A PRELIEWNARY aw; ”EX" {3? THE FOO?!- HABETS 0F M GREEN CRAB, CAECEMDEE dams film} WEEH EARTECULAE REEERENCE TO THE SOFT a SNELL CLAM, MYA AEENAEEA L‘ Thests 20:! 33MB Degree cf M. S EEC-E13233 3? 37E Dv”£’§3 TY thn E1. Dearbcrn 1957 . ll mm; HIZIIISIMHII Ln; 1m w @ gummy ”11M“ "1/ 'JUL 05 2008 inr- A PRELIMINARY STUDY OF THE soon HABITS OF THE GREEN CRAB, GARCINIQQS m (L.), WITH PARTICULAR mamas TO THE son-m GLAM, mm W L. By John H. Dearborn AN ABSTRACT OF A thesis submitted to the College of Science and.Arts Ruchigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of Master of Science Department of Zoology East Lansing , Michigan Approved John H. Dearborn THESIS ABSTRACT The purpose of this study was to learn more of the food.habits of the green crab, Carginides ggenag (L.), particularly in relation to the soft-shell clam, Mia aggng aria L. The most important objective was to determine the size groups of clams most vulnerable to crab predation, and the size of clams that a given crab could consume. Other factors that were studied included feeding methods, food preferences, effects of moulting on feeding, ingestion and elimination of shell bits, and the feeding habits of female crabs while carrying eggs. In addition, a brief description of the green crab, and its range and life history have been presented. Data for this study were collected over a period of nine weeks, during the summer of 1956. The laboratory work was done at the U.S. Fish and Wildlife Service station at Boothbay Harbor, Maine. Green crabs between 12 and 83 mm. were studied in the laboratory. In addition, numerous field observations were made on the feeding habits and activity of green crabs in the field. Green crabs were found to be most efficient predators on soft-shell clams and several other species of bivalves. yyg'between 4 and 20 mm. were the most heavily preyed upon of the sizes offered as food. Crabs between 61.5 and 81.5 mm. utilized the widest range of sizes of ENE in the labora- tory. Crabs in the laboratory showed a preference for soft- John H, Dearborn shell clams over other species of bivalves offered to them. Crabs in the process of moulting did not feed until the new exoskeleton had begun to harden. Shell bits were taken in by certain crabs While feeding, and this material was elim- inated by regurgitation or in the feces. Female crabs carry- ing eggs accepted food at regular intervals while under observation in the laboratory. A PRELIMINARY STUDY OF THE FOOD HABITS or THE GREEN CRAB, CARCINIQES mg (L.), wITH PARTICULAR REFERENCE To THE SOFT-SHELL CLAM, MIA ARENARIA L. By John H. Dearborn A THESIS Submitted to the College of Science and Arts Nuchigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Zoology 1957 5/wz/5‘ 7 ? A1517 TABLE OF‘ CONTENTS ACKNOWLEDGMENTS' INTRODUCT I ON C C O O O O O O O O O O O O O O 0 O O O O O O O O O O O O O O O O O O O O O O O O O O 1 WHODS. I O O O O O O O O O O O O O O O I O 0 O O O O O O O O O 0 O O O O O O O O O O O O O C O O O O 6 DISCUSSIONOOO.OO0...OOOOOOOOOOOOOOOOIOOOOOO0.0.0.000... 10 cellectirg AreasOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 10 General Description and Life History.............. 11 Feeding methodBOOOOOOOOOOO.0.00.00.00.000.0.000... 15 camabaliBmOOOOIOOOOOOOOOOIOOOOOOOOOOO00.0.0.0.0.. 16 Excessive Feeding................................. 17 Food PreferenCOOOOOOOOOOOOOOOO00......00.0.0000... 18 Berried FemaleSCOOOO00......OOOOOOOOOOOOOCOOOOOOO. 21 Moultirg and Food IiabitBOOOOOOOOOOO...00.0.0000... 23 Regurgitation and ExcretionOOOOOIOOOOOO.00.0.00... 26 Digestion RateOOOOOOOOOOOOO.OOOOOOOOOOOOOO0.0.0... 27 PredationonMOIOOOOOOOOOOOOOOOOO...00.0.0000... 28 Sex Differences in Feeding Habits................. 35 SW‘MARYAND CONCLUSIONS................................ 37 ILLUSTRATIONSOOOOOOOOOOOO0......OOOOOOOOOOOOOOOOOOOOOOO 4'0 LITERATURE CITEDOOOOOOCOO0000000000OOOOOOOOOOOOOOOOOOOO 46 ACKNOWLEDGMENTS Throughout this project I have received help and encouragement from a number of individuals. I am espec- ially indebted to John Glude and Walter Welch of the U.S. Fish and Wildlife Service for their permission to conduct this study and to make use of the laboratory facilities at the fisheries station at Boothbay Harbor, Maine. Both these men have given many helpful comments and criticisms and without their help the project could not have been undertaken. Dr. T. Wayne Porter of the Zoology Department at Michigan State University has been my advisor during my graduate work at this institution. He has given freely of his time and effort in my behalf and I am most grateful for this guidance. Dr. George Wallace of the Zoology Depart- ment and Dr. Gerald Prescott of the Botany Department at Michigan State have both read and ably criticized the text. Robert Hanks, Gareth Coffin, William Brown and John Repes, all with the U.S. Fish and Wildlife Service, have offered many helpful suggestions throughout this study. My parents, Henry and Dorothy Dearborn, have aided in collecting specimens and assembling a reference bibliog- raphy on the green crab. To them and to all of the others associated with