
back to supplement the movement of the leg.

Pleopods are not used for locomotion by undisturbed
individuals. Pleopodal beating may provide a water cur-
rent to aid movement of irritated specimens.

Uropods usually trail travois-style but the abdomen

is frequently flexed then extended causing the uropods to

,1 molar-mum:
)"1" 31'! {Rm
H'HF-I"fiq£ it u

1!." ’I["3?—2.*'

-50-

 

Fig. 33. Ganglion of Q, thomsoni showing
concentration of neurons (N) in the ventral
region. x 180.

 

-51-

provide some thrust. This movement is usually Sporadic
and sometimes very infrequent. When disturbed, Speed can
be increased by an increase in the magnitude and rapidity
of this flexing and extending movement which produces a
water current giving additional thrust. Usually Speed at
room temperature is approximately one-half to one centi-
meter per second. “When climbing, speed is slower and
movement more deliberate. They will often continue to
walk for hours at a time around the containers they are in
and at the Speed they travel could be expected to cover
approximately one-half mile in a day.

The antennae extend forward and apart at an ever in-
creasing angle until at the end the filaments form.an
angle that is usually over forty-five degrees from.the
anterior-posterior midline. Usually the right and left
antennae are alternating as they are waved up and down.
Antennules are projected out and up in a fairly constant
position.

Locomotion 93: A. communis.

A, communis has a rate of sustained movement that is

 

approximately the same as that of Q, thomsoni. However,

A, communis more frequently moves with Spurts separated

 

by stationary periods than by sustained movement.

-52-

The abdominal appendages are not used for locomotion.
The uropods are usually positioned at an angle to the
main axis of the animal and the rami are widely Spread,
giving a large area of coverage, perhaps for sensory pur-
poses.

Normal locomotion involves movement of the thoracic
appendages only. The position of the first four pairs of
legs is such that each succeeding leg moves in a plane
lateral and somewhat caudad to the preceding leg. The
fifth and Sixth legs have ranges of movement that do not
ordinarily overlap the position of other legs. The seventh
leg is moved very little during normal locomotion. The

plane of movement of the legs is diagrammed in fig. 32.

INTERNAL ANATOMY
Nervous System

Q, thomsoni has a large supraesophageal ganglion or
"brain" in the head capsule. Smaller segmental ganglia
occur at intervals along the paired tracts of the ventral
nerve cord. The cell bodies of neurons of a ganglion are
concentrated in the ventral part of the ganglion (fig. 33).
Lateral nerves arise from the ganglia. The nerve and

ganglia are located above the exoskeleton of the sternum.

-53-

The circumesophageal connectives of A, communis

 

extend abruptly from.the supraesoPhageal ganglion. The
cell bodies of the neurons of the posterior ganglia are
located in the ventral part of the ganglia.

Digestive System.

The hepato-pancreatic glands and the intestine (fig. 34)

fill most of the body cavity of Q, thomsoni. The hepato-

 

pancreatic glands open into the ventral surface of the
stomach. They are paired tubular glands. Two Short pair,
one of which is very Short, extend anteriorly. Three
pair extend posteriorly most of the length of the animal.
All open at the base of the gastric mill. The ventral
and lateral tube of one side join, then their common tube
receives the dorsal tube before entering the stomach.
This is similar to the condition found in Mesamphisopus
capensis by Barnard (1927), whereas Smith (1909) found
Phreatoicopsis terricola to have two pair. My dissection
of a specimen of‘g. terricola indicated there may be a
third short pair extending only about half the length of
the other two pair.

The intestine posterior to the stomach is of a uniform
structure. It is connected to the anus by the short rectum.

The large absorptive cells of the intestine contrast sharply

-54-

 

Fig. 34. Cross- section of abdominal region

of female Q. thomsoni showing hepato- pancreatic
glands (G), abdominal cecae of intestine (C),
heart (H), and typhlosole (T). x 80.

 

-55-

 

Fig. 35. Junction of intestine (I) and rectum (R),
cross- -section of male Q. thomsoni. x 360.

-56-

at each end of the intestine with the small cells of the
stomach or rectum as shown in fig. 35. The cells of the
intestinal lining possess very large nuclei and are of the
simple cuboidal type. The cells of this layer are thrown
intoa ventral loop extending dorsally into the lumen as
the typhlosole. This structure is not an artifact of pre-
servation. Fig. 36 shows the inner circular and outer
longitudinal muscle fibers of the intestinal wall do not
follow the intestinal lining cells forming the fold of the
typhlosole. In some instances the muscles do follow the
fold which is much more open. The Mount wellington speci-
mens examined by Barnard (1927) when he stated they do not
have one must have been of the latter type since the typh-
losole of those could resemble a wrinkle produced by
preservation. Thus Smith's (1909) statement is substantiated
regarding the presence of a typhlosole that was questioned
by Barnard (1927). The cells of the lining make up most
of the thickness of the intestinal wall. They are of regu-
lar arrangement (see fig. 37). In 25 terricola they can
be seen to present a rectangular appearance arranged in
rows visible to the unaided eye. Some‘specimens of

Q, thomsoni have a few paired ventral or lateral cecae in

 

the abdominal region (fig. 34).

-57-

 

Fig. 36. Cross-section of female Q, thomsoni
through oviduct (0) and genital opening (G).
x 80.

 

-53-

In A, communis there are two pair of hepato-pancreatic
glands that arise ventrally near the posterior end of the
gastric mill. There is a short median pocket extending back
from.the common atrium of the hepato-pancreatic glands.

The intestine of A, communis lacks a typhlosole and

 

is round in cross section. It has an inner circular muscle
layer and an outer longitudinal muscle layer. The lining
is a single layer of large cells with prominent nuclei.
Circulatory System.

Dorsal to the intestine of Q, thomsoni is the "heart"
or dorsal blood vessel extending through abdomen and
thorax. The position of the valves indicates the "heart"
is filled through the ostia (fig. 38). There are a number
of paired arteries (fig. 39) connected to the heart.

The heart of A, communis lies in a blood sinus in the
posterior thoracic region.

Excretory System.

The contorted tubular maxillary glands are considered
excretory structures of isopods by Needham.(1942a). The
glands are prominent structures in the head.

Exoskeleton.
The exoskeleton of a gravid female Q, thomsoni was

16 microns thick. The exoskeleton of a female, from a

-59-

 

Fig. 37. Tangential section of gut wall
of Q, thomsoni showing regular arrangement
of cells.

 

Fig. 38. Section through ostium (0) in heart
of female Q, thomsoni. x 80. .

 

Fig. 39. Section through arteries (A) leaving
heart of female Q, thomsoni. E, hypodermis;
S, exoskeleton. x 80.

-6l-

breeding pair of A, communis, was 8 microns thick. In
both Species the exoskeleton is colorless.

Coelom, Muscles, and Coloration.

 

Coelomic spaces extend through most portions of the
organisms. Striated muscles run in many directions through
the spaces. They underlie the hypodermis in many regions.
The hypodermis contains much brown pigment, especially on

the dorsum (fig. 39) of both Species. Q, thomsoni is

 

darker brown, although some Specimens in a late predmolt
condition have a banded appearance with a slight irrides-
cence which partially obliterates the brown. Such speci-
‘mens are usually in vegetation or on the surface of the
bottom, but rarely in the bottom mud.

No pigment occurs where the muscle fibers pass through
the hypodermis to insert, or originate, in the exoskeleton.
That, Needham.(l942b) has shown, is the cause of pigment
pattern in the English species of Asellus.

Reproductive System.

The paired tubular testes of the male Q, thomsoni
are located dorsolateral to the intestine. The vas
deferens descends from the posterior end of the testes
to open through the pene on the coxa of the seventh perio-

pod. The ovaries of the female occupy a position similar

-62-

to the testes of the male. The oviducts of the gravid female
descend from the posterior of the ovaries to open (fig. 36)
within the brood pouch near the posterior of it.

Eggg.

The eggs of Q, thomsoni are large (750 microns in dia-
meter), spherical, yellow, and few in number. From 11 to
17 were found in brood pouches of females. A 12 mm. female
that was dissected had 18 eggs packed in two columns exten-
ding from.mid-abdomen to the third thoracic segment. They
were in the position of the ovaries. The gut was empty and
her exoskeleton was in a pre-molt stage. She was collected
'May 18, 1957.

The eggs of g, communis are smaller (350 microns in
diameter), pale yellow, and more numerous. Brood pouches
contained from 95 to 168. Those dissected did not remain
intact for counting.

Deve10pmental Stages gad_£hg_Dorsal Appendage.
No dorsal appendage was observed in whole eggs and

embryos of Q, thomsoni that were examined with a dissecting

 

scope. Position of the embryo in the egg was somewhat vari-
able. In general they were sharply folded upon themselves
in the region of the last two thoracic appendages of the

embryo. The folding brought the dorsal surface of the pos-

-63-

terior region into contact with the dorsal surface of the
anterior region. variation involved folding similar in
position but with the lateral surface in contact, or an
intermediate position. However, the ventral surface always
faced the outer portion of the egg membrane, thus the devel-
oping appendages took a position at one end of the egg or

as a band enveloping it. Serial sections revealed a protru-
sion which represents the dorsal appendage. The dorsal
appendage is yolk filled and does not project much beyond
the circumference of the egg (figs. 40 and 41). The chorion
encloses the appendage without following its contours. Thus
except for position the embryo is much like that of @-
phisopus capensis figured by Barnard (1927). The material
from.which Barnard prepared his figure was very generously
loaned to me by the South African Museum and shows some
sections (fig. 42) in which the dorsal appendage is even
more prominent than in his figure.

The developmental stages of A, communis, as well as

 

the general internal anatomy, agree with that figured in
Sars (1867) for A, aquaticus. The dorsal appendage is a
prominent feature on the egg in the late stages of develOp-
ment and is shown in fig. 43. Histologically the appendage

is identical to the adult pleopod. The ventral surface

-64-

with the developing appendages of the embryo occupies the
surface position. The folding along the dorsal side causes
the dorsal appendages of the embryo to appear at opposite
sides of the egg. Sars (1867) has shown that the chorion
has disappeared from the egg of g, aquaticus at this stage.

The newly hatched young of both A, communis and Q.
thomsoni have a general resemblance to the adults but have
only six pairs of walking legs.

An adult female of Q, thomsoni is illustrated in fig.

44.

-65-

5'00“

 

 

Fig. 40. Drawing of an embryo of C. thomsoni
(longitudinal section). StipplingT'cellular—area;
circles, yolky area; A, dorsal appendage; C,
chorion. Drawn with the aid of a camera lucida.

-66-

3’ 500M

 

 

Fig. 41. Drawing of an embryo of C. thomsoni
(cross—section). Stippling; cellular area;
circles, yolky area; A, dorsal appendage; C,
chorion. Note that both dorsal appendages are
visible and that embryo appears in two portions
due to folding. Drawn with the aid of a camera

lucida.

-67-

‘loo M

 

 

 

Fig. 42. Drawing of an embryo of M. ca ensis
(longitudinal section). StipplingT ceIEuIar area;
circles, yolky area; A, dorsal appendage. Drawn
with the aid of a camera lucida from a slide
’prepared by K. H. Barnard on 20/12/13; on loan
from the South African Museum.

 

Fig. 43a. Dorsal appendage (A) of A_. communis
showing relationship to egg (E). x 80.

 

Fig. 43b. Dorsal appendage of A, communis.
x 360. ’

-69-

//’

4.

44mm.- are:

 

.HSOmaoau .M madam“ pw>muwucoa .uaopm mo wnw3mup 3oH> Honouma

  

Whirl \11...
9%....39».

.ee .wam

\\l
......

NATURAL HISTORY OF COLUBOTELSON THOMSONI

Qigg Distribution.

Examination of collections usually showed three size
classes. This is partially evident in the graph (fig. 45)
of the size classes of collections. Specimens were mea-
sured for length when flattened along a straight edge in
a manner such that the intersegmental membranes did not
Show. ‘Measurement was to the nearest one-half millimeter
(from.the anterior edge of the head capsule to the pos-
terior end of the telsonic projection. From.the graphs
(fig. 45 and fig. 46) it is obvious that I tended to
gauge length as being closer to the nearest millimeter
than to the fractional measurement. The presence of
three distinct size classes (fig. 47) is more obvious
than the modes in fig. 45 indicate.

Laboratory Rearing.

Specimens maintained in glass tanks in the laboratory
had a high mortality. No young were reared to maturity.
Those that hatched in the laboratory did so at an earlier
time thanroccurred in nature. Perhaps higher.laboratory
temperatures speeded some developmental processes faster

than others with reduced vitality as a result. Laboratory

-71-

Fig. 45. Size distribution of Q, thomsoni.
Ordinate, number of individuals; abscissa, length
in mm. Diagonal ruling, males; stippling, females;
clear, sex undetermined.

(swam UUlVQD 040140

0-1me

-72-

 

 

NOVEMBER 9, I956

 

 

 

 

 

 

 

 

 

 

 

m
? 5.55.1” €355:
fl )‘El‘iV/xé ,

 

 

 

MAY 18,1957

 

l—lfl

 

 

rfl

JULY 6, I957

 

 

 

 

 

 

[— .1

 

mm

 

 

AUGUST 16, 1957

 

 

 

 

 

 

 

 

 

 

\\\\\\\\\

 

 

 

 

13' 3

 

 

-73-

Fig. 46. Size distribution of Metaphreatoicus
magistri collected on Bruny Island, Tasmania on
February 19, 1957. Ordinate, number of individuals;
abscissa, length in mm.

 

33
29
25

21

FEBRUARY .19,

 

 

 

 

 

 

 

 

 

 

 

 

 

I957

 

 

-75-

HJ) ’ ’WJ "" ‘ ”’\‘k

NOVEMBER 9. I956

fl

H... I.“ a) ”m "“.

HAY IB, I951

 

 

 

m
"WhMbv “)’ ‘ ‘

JULY 6. I”?

amwunmw

mnuau

AUGUST l6. I957

 

  

 

 

- _,__- _ y— .--—o—-
I

 

Fig. 47. Photograph of specimens used for size
distribution in figure 45. x 5/7.

-76-

 

 

 

 

 

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ohms same 03» .mQOpmhoan on» Ca opaam puma messaomom condom mo .Nmma .ma has
.esHosmame madam cm can mamas“ mco .opscde a camp mama ca o>auoa madame hon» .mcmEa
nomam nmnpo mm Ham: on .mcmedomom pended one on cocoa mm: poems con; .eoupoo Hood
gateways one ca mpdpomca who; made was» no pmpomaaoo mcosdomam .Nmma .m nohwz .
.casmo omnamo mam: madma can mo omega Nmma..am .no& :0 .onth one ocsoam pmaaso can
case who nouns cmucamomm gamma use nonumwou Hades was sand was om .nm& :0 .mmwo
accedpduom 03p mom was and mcaaoaaoh on» madam 03» .zmc awoc one anemone Adana mnmz
madam mouse .anOpmnonoa on» Ca m>aam paox ohms mnawa such .Nmma .ma announce
.madsmhme on» puma can .mamamu oco ca ozu pomoxm .mcsom on» and use: one .m>aam pom:
pew pmpomaaoo ohms sausages :a mono» nods moaoEmm 039 .Nmma .:H.mamscmh
.maQSmnme show no: cap moans nonmad amwmpmoo Hanan weanmmn mums mmma .mm
.omm so was condos adamspcm>m manomams and: mmHmEom omega .wmma .m nooao>oz
.mopoc hampcmaoaansm

 

mm» we» mom on on oc mm» 0: mo» mom mm» mm» mm» popomaaoo
moamsmh oa>mnw

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ms¢ and as: so< Adz mom can can >02 poo poo new new

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-77-
temperatures ranged from.42° F. to 92° F. while midday water
temperatures in their native pool on Mt.‘Wellington were

from 32°F. to 70° F.

Reproductive Cycle.

Table 1 contains data Showing only one breeding cycle
per year in Q, thomsoni. Pairing begins in February and is
a prolonged process which even the dormancy of a dry period
did not stop. The fourth walking legs of the male are used
to carry the female beneath him. Pairing may last three
months. Following mating, eggs are laid about June and are
carried for months until the miniature adults hatch and are
released about December.

With such a long reproductive cycle, in which all mem-
bers are at about the same stage, there can be only one large
influx of small individuals into the pOpulation per year.
Thus each major peak in the curve should be chiefly indivi-
duals released at the end of the yearly reproductive cycle.
The sudden influx of the smallest size class at the end of the
cycle is obviously from.the most recent year, the next larger
Size class those from the previous year, and so on. Inspec-
tion of the data reveals at least three size classes. Thus
it takes a minimum of three years for Q, thomsoni to reach
maturity. ‘Mortality and slower growth of the larger sizes

could obscure what is possibly a longer cycle of life.

Barnard (1927) used a similar analysis of size classes of

”J .' .I." "up. urn-51mm 'u’l Ir (V'aJl; "ahn

.u m“; ._:A ‘1': Josue?! not: em. 1013 212mm .‘r ".7
2.. .m. a: hm. .1951: suck-mm ad: mfl"uodu gem
"-.' . Angel.) 531:. 3311!” 38m! 38 I‘Mliw $113
w: 3 2362mm: .2 303‘ case! $911” lfqdupinlm n an.
..;r.: reg-mi an; is Mum-:3 touch 1933211131108 .-(_.;
.3111 in sing: maggot a 1161”“; .§_ um say-12'?

1c. 2593qu am. ~10 autism nlmpg‘psw ($381) 9

-73-
collections of Mesamphisopus capensis in South Africa and
found that member of the Amphisopidae took approximately two

years to reach full size.

Paired specimens were first collected on February 15.

Specimens were still collected in the paired condition as
late as May 18, with one pair being found on July 6. Pairs
brought into the laboratory would not always remain paired.

Two pair brought in on May 18 were still paired nine days

later. Females from.ather pairs brought in at the same time
had separated from the males, completed their molt, and dev-
eloped marsupia by May 27. Data in Table 1 indicates that
c0pulation occurs in April or May.

Incubation.

The eggs are carried through the winter and well into
the summer before they hatch. Many gravid females were col-
lected on September 3. All of them died by October 9, pro-
bably from.the high temperatures which occurred in the lab-
oratory. Development of the eggs seemed to occur more
rapidly than in the natural habitat. None were reared suc-
cessfully. The few that did hatch died shortly after hatching.
A considerable quantity of yellow material, presumably unab-

sorbed yolk, could be seen in the thoracic region of the
young which are unpigmented when hatched.
As with other isopods, the young resemble the adults but

have one fewer pair of legs and fewer antennal segments,

-79-’

as well as lacking the brown marbling seen in the adults.
Of two females with marsupia collected on January 14, one
had an empty marsupium and the young from.the other had
nearly all left by the next day. Nearly all eggs develOp
successfully.

Ecdysis.