this study I am most grateful. . w—T—A . P.l. Last complete sentence should read as follows: The blue crab, Bath. is another common member or this family found on the Atlantic Coast. Note: anggg‘ggn. are in the Family Cancridae. INTRODUCTION In July, 1956, a laboratory project sponsored by the U.S. Fish and Wildlife Service was begun to determine the effects of the feeding habits of the green crab, gagginiggg maenag (L.) on certain shellfish, notably the soft-shell clam, ENE aggparia L. This project was carried out in con- junction with a larger study being conducted by the Fish and Wildlife Service to determine means of increasing clam populations to insure profitable commercial digging. Data for my study were collected over a period of nine weeks. The laboratory work was done at the Fish and Wildlife Service station at Boothbay Harbor, Maine. The most important objec- tive of this part of the study was to determine the size groups of clams which are most vulnerable to crab predation. Other factors with which I was concerned included possible variations in food habits between the sexes, ingestion and elimination of shell bits, food preferences, and food accep- tance during moulting. Also, the feeding habits of females carrying eggs (berried) were included in this study. The green crab or "Joe Rocker" is a member of the fam. ily Portunidae which comprises the true swimming crabs. The blue crab, gallinggggglgapigug Rath., rock crab, Cancer irroratug Say and Jonah crab, Cancer bgrgalis Stimpson are a few of the other common members of the family found on the Atlantic Coast. The green crab is set apart in the sub- family Garcinidinae and belongs to the monotypic genus gagginiggg Rathbun. The taxonomic aspects of the species gaggipig§§_m§§ga§ (L.) have been reviewed by Rathbun (1930) in her monographic study of the cancroid crabs of America. The literature on,§aggin1g§§ maenas is extensive in respect to its physiology, embryology, growth and develop- ment, and general natural history. In reviewing the back- ground material for this study the following papers were particularly helpful. Bateman (1933) and Webb (1940) have reported their work on osmotic and ionic regulation in this species. Respiratory processes have been discussed by Lim (1918). Oogenesis and yolk-formation were described by Harvey (1929). The physiology of the male reproductive system has been studied in detail by Spaulding (1942). Borradaile (1922) has described the mouth parts, their anatomy and function. My review of the studies on growth and development have been particularly rewarding. Those reviewed included Williamson (1903), Labour (1928, 1944, 1947), Huxley and Richards (1931), Shen (1935), and the outstanding paper by Broekhuysen (1936) which included details of growth, deveIOpment and distribution. This work is the best general reference on these phases of our knowledge of green crabs, and also gives a most helpful list of related references. Punnett (1904) presented an interesting publication on the distribution of the sexes among green crabs. Vles (1909) provided an excellent mono- graph on the soft-shell clean ENE arenagja used in the present study. The range of the green crab on the Atlantic coast of .America prior to 1900 was apparently limited to the region from Cape God to New Jersey. Leidy (1855), Smith (1879), and JRathbun (1887) all gave this approximate range. Rathbun (1905) and Morse (1909) noted the range had been extended to Casco Bay, Maine, by the early 1900's. The relatively rapid north- eastward movement of this species has been well documented ‘by Scattergood (1952) and Glude (1954). This expansion of the range of the green crab north of Cape Cod brought rather drastic results to the commercial clam industry of the area. Goucher (1951), in a fascinating narrative, has shown the effect of the green crab on clamming operations in the town of Essex, Massachusetts, and the surrounding area. Glude (1954) has outlined the predatory role of Carginide§,mg§pag and has stated the necessity for increased research activity not only concerning methods of control but also the environmental factors governing dis- tribution. The abundance of green crabs in the northeastern areas of New England has prompted Canadian workers to begin a series of studies. The potential seriousness of the problem in respect to the important bivalves there has been stated by Medcof and Dickie (1955). In the late 1940's and early 1950's experimental clam farming and green crab control studies became increasingly important. The U.S. Fish and Wildlife Service and various state fisheries departments began projects designed to bring the commercial flats back to a profitable production level. Smith (1953), Glude (1955), and Smith, Baptist, and Chin (1955) have outlined several control measures particularly in respect to fencing as a barrier to crabs. At present the Fish and Wildlife Service is coordinating a number of pro- jects designed to give a much greater insight into the nat- ural history of the green crab. Whereas a number of physiological and morphological studies on green crabs