After the young escape, the subsequent molt of the
female replaces the large oostegal plates forming the
marsupium with small plates similar to those of the pre-
reproductive period. The exuvia of the youngest specimens
are shed in such a fragmented filmy condition that the
parts are not distinguishable from the debris the isopods
must be fed to maintain them, The larger specimens molt
and leave an exuvium which is usually in two pieces. The
break usually occurs behind the fourth thoracic segment.
Specimens are seldom.found exposed in their natural en-
vironment but are usually in the mud, under rocks, or in
vegetation. However, numerous exuvia could be seen on
the surface of some moss in which they were reasonably
abundant. One was observed once kicking at its head with
its uropods, until its molting was complete. Specimens
are often found in which the anterior half is a lighter
. silvery-brown color due to the presence of the front half

of the exuvium. It would be impossible to calculate the

.r' .a. 1‘). awards 21:11»): on: bald-41 en's-n {ileum ”as
"Til Intruder: (sled-4, n5: bosom brand nobles an. assuage

ni— 1n 3ng 15:an bus, 5%“ (flaunt: 9m and anemic-1

 

 
 

W was maid-quasi

“his!!! 31‘ mutvuxs 9:13

I ,

-30-

number of molts from Size classes as can be done for
Tardigrades (Higgens, 1959) and Ostracod instars (Kes-
ling, 1951) since the isopods' segmentation makes the
measurement of length so variable.

Q; g Cycles 2: Related Species.

Limited data (fig. 46) indicates Metaphreatoicus

 

magistri may have a life cycle similar to the closely
related Q, thomsoni. There was an absence of gravid fe-
males at the same time in each population and the graph
of the population size classes shows peaks indicative of

a three year life cycle. This occurs in a markedly warmer
habitat with no freezing temperatures. Other species col-
lected in other parts of Tasmania seemed to be at the same
phase of their reproductive cycle as Q, thomsoni. Barnard
(1927) in his study of Mesamphisopis capensis made the
only previous investigation of the general biology of a «
phreatoicid. There are several differences between the

life cycle of Q, thomsoni and the life cycle of y, capensis

 

 

(Barnard) Nicholls, 1942.

Barnard interpreted his data as indicating the South
African phreatoicid matures in less than two years. He
also found that the mating, or pairing, began much later

(in early May) but lasted even longer (about Six months).

-31-

However, the eggs were deposited in the marsupium and
hatched before two months had passed after pairing. Their
release occurred in late December which is comparable to
the time of release of Q, thomsoni. They produce 7 - 15
eggs but usually fewer (5-12) reach maturity. Barnard
concluded that the female produces only one brood on the
basis of Size data and the absence of females among the
young and males shortly after hatching. He was not suc-
cessful in rearing them.under laboratory conditions for
more than a month and frequently gravid females died before
the eggs hatched.

Sex of specimens reported in figure 45 includes 45
males and 21 females with 57 too immature to sex.

General Ecology gf Q, thomsoni.

The pools on Mount wellington in which Q, thomsoni
was taken were clear and shallow, mostly less than one
foot in depth, and in only a few places wider than one
could step across. The collecting site is indicated on
the map of Mount wellington (fig. 48). The water supply
from.rain and melting snows percolates down through the
scanty vegetation and high-moor soil covering the dolerite
peak. The flow of water is slow in most places due to the
slight slope, rock obstructions, and the bolster plant

(Abrotenella Fosterioides) which by its damming gradually

-82-

 

 

 

 

 

 

 

     

 

 

Fig. 48
S E
TA 5 M ANIA
IMILE
H /
w E / /
S
comoua INTERVAL ' V o
0‘ INSET. zoo rcn‘ / ‘2,
< 2
m
’ z
/ —I
LONGLIY é
KINGSTO T
:37
BRUNY
. ISLAND
I
t I MILE \%N\ 14”
MT. WELLINGTON

 

 

 

 

 

 

 

 

 

 

-83-

alters the flow pattern of the water (Martin, 1940). The
rainfall is about fifty-five inches per year, with monthly
means ranging from.3.36 inches in February to 6.04 inches
in October. The temperature of the air at a location 1,600
feet below the collecting site has a mean maximum of

52.40 F. and a mean minimum of 39.60 F. For February, the
'means are 61.7 and 45.7, for October 52.2 and 38.0. July
has the lowest means, 43.90 F. and 34.90 F. Thus with the
seasons having a six months lag or advance in the Southern
Hemisphere, the higher insolation of summer with a slightly
lower rainfall and higher temperatures frequently causes
the pools to dry up during January, February, or March.
During 1957 the pools were nearly dry on January 2. On
January 14 some pools were still present but there was no
appreciable current left. By February 15 the situation
was still similar. On March 6 they were dry with the mud
still moist but not wet. The filled pool is shown in

fig. 49 and the same area is shown in fig. 50 during a

dry period. During the dry period the mud remained at
least slightly moist although cracks appeared on the sur-
face. Inactive specimens were found in the dry mud.

Some were in small chambers not much larger than the indi-
vidual. No structural material other than the mud could

be found forming the wall of the chamber. The isopods

 

 

 

 

Fig. 49. Mount Wellington collecting site
with pool full.

 

Fig. 50. Mount wellington collecting site
with dried pool bottom.

  
  

4 ,4 «$1.: $3 - "7

Fig. 50a. Mount Wellington collecting site
showing dried pool bottom.

Fig. 50b. Mount wellington collecting site
as viewed from Battery Point, Hobart.

.
‘T—al ,1

.1. '

-86-

 

Table 2. Temperature of pool, Mt. Wellington and
remarks about collection of Q. thomsoni.

 

 

 

Date Temperature Remarks
of water, °F.

 

 

Sept. 3, 1956 -— Sunny, snow on ground, phreatoi—
cids found in moss, under cush-
ion plant, under stones, etc.

Sept. 12, 1956 34° None found. Some unmelted snow
in pools.

Sept. 22, 1956 739° Found in moss.

Oct. 6, 1956 42° Mostly gravid females in cushion
plant.

Oct. 17, 1956 44° Males found in loose moss.

Oct. 30, 1956 45° Gravid females in cushion plant

and moss.

Nov. 9, 1956 56° Gravid females mainly in moss.
All others chiefly in mud.

Jan. 2, 1957 650 Found in mud. Pool nearly dry.

Jan. 14, 1957 70° No current left. Specimens
found under rocks, with a few
in moss and mud.

Feb. 15, 1957 54° Many paired in moss.
Mar. 5, 1957 -- Pool dry, none collected.

Mar. 6, 1957 -- Dormant Specimens found in damp
mud and under cushion plant.

Apr. 15, 1957 44° Some still paired under cushion
plant. Others in mud.

May 4, 1957 42° Large ones found in pool ten
feet from summer low point.

May 18, 1957 40° Paired Specimens under cushion
plant.

July 6, 1957 32° Pool ice-covered. Few phreatoi-

cids found.

Aug. 16, 1957 33° Pool ice—covered. Fairly abun-
dant in mud.

 

 

 

 

 

 

-37-
collected under these conditions were moist but exhibited no
activity. When they were placed in water they became active
within a minute and were not noticeably different in behavior
than the ones collected in wetter circumstances.

By April 15 the water level was back to normal. The
pools were usually ice-covered during July, August, and Sep-
tember. Data regarding temperature of the pool is in Table
2. Further information regarding rainfall variability is
provided by Scott (1956). Davies (1958) discusses the sur-
face geology of Mount wellington. Martin (1940) treats the
physical factors as well as the vegetation of Mount welling-
ton. Aquatic vegetation was of only two Species. One,
Drepanocladus Sp. in some areas made extensive beds which
frequently contained mature phreatoicids. The other, a tiny
leaf liverwort, Jungermannia Sp., occurred in much smaller
patches which occasionally contained phreatoicids. The phrea-
toicids, apparently, were feeding on the periphyton. In the

laboratory the Drepanocladus was left fairly much intact,
although its periphyton was removed by the feeding phreatoi-
cids. In the process of feeding on the liverwort, Junger-
mannia Sp., leaves were usually separated from the stem.

The bolster plant, Abrotenella Fosterioides grew in and at
the edge of the pools with its base in the water. On the

underside of clusters of them.I have found paired

-33-

adults and at other times gravid females. Astelia alpina
made up most of the remaining vegetation of the edge not
occupied by A, Fosterioides, but it was less in the water
and I found no association between it and phreatoicids.

In addition to the moss and vegetation there were other
habitats for the phreatoicids. Sometimes an individual
would be found crawling along the bottom. Often they could
be found on the undersides of rocks. But most frequently
they could be found in the soft organic mud which filled
the lower spots where the current was slow. The mud was
fine, black and almost flocculent. All sizes (but few
gravid females) could be sifted from such mud on most
occasions. Immature forms were rarely found in places
other than the mud. Phreatoicids were seldom.found in mud
denser with inorganic particles. No leaf litter occurred
on the pool bottoms. Many broad leafed plants occurred
nearby but were of the evergreen type so the seasonal
accumulation of leaf litter did not occur. This is pro-
bably a widespread condition in the Southern Hemisphere
areas where the Myrtaceae is the most important woody plant
family. Other species are commonly found among submerged
Vegetation and roots or under rocks. More unusual loca-
tions have been crayfish burrows (Hypsimetopus intrusor,

Sayce, 1902); wooden reservoir pipe (Colubotelson setiferus,

-39-

Nicholls, 1944); wells (Chilton, 1894); a hot, saline bore
(Phreatomerus latipes, Chilton, 1922); subterranean burrows
(Phreatoicopsis terricola, Spencer and Hall, 1896); and the
stomach of trout (Mesacanthotelson decipiens, Nicholls,
1944). Phreatomerus latipes was taken swimming in the hot,
saline bore, and to my knowledge is the only record of a
swimming phreatoicid. Colubotelson thomsoni was never ob-
served swimming under either field or laboratory conditions.
Animals associated with Q, thomsoni were mostly of
the type common to vernal ponds and small bodies of water
throughout the world. However, there were perhaps fewer
species and fewer individuals present. Those collected
at the collecting site are listed in Table 3. There was
a notable absence of larger predators. The red-spotted
frog Crinia tasmaniensis of Mount wellington has been
found to have phreatoicids in its stomach during the
breeding season by John Hickman (personal communication).
Although I have found the frog and tadpoles in pools on
‘Mount wellington, I have never found it in the sample
area. There were no mollusks in the pools. Anaspides

tasmaniae is found in the deeper pools farther down the

 

Northwest Bay River on Mount Wellington, but I have never
collected them in the study area either, although the

literature indicates they occurred there in other years.

-90-

Table 3. List of the taxa of invertebrates, other than
Q, thomsoni, having examples observed in the sample pool

on Mt. wellington, Tasmania.

 

 

 

 

Protozoa
Peritrichida. Vorticella Sp.
Platyhelminthes
Tricladida. Planariidae.
Anellida
Oligochaeta.
Plesi0pora. Naididae. Pristina Sp.
PrOSOpora. Lumbriculidae.
ArthrOpoda
Crustacea
Cladocera.
Copepoda. Cyclops albicans Smith.
Ostracoda. Cypridae.
Amhpipoda. Neoniphargus montanus (Thompson).
Insects
Collembola. Hypogastrura?
TrichOptera. Limnephilidae.
ColeOptera. Helodidae. Dysticidae (two Species).
Diptera. Tendipedidae. Tipulidae?
Arachnida

Acarina. Hydrachnellae.

-91-

A, tasmaniae and Q, thomsoni would seldom be in direct
competition if both were present due to different feeding

preferences. The amphipod Neonjphargus montanus (Thompson)

occurs abundantly in the pools. E, montanus is much

 

smaller than Q, thomsoni and more active, Spending much

of its time in the open. In contrast to Q, thomsoni which

 

is a dull brown-black, the color of the mud, the amphipod
is yellowish-orange. A small ostracod with bluish-green
valves is common on the surface of the mud. A copepod
was frequently found in the moss. A light red mite with
dark markings was found in the pool during the summer.
Adult and immature dytiscid beetles were common. Only
one large (over % inch) adult dytiscid was found. When

it was placed in the tank with a phreatoicid the phreatoi-
cid was found dead the next day with many of the dorsal
tergites missing. Small adult dytiscids (less than %
inch) were common. On one occasion one was observed to
bump the posterior of a small phreatoicid just at the sur-
face of the mud. The phreatoicid spurted forward as if
he had been stung. Trichoptera larvae were sometimes
present. Tendipedid larvae were abundant. A long red
oligochaete was very abundant in the more compact bottom
sediments which seldom contained phreatoicids. The floccu-

lent, phreatoicid containing sediments also had two Species

-92-

of colorless small oligochaetes. A planarian, Dugesia
Sp., was abundant until summer. It was grey-black with
round auricular sense organs and the pair of eyespots
‘were posterior to the area between the auricles of the
triangular head. The phreatoicids and the large red oli-
gochaete were the dominant animals in the pool. Each
appeared to prefer a different type of bottom sediment.
The planarian and the amphipod were at times abundant in
the area above the bottom.mud. One interesting point
regarding the isoPOds, amphipods, and ostracods of the
pool was that they were never observed to swim.in either
field or laboratory, but always crawled or burrowed. Thus
the only forms observed swimming were the adult dytiscids
and the few copepods. Vorticella Sp. was sometimes very
abundant as epizooites on specimens maintained in the
laboratory. Occasionally laboratory specimens would
develop a dense growth of Saprolegnia sp. around them
within a day of death. The rapid growth of the mold made
it appear that the mold was established before death and
perhaps caused death. Dead specimens, not removed imme-
diately from.aquaria, were often filled with many Euglena
Sp., Colpoda? Sp., and bacteria. Specimens of Q, thomsoni
put in an enamel tray for sorting from debris would usually

remain motionless and curled for some time. In fact,

-93-

curling seemed a more frequent response to disturbance
than escape movements.

Isopods, in closed vials of water, increased the fre-
quency and magnitude of pleopodal beating in the early
stages of asphyxiation. When Q, thomsoni and A, montanus
were placed in the same vial the amphipods would succumb
before the isopods. The isopods could be revived more
readily than the amphipods when placed in cpen containers
with little water.

There was an incomplete correlation between pools
which appeared suitable for phreatoicids, and the presence
of phreatoicids. Incomplete observations indicate that
pools containing isppods may have been pools with greater

resistance to drying during drought periods.

NATURAL HISTORY OF ASELLUS COMMUNIS
gigs Distribution.

Collections of A, communis were of one size class, two
size classes, or a wide range of sizes. The smallest sizes
were collected in May and August, larger sizes were present
in all collections (Table 4). The adults of the Spring
population were easily distinguishable from.the juveniles
of the spring pOpulation by a minimum size difference of
3 mm. 'Most young were added to the population in early
May. The young of the Spring were the major part of the
late June population; adults were then very rare. More
juveniles were added to the population in August.

Pairing 229 Egg Deposition.

Pairing of adults was relatively Short and probably
was of less than a month's duration. A large number of
breeding pairs were found in the spring (the juveniles of
the May collection were the progeny resulting from that
breeding period).

Laboratory observations confirmed the Speed of their
reproductive process. Paired specimens were seldom.to-
gether long after being brought into the laboratory. They
soon separated and the females molted to produce a marsup-

ium Which they filled with eggs.

-94-

 

 

 

 

-95-

Following are observations made on April 2, 1958.
A pair was isolated at 3 p.m. The male was considerably
largerthan the female and held her with his fourth per-
iopods between her third and fourth periopods on the
thorax. During more than half an hour of observation at
15X magnification with a binocular microscope, he attempted
to, and perhaps did, fertilize her from each side by
arching his body in the posterior thoracic region and
bending his posterior around to her side. His first and
second pleopods would often quiver violently and move back
and forth rapidly but no transfer of material could be
seen. This was repeated several times, usually first on
one Side and then the other in rather quick succession and
then a longer rest afterward. At times he would bend his
head down sharply over and in front of her. At least once,
he was observed holding the base of an antennae of hers
with his gnathopod. The antennal base seemed to fit be-
tween the two palmar processes. At times she would
vibrate up and down beneath him. At least some of the
time, he was causing the vibration with his fourth perio-
pod. That was apparent because there was some slack in
his grip with greater movement of his periopod than her
body. He moved very little at the time. I thought that

he was being unsuccessful and would keep trying Ag infinitum

 

 

_-96-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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-97-
so I stopped observing. Seven hours later they were separa-
ted. The anterior part of an exuvium was present and presum-
ably belonged to the female for the male did not appear to
be changed. Her brood pouch was already full of eggs, none
were seen on the bottom of the dish. She still possessed
both antennae and uropods.

Specimens are frequently found with an antenna or a
uropod missing. Loss of an antenna could easily result from
mating. Eggs are frequently drapped from.the marsupia of
Specimens being reared in the laboratory. A female with an
empty marsupium.was isolated. Seven hours later the marsu-
pium was full but the lamellae of it were ruffled to one
Side although no Opening was visible. On the bottom of the
finger bowl were 36 to 40 eggs. The eggs in the marsupium
seemed tightly packed in.

Incubation ABQ_Growth.
Observations on the incubation period were in fair

agreement with Ellis (1961). He found A, militaris (i.e.,

A, communis) had an incubation period of 10 or 11 days and

 

A. intermedius 12 or more days, depending on temperature.

 

I found A, communis had completed incubation in 15 days at

room.temperature and one day later had no marsupium, i.e.,

the largest oostegal plates were replaced with small ones
by a post-incubatory molt. Table 4 indicates the wild

population grows to parental size in approximately three

-98-

months during the period of May to August. The pool was
dry during September and the larger sizes seemed to sur-
vive this period best. I do not interpret my data to
indicate a growth during this period as suggested by Kenk
(1949). On September 5th I was able to find seven gravid
females in moist depressions in the dry pond. The females
became active when placed in water. Along with them.were
four tiny specimens about one millimeter long which were
the same Size as others in the brood pouches. On Septemr
ber 19, 1959 one came out of a sample of dirt to which
water was added.

A female collected through the ice early in winter
was placed in a jar in the refrigerator with several other
iSOpodS. Although inadequately fed to produce normal yolk
filled eggs, she had a marsupium.at the time in the spring
when they were most common in the natural pOpulation.

Specimens maintained in the laboratory had variable
growth rates depending on the amount of food supplied.
Thus specimens with little food exhibited little or no
growth but resumed rapid growth with adequate food. Thus
it would seem.that the slow growth reported by Johansen
(1920) was the result of an inadequate food supply.

Number 2: Broods.

Examination of the size distribution in Table 4

-99-

indicates that only one brood was produced in the spring
breeding season since the period when the Shortest size
was collected in was of such short duration with no recur-
rence of that size until August. Apparently the August
broods came predominantly from the young of the year as
indicated by the declining size of gravid females in the
fall (Table 5) and the sparsity of large size females in
July collections (Table 5). But examination of the res-
ults in Table 5 indicates that females may produce a
second brood since the majority of larger females in the
June-July period were gravid. Their numbers were small
and of minor importance in the population. These obser-
vations agree with Allee (1912b) on number of broods, but
the Size distribution indicates it is doubtful that any
adults from.the spring survived to the following year as
he stated they may. Paris and Pitelka (1962) found a
somewhat similar pattern of brood production in the terres-

trial isopod, Armadillidium vulgare.

-100-

.omehooms hopes: on» cH
pm©5HocH poo BSHQSmhms madam as csz moo omchpcoo COHpooHHoo mHzp *

soap

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ascend Hmpoe

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Is as I» I » m w I» Iw I» I» I so H.ee cH-

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oH>mnw whoz use»

mm o o o ooH om 00H 0 OOH 00H om OOH mm mm om o .
mm

 

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-101-

megs.-
Ellis (1961) found that brood size was proportional

to length of parent in A, intermedius. Hatchett (1947)

made similar observations on A, communis. Two selected

A, communis with apparently undisturbed, full marsupia had

 

brood sizes that were within the Size ranges they found in
A, intermedius and A, communis. One 9 mm. long had 168
embryos, one 7% mm. long had 95 eggs. Ellis (1961) found
gravid females from.leSS than 5 mm. to almost 13 mm. in
length. The size range of gravid females for A, communis
has less variability in my data (see Table 5). The range
is from 6 mm. to 10 mm. In the spring the length is nearer
the median value, whereas in the fall they were mostly 6%
or 7 mm. long. Hatchett (1947) found that brood size
ranged from.53 to 235 in females varying from 7 to 11% mm.
long. Within the same brood all eggs, or embryos, were very
close to the same stage of development. Non-developing
eggs were never observed in marsupia. I suSpect that
under natural conditions a very high proportion of eggs
retained in the marsupia hatch.
seam.

Sex of Specimens determined from 13 months collecting
beginning April 25, 1959 included 113 males and 107 females

with 388 immatures.

-102-

General Ecology g; A, communis.

The vernal pond from which the Specimens of A, communis

 

used in this study were taken had maximum dimensions of
over 100 feet by over 60 feet in the spring when its great-
est depth was over three feet due to flooding by melting
snow and heavy spring rains. This size was not maintained
long and a more natural level appeared to about two feet
of depth which gave dimensions of about 60 feet by about
40 feet (The area in zones B and C of fig. 51). Following
foliation of the trees and the resulting transpiration
the level dropped rapidly until the depth was approximately
one foot, except for occasional partial refilling by heavy
rains. The last foot of water was quite persistent until
prolonged hot dry periods came and the water disappeared.
In the year of 1959 the pool became dry in mid-July for a
short period and again during September and early October.
As Allee (1912b) stated, specimens can be found in
the mud during droughts. Specimens which I found during
such periods were in depressions filled with leaves and
mud. They were inactive but became active shortly after
water was added to the container in which they were placed.
The Specimens thus recovered on September 5, 1959 were all
gravid females. Litter and mud from the same depression

also harbored many Chaoborus Sp. larvae, many aquatic

-103-
oligochaetes, and some tendipedid larvae which became active

when water was added.

From December 1959 to April 1960 the pond was ice-
covered. During this period the water develOped a discolo-

ration and foul smell. The few live asellids collected then
were in a rather sluggish condition. 0n the thirty-first of
January, 1960, there were about three inches of snow cover.
There were three inches of surface ice over about five inches
of water which covered a second layer of ice similar to the
condition Kenk (1949) encountered in his Pond I.

Temperatures encountered in the pond are recorded in
Table 6. Average monthly temperatures and precipitation
recorded in Table 6 are from.stations located a few miles
from.the collecting site in Baker WOodlot. The pond was

shaded most of the day by the canopy provided by trees which
are predominantly American elm, swamp white oak, black ash,
and silver maple. These also contributed the majority of
the leaf litter along with a strong admixture of beech and

maple leaves from.the trees on higher ground around. No
rooted aquatic plants occurred in the pond. A slight algal
growth consisted principally of Spirogyra Spp. The central,
more persistent portion of the pond (Zone C, fig. 51) had
no herbaceous vegetation when the pond was dry. In

addition to the small trees which are indicated on

 

 

-104-

Table 6. Temperature of Baker Woodlot pond and temperature
and precipitation records taken at the Horticulture Farm,
East Lansing (Hort. Farm records reported in the U. 8. Dept.
of Commerce annual summary of climatological data for Nich.).

 

 

 

 

 

 

1959 East Lansing Horticulture Farm Baker
Month Day Inches Aver. Temp. °F. Woodlot
.of Air Water in pan Pond
Precip. Evap. Max. Min. 0F.
Jan 1.95 18.6
Feb 1.76 22.7
Mar 1.83 32.6
Apr 4.24 4.09 47.8 61.5 39.0
'May 2.74 6.31 62.7 78.2 54.4
2 72
9 66
30 70
Jun 2.16 8.16 69.0 85.9 60.9
6 70
14 60
20 67
27 76
Jul 5.59 71.4 88.4 62.7
4 73
11 65
18 (dry)
25 68
Aug 4.54 5.87 74.7 89.7 67.3
1 67
8 68
15 76
22 77
29 77
Sep 2.76 4.93 65.7 79.9 56.2
5 ' (dry)
Oct 5.10 2.28 50.3 59.2 43.2 I
3 (dry)
18 47
N 2. 2 4.
0v 6 7 3 9 41
Dec 1.88 32.6

 

-105-

the map there were a few grape vines and Small oak seedlings.
Zone B supported a Sparse growth of seedlings, moss, and
dandelions after the water receded. Zone A, which was inun-
dated for only a short time in the Spring, had a luxuriant
growth of grasses and herbs as well as much poison ivy.

The litter in the pond, which provided the cover for A, 22m:

munis, came principally from the elm, oak, ash, and maple

 

canopy of the pond.

Rotting leaves from.the pool were used as a source of
food for asellids kept in the laboratory. Maple leaves were
utilized rapidly and oak leaves only slightly. Gibbons
(1960) reported that aerial Spraying of DDT reduced the
pOpulation of insects that he was studying in Baker woodlot.
DDT may partiallthave caused the changes that have occurred
since field work for this study was done. Isopods have
been harder to find and algae more abundant during recent
checks on the pond. Since this is not a quantitative study
this cannot be verified. An ecological Study was done in the
area by wacasey (1961). His study of salamanders from
rather Similar nearby ponds which also contained asellids
revealed no Asellus Spp. among the food items used by
either Ambystoma maculatum or Ambystoma jeffersonianum.

Vertebrates were rarely observed in the pond in which

the asellids of this study were collected,

-106-

Fig. 51. Map of vernal pond at edge of
Baker Woodlot.

 

Bottom depth below surface,
0 to 1 foot when pond is full
(Zone A).

 

 

 

Bottom depth below surface,
1 to 2 feet when pond is full
(Zone B).

 

 

 

 

 

Bottom depth below surface,
2 to 3 feet when pond is full
(Zone 0).

 

 

 

 

4—~*+* Fence.
American elm.

Black ash

 

Swamp oak.
Silver maple.

9221.2 81»

“1111100 (D >

Dead oak.
' 2 to 6 inches in diameter.
" 6 to 12 inches in diameter.

I" 12 or more inches in diameter.

 

 

-lO7-

 

 

 

 

 

 

 

 

 

E _
TI _ s H. Du O
_ III/ E _ r
w IIII // H
\.\ . /
c. O
\ / B
t o \IIIIIIIII I
x . l . I
\ a c I /
\ ‘. ‘Il! o u .l/ I
. \ l- l . I I,
\ u. _ \ I / c / Wl/
\ A .\m IA. . C . /
I \ IIIIIIII l
\\ «(A \\. \.\ . .l /. /
I \\ \.. . .. / x
\ \o l .W... . c. ..l/ .II‘J O
c \ \ o . o n n a/ o a 1
l C t ‘ Q Q A
\ \.\. .-\. .. o... .....\.». ..\ A x 5
\ .\.\ _. .8.. . . .... \. \\ \ o
. o .. . . . \ \
\\ ~8. .. . .a. . ..\ \\u \\.\ MUD
\ _w J n u a o . o-J\ ' ' \‘\
. . . . .. . o N A \ F
a H 3 . .\. A \\
I / . . . .. \ x
m. / II A . \ \
/ B. c ,A . I. 8. \
III I - uuzuu

 

1".

l

l

J

l" i

IIIII-IIII I- Ila/Sm m2.) 2.2.1--..I... I I I.

I

l

 

 

 

 

 

 

 

 

-108-

cutly'the leopard frog and the green frog and many unidenti-
fied tadpoles were observed. The invertebrates found in
the pond are listed in Table 7. The planarians were very
almindant (usually many Specimens in a sample) in the spring
tuit disappeared in the summer. Daphnia Sp. was very abun-
<iant in the spring as well as smaller copepods and ostra-
<:ods. A member of the lumbriculidae was usually very
abundant among the litter and bottom mud. Lymn_aea Sp. was
abundant on the surface until later in the year. Immature
aquatic insects were usually present but the adults were
:most prominent in the springtime. water mites, leeches,
hydras, and rotifers were sometimes very abundant. Large
predaceous aquatic insects were present but were never
observed preying upon asellids. Allee (1929) has stated
that dytiscids and crayfish feed on asellids. Dytiscids
‘were abundant (i.e., present in most samples) in the
springtime as adults although larvae never became very
abundant. A larval dytiscid was placed in a finger bowl
of water with a male asellid on April 13, 1958; on April
14, only the dytiscid remained. Only one small crayfish
'was observed in the pond. A possible instance of canni-
balism was noted as follows. The male and female which
had been observed during the fertilization process were

left in the same finger bowl. Since the ages of the female's

 

-100-

iPatde 7. List of taxa represented in the observed biota
(of Baker Woodlot roadside pond, other than A. communis.

ChlorOphyta
Siphonales. Vaucheria Sp.
Zygnematales. Zygnema Sp. and Spirogyra spp.

Protozoa
Testacida. Arcella discoides E. and Difflugia oblonéa F.

Peritrichida. Zoothamnium Sp.

 

(Joelenterata
Hydrozoa. Hydra Sp.
Platyhelminthes

Rhabdocoelida. Dalyellia Sp.
Tricladida. Planariidae.
Rotifera. Sinantherina Sp.
Annelida
Oligochaeta
PlesiOpora. Aeolosoma variegatum Vejd.
PrOSOpora. Lumbriculidae.
Hirudinea
Rhynchobdellida. Helobdella stagnalis L.
ArthrOpoda

Crustacea
Cladocera. Daphnia longispina (0.F.N.)

COpepoda. CyclOps7
Ostracoda. ypr ae.
Conchostraca.
Decapoda. Cambarinae.
Insects
Odonata. Lestidae.
Hemiptera. Corixidae and Gerridae.
Megaloptera. Chauliodes Sp.
ColeOptera.
HydrOphilidae. Berosus Sp.
Dytiscidae. Coptotomus Sp., Bidessus sp.,
Laccophilus Sp., and HydrOporus?
Diptera.
Culicidae. Culex tarsalis?, Anopheles Sp.,
and Chaobbrus Sp.
Tendipedidae, Tendipedinae.
Arachnida

Acarina.
Hydrachnellae. Limnochares sp. and others.

 

Nollusca
GastrOpoda
Pulmonata. Lymnaea sp. and Aplexa hypnorum (L.).
Chordata
Amphibia -
Anura. Rana clamitans Latreille and R. pipiens
Biplane Schreber. "

-110-

eggs were known they were being removed at intervals and
prweserved for later study. But six days after fertiliza-
tion the female was gone, two pleopods were floating on
time surface, and a few exoskeleton parts were on the bottom.
Presumably the male ate her Since the exoskeleton normally
remains intact on dead Specimens.

Although asellids were an important invertebrate group
in the study pond, they never reached the concentration
reported in Ellis (1961) or Allee (1929). Ellis found
them as numerous as 15,600 per sq. foot based on a 9 sq.
inch sample area. In a larger sample area (exact Size

not determined) I found a maximum of 133 individuals.

 

TAXONOMICAL DISCUSSION

Colubotelson thomsoni Nicholls, 1944, is morphologi-
cally distinct and allopatric with related species. The
species of the genera Colubotelson and Metaphreatoicus
form a closely related complex in the sub-family Paraphrea-
toicinae. Nicholls (1944) uses only the morphology of the
tailpiece to separate the two genera. He states that Q,
thomsoni "closely resemblele. joygeri, but in the condi-
tion of the tailpiece seems intermediate between that
species and M, magistrif’The limited generic descriptions
he has provided are insufficient to determine the genus
of Q, thomsoni which has intermediate generic character-
istics. The Specimens, described as Q, thomsoni in this
thesis, fit the species description of Q, thomsoni and
are from.the type locality of Q, thomsoni. The latter two
facts indicate the identification is correct although the
generic descriptiOns of Colubotelson and Metaphreatoicus
need improvement. Since the Species of the two genera
almost form a continuum, with respect to the distinguishing
features of the genera, it is unlikely that a satisfactory

.generic description can be written to differentiate between

-111-

 

-112-

thpse close to each other but separated by the generic
boundary.

Asellus communis Say, 1818 is a somewhat plastic Species
that is widely distribUted in the United States and Canada.
The Specimens in this study are probably similar to those
referred to as A, militaris by Ellis (1961), Kenk (1949),
and others. A, militaris was described by Hay (1878).
Later he (Hay, 1882) referred to it as A, communis with the
following statement:

"I am now pretty well convinced that the form

that I described as cited above is the same as

the Eastern Species. It differs certainly from

the Eastern Specimens in the armature of the

hand, in the form of the genital plates, in

Size, and in some other respects; but I do not

believe that these characters are sufficiently

marked and constant to enable us to found species

on them."

Since then both major monographs on the Isopoda of the
United States (Richardson, 1905; van Name, 1936) have

treated it as a synonym of A, communis.

Q, thomsoni is in the isopod sub-order, Phreatoicoidea.

A, communis is in the isopod sub-order, Asellota. The rela-

tionship to some other crustacean taxa is shown in fig. 52.

 

-113-
0nly one distinct )(
abdominal segment

Male Opening on first
abdominal segment

Body not greatly
flattened

UrOpoda styli form X X X

UrOpoda terminal

UrOpoda flattened and ><
hinged to side of telson
PleOpoda fitted for X
air breathing
Mostly parasitic )(
s
a)
p
H
as o: O
Q) 0) O (3
o o s H-- a
«4 r4 +3 O Q)
0 O O P 'H
n o a m H
a m .4 o >
C). H 0) 34 H
O c: In .G as
m o 4 m >

Fig. 52. Diagram of ISOpod Relationships.
Modified after Richardson (1904), Hale (1929), and
Nicholls (1943).

Flabellifera

D IS CUSS ION

Anatomical Comparison.

An understanding of some principles underlying anatomy
is desirable before embarking on a comparison of the anatomy
of these two related animals. The Structure, or anatomy,
of an organism is the expression of its genotype under the
influence of its environment. Even identical genotypes may
result in different forms under different conditions as the
cyclomorphosis in the helmet Shapes of Cladocerans illus-
trates. Thus with acknowledged differences in the environ-
ment of the two Species of isopods under comparison, it

would be pointless to repeatedly contrast the many minor
variations that might occur.

Each Species comes from a different distinct group with
Inany other Species. One of the groups is proven ancient by
the fossil record. Similarities could indicate Similar
Selective processes have resulted in the retention (parallel
eVolution) or development (convergent evolution) of similar
Structure. That the retention of the Similarities,
rather than the development of them, has occurred is
itldicated by the little change in phreatoicids since

Triassic times. Thus general similarities are useful

-ll4-

 

-115-

indicators of the parts of the organisms that have been
subject to Similar selective pressures of a relatively
unchanged type. Such pressures would be capable of preser-
ving genetic configurations derived from their common
ancestor, whoever it may have been. Conversely, differ-
ences of a major nature probably indicate the long term
Operation of different environmental pressures. Environ-
ment may include the internal environment of the organism.
Form is related to function. This statement is docu-
mented in a general sort of way by Thompson (1961) and

will be further demonstrated in this study.

Gross Anatomy

Table 8 is provided for easy comparison of some feat-
ures of A. communis and Q. thomsoni. The gross aspect of
Phreatoicid and asellid is markedly different due to the
dorso-ventral flattening in the asellids contrasting to
the somewhat lateral compression of the phreatoicid
(Chilton, 1894). The lateral compression of phreatoicids
is an amphipod-like characteristic that caused confusion
in the first concepts of their phylogeny. The different
direction of flattening is a consistent external differ-
ence although it is not accompanied by much internal

difference. The difference in the direction of flattening

 

-ll6-

‘is primarily due to a change in the position, or rather
anngle, of the pleura. The rotation of the asellid pleura
to a more horizontal position may be an expression of the
influence of the habitat in the following respect. The
leaf litter of asellid habitats studied consists partially

of flattened leaves which give a lamellar Structure to part

 

of the substrate of temperate woodland pools during the

greater portion of the year. Thus the flattened asellid

*would have his legs pressed against the solid portion of

the substrate as he made his way among them motivated by
11is negative phototaxis or search for food. The phreatoi-
<:id with his Shape and habit of curling up isopod-style
would have a difficult time keeping his legs in solid
contact with the substrate for efficient locomotion.
Conversely, the more equidimensional cross-section of the
I>frreatoicid would have a generally greater efficiency for
movement in random or fas icularly arranged branches of
moss, etc. or the amorphous flocculent detritus it com-
11101:11y inhabits.

The thicker exoskeleton of the Slowwmoving phreatoicid
Would offer greater protection needed with its habit of
Cut'ling up when disturbed. The reduced protection provi-
ded by the thinner exoskeleton of the asellid may have

hastened the selection of the speedier asellids. The

 

-ll7-

 

mnouoaa_m moouoflfi 0H
Maud N name m
unease “douche
huomoom hhouoaoooa

pom .wSOH .uopooam pom .uuonm .usoum

Hw>OU

m>auoououm m on poawwpoa muonuo on Hmaflaam

Aoamaom SH woammas name sum pom sud
ocoV moomooam umoHHmam .pum ou HmHHEHm onwm
venom oumnmoom

mommmooum m no N moaamm oumHSoHuGoo m

llllll llllllll IIHmHflEHw hHmmOlellllll lllll I

A.xoumomv NH A.xoumamv om
IIIIIII IIIIIIIIIHoHHEHm hammouwIIIIIIIIIIIII
dowwou moon umozouumo hoop mo upon masses

haamuouma odouxo haamuuom> pamuxo

.2 H .5 wN
.ss OH .aa NH
.88 wma .EE ma

mannaaoo .<

 

 

.mmocouommao .wwnaaaoo .< paw «domfioau

 

anon-moan .w.
.w

mo acmaummaoo Hmaonma

:ouoaoxmoxo mo mmooxoanu
nonmaw oHumouocmmIoummos

oHOmoHsmhu

moomons

pomooam ohm mo ouwoomoxo

moomooam pom pom umH
muomawom nomad
pomosumow mama mo EHod
mowmpoommm oHomHonu
who won HHHooo
wowmpooomm poo:

supa3 poms

snowed mo coaumuooauo
Hos-CH >305 u ooze-um
mamaom uwowooa

mama ummwaoa

.m mHnse

-118-

 

 

ABSEHGHEV pounce woman ABSEHGflEV munch o3ua.xoummmv muauouma amoxmm an own
axoo3.o3u .xowoom amok mamouoco moaned cowumoooaw
on »H Apouo: .Nmfiv achuofio mo Hones:
monounfioa_wwo
Canoes omaon oHomHuo> unmowaond .moAOH umam m chuofio mo owmpnommo Hmmwoo
mcouoaa 0mm moonofifi on» www mo Houoamao
mason-Boo .I-m anon-noon .m

.AomsoHuGOOV m oHomH

-ll9r

reverse could as well be true, increased Speed may have
reduced selective pressure for a thick exoskeleton.

The shape of A, communis, narrow anterior and wider
posterior with antennae-like uropods, may aid the isopod

in escape from.ehemies which may have mistaken front for

back. ‘
II

Appendages 4.
Extremely close correspondence is shown in the appen-
dages. Some modifications have occurred but the Similari-
ties are even more remarkable. There is even a great
Similarity in their setation. Most impressive are the
existence of similar specialized setae, e.g., on the
antennule are olfactory setae (figs. 2 and 3), the lateral
lobes of the second maxilla have pectinate setae which
bear their teeth and curve in the same direction, the
maxillipeds (Chilton, 1894) have stout coupling Spines
(figs. 12 and 13). In addition, general chaetotaxy is
remarkably similar (compare figure 22 with figure 24).
Some divergence is shown in the following aspects of
the appendages. A, communis has large processes instead
of spines on the palmar surface of the propod of the
gnathOpod (the general similarity of gnathOpod was noted

by Chilton, 1894). The gnathOpods of female asellids are

-120-

much less chelate. The first two pair of pleopods are
greatly reduced in A, communis, while the third pair has
the ex0podite modified as a protective cover. The urOpodS
of A, communis are less robust, more movable, prOportion-
ally longer, and are attached caudally rather than latero-
ventrally as in Q, thomsoni.

The difference in the gnathOpod is difficult to
explain. The processes on the gnathOpod of the asellid
male provide a grasping mechanism for holding the femalels
antennae during coitus. The phreatoicids have their palm
well supplied with Spines in both male and female. Since
the A, communis females have nearly lost the chelate con-
dition of the gnathOpod, perhaps the chelate condition of
the gnathopod is primarily of sexual importance and of
little value for feeding or locomotion.

Chilton (1894) noted the pleon in the Asellidae was
small, flattened, and usually of only one segment in
contrast to large segmented pleon of a phreatoicid.

The condition of the abdominal appendages is probably
related to the flattened aspect taken on by A, communis
as is the absence of segmentation for most of the pleon.
First, the flattening reduced the protection provided for
the delicate pleopods by the downward projecting abdominal

pleura of the phreatoicids (see fig. 35). The develOpment

 

-121-

of the protective function of the third pleOpod allowed

the second pleopod of the male to remain free to continue
its reproductive function. Reduced Size of the two pair
of exposed pleopods probably had selective advantage for

preventing serious injury to them. Flattening caused

“I

reduced structural strength of the sort needed for rapid

flexing and extension of the pleon. ‘Without the ability

I‘Z“_"
I."

to flex in this manner the locomotory function of the
abdomen is minimized and associated locomotory structures
are no longer under selective pressure to maintain their
locomotor efficiency. Thus the uropods of asellids have
retained and probably improved their sensory function at
least as far as mobility and coverage is concerned since
they no longer need the strength demanded of a structure
bearing the load of an amphipod-like kick. This has
allowed develOpment of their slender uropods. The loss of
unneeded segmentation would be rapidly selected for if the
chance of injury to intersegmental regions is as great as
the infolded non-sclerotized membranes would indicate.

The brood pouch and coxal lobe of the maxilliped of
ovigerous females are similar. This was reported by
Barnard (1927) and Sheppard (1927).

DeveloPmental Stages

The dorsal appendage represents an extremely interesting

-122-

cenogenetic characteristic. In the phreatoicoidea it is

not much more than a yolk filled bulge on the dorsal sur-
face of the embryo. Its integument is very thin in contrast
to the limb primordia which are formed by a high concentra-
tion of cells. The appendage encloses, or stores, yolk

that cannot be contained in the embryo as it develops its
peculiar folds within the limiting membranes of the egg.
Since the chorion encloses the appendage without following
its contours closely the appendage is of limited value for
diffusion of dissolved gases, wastes, etc.

In A, communis the dorsal appendage is a highly
organized structure which projects far beyond the outer
surface of the egg. The histological Structure of the
appendage is Similar to the respiratory portion of the
adult pleopods. The appendage does not appear to be yolk
filled. The respiratory function could allow a more rapid
metabolism for the speedy develOpment which occurs in
A, communis.

The qualitative change involved in evolving from a
storage organ to a respiratory organ is remarkable in
that it occurs in an embryonic Structure.

A. communis has more eggs than Q. thomsoni. Q. £132-
£223 has larger eggs than A, communis. Volume of eggs

produced appears to have a close correspondence. The same

-123-
amount of egg material would obviously produce bigger eggs
if the material were distributed to fewer eggs. Mere eggs
could be an answer to a higher mortality rate. Smaller eggs
could allow more rapid development in response to the same

pressure. The phreatoicid, Phreatomerus latipes, of Central

Australia, produces up to 109 eggs (Nicholls, 1943). The ;
EFF

life cycle of g, latipes has not yet been described. Per- I;

haps g, latipes has a more rapid development than Q, thom- ii

£221 since Size of egg and rate of development are often
inversely related. A change in the rate of development
might be accompanied by the modification of the dorsal
appendage. Nicholls remarks about the large Size of P,
latipes marsupium. That might be an indication that there
was no reduction of egg size with increase in number of eggs.
If there were no reduction of egg Size I would not exPect
‘more rapid develOpment on the basis of the inverse relation-
ship noted above.

Sensory structures are Similar. The antennules of both
bear olfactory cylinders or Specialized setae (fig. 2 and
fig. 3), of small Size which have a Short basal portion and

short blunt terminal portion. The eyes of each are composed

of a small number of ocelli set nearly flush with the
integument.

Both are dark brown; a color Similar to the substrate

-124-
they usually inhabit.

The internal anatomy is grossly Similar. Barnard (1927)
noted the Similarity of the gastric mill in the two groups.
A, communis has only four hepatic cecae, whereas Q, thomsoni
has eight.

The absence of a typhlosole in the asellid is probably
not the result of a modification of the asellid, as most
differences were interpreted to be. Since the presence of
a typhlosole is not a characteristic of all the Phreatoicoi-
dea, the typhlosole appears to be a character evolved Since
the divergence of the Asellota and Phreatoicoidea. If the
typhlosole had been a character of the ancestral type it
would probably have had a selective value due to the greater
surface provided for alimentary functions.

Cell bodies of the neurons occupy the ventral portion
of the ganglia in both species.

Qéfig'Cycles.

The most obvious difference in life cycles is the rela-
tively long time it takes Q, thomsoni to rear young and for
them to mature. A, communis reproduces rapidly and perhaps

would rear more broods in a year's time under favorable

conditions than the probable minimum.of two broods a year.
One factor in the shorter incubation time of A, communis

could be the smaller egg Size (half the diameter) which accom-

-125-

panics the larger brood Size. Unfortunately, nothing is
known about the life cycle of phreatoicids such as ggggm;
phisoPuS palustris and Phreatomerus latipes which have
larger brood Sizes than Colubotelson thomsoni. In a Stable

pOpulation replacement must equal mortality. Thus is seems

M __-s.l-

that individuals of A, communis are subject to greater pre-
dation or are physiologically less well suited to the
environment. Since most eggs in each Species hatch suc-
cessfully it must be that the mortality comes later and
that A, communis requires more individuals to carry it
through the unfavourable season. Since a maximum of 17
eggs was observed in the brood pouch of Q, thomsoni, and

A, communis had an average of 117 eggs (Hatchett, 1947) at
a pace Six times as frequent as Q, thomsoni, it is evident
that A. communis has a reproductive potential over 40

times as great as Q, thomsoni. With a large population it
is more likely that enough of the pOpulation will be in

the depressions most suited, to retaining sufficient mois-
ture for life during dry periods, to maintain the population.
Since many of the isolated pools further from.the main
stream on Mount Wellington contained no phreatoicids,
though otherwise indistinguishable from.pools containing

them, the phreatoicids may have been unable to weather the

-126-
dry Spell in them. Higher population levels may have enabled
them to exist there by increasing the probability of some
being in the moister part during the dry season.
Barnard (1927, p. 152) considered the time of year the
young emerged as important. He stated as a general rule

that young of animals are produced during the season most
favorable to their further growth. Thus the life cycle of
A, communis, Q, thomsoni, and the South African phreatoicids
have the following in common. Maximum produdtion of young
following the cool season and before the dry period. Thus
the young have time to deve10p sufficient vitality for suc-
cessful aestivation. Although the period of greatest repro-
ductive activity of A, communis does conform to this the
recovery of such a large percentage of aestivating gravid
females may represent an abnormal circumstance.

Sexual Differences.

Niiyama (1956) has demonstrated an X0 sex mechanism in
the isopod, Tecticgps japonicus, and Stated that Dworak had
reported one from Asellus aquaticus with a male diploid
number of 17 chromosomes. Such a mechanism would give an
expected sex ratio of one male to one female. Barnard
(1927, p. 151) inferred the existence of such a ratio in

M, capensis because of the absence of single males or

females during the period of pairing. Allee (1929)

 

-127-

found approximately equal sex numbers during collecting at
periods and places other than those with the large aggrega-
tions which were predominantly male. Sexes reported among
the phreatoicid species Show great variability in ratios
of males to females. Males predominate in the collections
1 made but some types of collecting areas yielded more
females. Probably the gravid females avoid the mud, which
was used for pOpulation samples, and thus cause the unequal
sex ratio. Sexual differences in habitat selection by
M, capensis were probably a cause of Barnard's (1927) ob-
servations of great fluctuation in sex ratios in many
collections, some of which were mostly gravid females.
Thus the absence of females after emergence of young was
attributable to death of the females, but may have been due
to other causes. Collections of A, communis in this study
were made in an area of considerable uniformity which
presented fewer sampling problems, thus the ratio reported
comes close to the theoretical one to one.

‘Why the gravid female of Q. thomsoni perfers to
remain in places other than detritus is not certain.

A trait common to both groups is the method of pairing
in which the male uses his fourth walking leg to carry

the female until She is ripe for fertilization. The only

 

I!
.II
III
lull-i
ll. II

 

 

-128-

considerable difference noted about this process was the
longer time the whole event takes for Q, thomsoni. This
appears to be a very primitive characteristic since it is
quite similar to the activities of members of the Gamma-
ridae in the order Amphipoda, another pericaridan order.

Both Q, thOmsoni and A, communis seem to be able to
produce more than one brood Since post-gravid females
were able to molt their large oostegal plates and return
to the small size oostegal plates of the non-gravid female.

Both share the isopodan characteristic of larvae with
Six legs. As far as was observed, both have Similar methods
of molting.

Comparison o_f 13.92 2921.3.

Temperature in both pools followed an annual cycle
with cold temperatures and ice cover in the winter and
high temperatures in the summer. The Michigan pond had a
warmer summer temperature. Probably the Slight flow in
the Tasmanian pool helped keep the pool cooler in the
summer.

The higher altitude of Mt. Wellington tended to
counteract the general maritime climate of Tasmania.
Fluctuations more comparable to the pond in continental-
climated'Michigan occur in Mt. Wellington waters than

occur in waters of lower altitude in Tasmania. Since

 

-129-

there is less than nine minutes of latitude difference in
their distance from the equator they have comparable day
lengths. Higher altitude results in less atmospheric
filtration of light at the Tasmanian location. The Tas-
manian pools were not Shaded by a tree canopy. The greater
insolation falling on the pool bottom may have raised the
micro-environmental temperature of the detritus closer to
the temperature of the pond occupied by A, communis. The
differences in light and temperature are slight and the

dry periods seem comparable in the two areas.

The vernal pond, Since it lacks an outlet, is probably
much richer than the Tasmanian pools in nutrients for pro-
duction of a rich microbiota in the detritus and forming
the aufwuchs which are the sources of much isopod food.

Its richness would result from its lack of an outlet for
particulate matter and a greater annual increment of
detritus forming organic matter from the surrounding deci-
duous trees. This factor and the flow factor made the
vernal pond richer in planktonic life than the mountain
pool. The flow supplying the pool came from the rocky
sparsely vegetated top of Mt. Wellington in contrast to

the run-off the vernal pond received from.the richer nearby
beech-maple forest in periods of high rainfall. This

probable difference in trophic Structure at a primary

....-...4__‘

an.

 

-l30-
level may mean that the long life cycle of Q, thomsoni rep-
resents an adaptation to a habitat poor in nutrients more
than the short life cycle of A, communis indicates an adap-
tation to high mortality rates since life cycles of small
aquatic invertebrates of comparable size rarely take more
than a year to complete.

Both locales had seasonal dry Spells resulting in dried
up pool bottoms. That these two animals, found living in
areas subject to such conditions, have the ability to wait
for rain in an inactive State is not surprising. If they
didn't, a yearly reinvasion would be necessary.

Faunal Differences.

In general, the fauna of the Tasmanian Site seemed
rather depauperate. Only amphipods were present there in
abundance, but absent in the Michigan pond. Perhaps the
low number of asellids may have been one reason for the
absence of large numbers of amphipods and mayfly larvae,
Since Minckley (1962) and Brown (1961) have noted the impor-
tance of asellid feces in detritus fed upon by these forms.

On the other hand, the vernal pond had two snails and
a fingernail clam.compared to no mollusks in the mountain

pool. It was also the only place to have hydra, leeches,

and a crayfish.

. 3

I;— ~——.~

bih

-131-

Both had planarians in abundance in the springtime
although the vernal pond had two more Species during the
summer. Both had a variety of aquatic insects although
the vernal pond had greater variety (Table 3 and Table 7).
They were primarily dytiscid beetles and tendiped larvae
in Tasmania while the Michigan Site had, additionally,
other dipterous larvae and hemipterans. Both had water
mites, cladocerans, copepods and ostracods as well as
small, colorless oligochaetes and a larger reH-blodded
one. Frogs are known from.both areas, one small species
on the mountain and several Species in the woods, two of
which were found in the pond. In both areas frogs are
known to eat aquatic iSOpodS.

In summary, the effect of the fauna in Michigan would
probably have a greater impact on A, communis because of
competition and predation, particularly competition Since
the numbers, sizes, and variety of competitors (i.e., the
fauna) are greater. Direct competition for food seems
quite low in both areas, particularly in Michigan where a
surplus of leaves is evident. Because of its habits
Q, thomsoni would be less exposed to predation by frogs
than A. communis.

Since both areas were covered by glaciation in the

Pleistocene, both forms must be relatively recent arrivals

-132-

in their respective areas. Invasion routes would probably
have been rivers for the most part, unless passive tranSport
by birds, etc., is more common than active migration.

Q, thomsoni would only have to go upstream a short dis-
tance since that area had only mountain glaciation. In such
waters Q, thomsoni would have been exposed to a lesser
variety of fish, amphibians, reptiles, Since Tasmania is
notably poor in fresh-water native fish, has only eleven
Species of frogs (Moore, 1961), and no salamanders, turtles,
or aquatic snakes. In contrast, North America has a rich

fresh-water fish fauna, and many frogs, salamanders, turtles,

and aquatic snakes. Many of these may be capable of preying
on such organisms as Asellus Spp. Ellis and Gowing (1957)
have demonstrated a preference for Asellus intermedius by
brown trout (an introduced Species) in Michigan. Forbes
(1878) found Asellus Spp. in the stomachs of native fish in
Illinois. Most faunal differences are long Standing ones and
could have placed a premium on high reproductive potential
for Asellus spp. Thus many eggs, rapid development, and
early maturity may have enabled them to survive. Their
Speed, coloration, retiring habits, and head-like posterior
would reduce the fatalities from predation. The dorsal
appendage of the embryo may have Speeded developments

sufficiently to allow survival, or it may have permitted

-133-
development in an environment that is subject to periods of
low oxygen due to decomposition of abundant decaying organic
matter.
Evolutionary Considerations.

The effect of evolutionary change on the asellids would
seem to have been much greater than on the phreatoicids if
Glaessner (1957) is correct about the phylogenetic position
of the phreatoicids. Actively meiotic cells are more sub-

ject to mutation than resting cells. Thus the asellids, with

reproductive periods at least six times as frequent as the

phreatoicids, would seem to be more subject to mutation.
Also the opportunity for recombination would be Six times as
frequent. Thus selective pressure could be effective on new
genetic combinations six times more often.

Another factor which might affect evolution within the

phreatoicids is the small size of the breeding population.

The immature Stages make up a large prOportion of the phrea-
toicid population over the year,.whereas at some periods

the asellid population is nearly all mature individuals.

A small breeding population can carry fewer genetic combina-
tions than a large one. Thus the possibility of loss of
factors of low frequency appears to be greater in the phrea-
toicids. At any rate, the Triassic phreatoicid indicates

changes in the group have been very Slow.

-l34-

Since phreatoicids were already extant in the
Triassic, 100,000,000 years would be a conservative esti-
mate of the length of time the phreatoicids and asellids
have been pursuing a separate evolution. In the last
chapter we saw that some morphological characteristics
had undergone a parallel evolution or remained unchanged.
Other morphological characteristics diverged greatly. 3
Most Characteristics underwent an intermediate amount of
divergence. Characteristics of physiology and behavior
have undergone a Similar range of changes. Similar are
food preference, habitat preference, and method of pairing.
Different are reaction to diSturbance, duration of pairing,
and speed of movement. Somewhat modified are use of
walking legs as well as other structures which have under-
gone structural modificationn.The fact that behavorial
characteristics cannot be visualized, as structures can,
does not mean they are less important as an integral
part of the whole organism. Both types of characteris-
tics have been the product of long periods of selection
and in a Stable environment should have high adaptive
value.

Temporal Reproductive Isolation Agg_Speciation.
An interesting Situation exists in the phreatoicids.

It is possible that three genetic lines could be estab-

 

 

 

 

-135-

lished in the sympatric population. There would be complete
reproductive isolation of the three lines A; there were no
success in breeding the fourth year of their life and none
matured and bred the second year. A longer reproductive
life would not necessarily make the gene flow between the
lines complete since the mortality would probably restrict
gene flOW‘Wlth a direct prOportionality. Thus Speciation
could occur without geographic or niche separation. It

is in effect a temporal isolation. The existence of several
Species in Great Lake, Tasmania (Nicholls, 1944) may be a
result of this ability. Of course, a Similar Situation
occurs in cicadas and other insects with a long life

cycle. Alexander and Moore (1962) point out that, although
the genus Magicicada has Speciation aided by temporal iso-
lation, due to long life cycles the rate of Speciation is
Slow.

Marine Relatives.

There is no evidence of marine forms among the phrea-
toicids. The one from brackish wells in Central Australia
is perhaps more closely related to marine forms than those
found within sight of the sea. The family Janiridae in
the Aselloidea has marine members which strongly resemble
Asellus Spp. So do members of the family Stenetriidae.

The relict nature of the phreatoicids and the variety

 

 

-136-

within the asellots hide their origin. However a fresh-
water origin appears more likely in the light of over

fifty Species of phreatoicids Still extant in fresh water.
Thus marine Asellota may represent a return to that environ-

ment from fresh-water ancestors.

 

 

 

5%.:

 

 

SUMMARY

1. Colubotelson thomsoni is Shown to have a long life
cycle, with pairing and development of eggs taking sev-
eral months each, with growth to maturity taking at least
two years.

2. Frequency of bnood production in.Q, thomsoni is shown F“

 

to be a yearly phenomena.

I'T—“T
an

3. A general description of the environment of Q, thomsoni
and its relation to it is presented. Seasonal sexual
differences in habitat selection as the cause of aberrant
sex ratios is indicated.

4. various features of the external and internal anatomy
of Q, thomsoni have been presented and the controversy
about its typhlosole resolved.

5. ASpectS of the biology of Asellus communis, corres-
ponding to those presented for Q, thomsoni, are presented
based on the literature and original observation.

6. Their taxonomic positions are discussed and their
general phylogeny presented.

7. Aspects of the anatomy, natural history, and ecology
of each are compared and discussed. Some mechanisms of

evolution are suggested.

-137-

 

-138-

8. The comparison of the differences in the two animals
is interpreted as indicating the following environmental
differences have exerted the most effective selective
pressure.

a. A, communis is in a structurally different
environment caused by the annual addition of leaves from
deciduous trees.

b. A, communis is subject to greater predation.

c. C. thomsoni lives in an area with a poorer

traphic stricture.

 

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