have been conducted, as previously noted, specific information on certain phases of the life history is woefully lacking. Only the briefest mention has been made of the natural foods of this species in the liter- ature. In their observations on green crabs in Maine, Dow and Wallace (1952) discussed briefly the feeding habits and noted that shell thickness of bivalves and digging condi- tions for the crab in the substrate were two factors that help to determine the feeding habits of crabs on the clams in the field. MacPhail, Lord, and Dickie (1955) have pub- lished a paper on the green crab in Canada and as yet this is the only reference that I have found pertaining to eXper- imental laboratory work on the feeding habits of green crabs in relation to bivalve mollusks. John Ropes, a biologist with the Fish and Wildlife Service at Kingston, Rhode Island, has been conducting a detailed study of the food habits of green crabs based particularly on analysis of stomach con- 'tents, but as yet none of his findings have been published. The present study is an attempt to determine in greater detail the feeding habits of the green crab. Emphasis has been placed on the relationship between this species and the soft-shell clam, Mya azgnagia L. The generic term My; used throughout this paper refers to the one species of clam men- tioned above. No other bivalves in this genus were offered as food. Notes have been taken concerning a number of other aspects relating to the feeding habits of green crabs both in the laboratory and in the field. The author hopes that this project will not only present information of a practical nature but will also provide spec- ific information on a rather neglected phase of the life history of Gagginidggwflééflifi (L.). METHODS Crabs were collected from two localities in Lincoln County, Maine. These were at Powderhorn Island, located in the Sheepscot River northwest of Southport Island, and at Sam's Cove in Bremen. In the first area both traps and hand collecting were utilized in obtaining specimens, whereas at Sam's Cove only hand collecting was necessary. The ecological aspects of these areas are discussed under a separate heading. The crab traps (Fig. 9) used in this study were made by William Brown, Fisheries Aide at the Boothbay Harbor Station. The trap dimensions at the base were approximately 36 x 18 inches with the sloping sides narrowing the width to about four inches at the top of the trap. The wooden. frame of the trap was covered with wire mesh and one side could be opened to remove the crabs. A long narrow top open- ing lined with sheet zinc formed the entrance. Six or eight frozen alewives approximately 10 or 12 inches long were used as bait in these traps. Crabs readily ate these fish but would not accept any putrid bait. If these fish were not consumed within several days they had to be replaced with fresh bait in order to attract green crabs. The large numbers of soft-shell clams, M19 arenagia L. (Fig. 10), of a wide variety of sizes, used in this study were obtained from Robinhood Cove and Sagadahoc Bay, both on Georgetown Island. Mbst of the other invertebrate food offered was collected from the above localities or in the immediate vicinity of the Fisheries Laboratory at Boothbay 6 Harbor. Crabs were kept in the laboratory in large wooden tanks provided with a continual flow of sea water. This water was maintained at a depth of about six inches. The crabs were stored for at least 24 hours in these group tanks before being isolated. As they were needed, individual crabs were measured across the lateral carapace spines at the widest point, sexed, then placed in separate containers. These var— ied from wooden tanks approximately one foot square to a variety of beakers and finger bowls, the container depending upon the size of the crab. Each container had sea water cir- culating in it, except the finger bowls which were refilled at regular intervals. Certain containers were fitted with screen covers to prevent the crabs from escaping. Solid covers were used when necessary to shade finger bowls from direct sunlight. ' The water temperature in the laboratory was taken each day, with a few exceptions, and varied from 11.9 to 18.40 C. These temperatures are well within the extremes tolerated by the crab in nature and temperature was not considered an important factor in this study. Several salinity readings were taken in the laboratory throughout the summer and these correlated with the past data on record with the Fish and Wildlife Service for the area during that time of year. Thus no special consideration was taken for either temperature or salinity as factors influencing the selection of clams as food by green crabs in the laboratory. The scope of the problem did not dictate close physiological control although of course both the above factors are vital in the ecology of the species. The most important part of the project was to determine the size groups of clams that were most frequently utilized and the extreme size limits that a given crab could consume. After a minimum starvation period of 48 hours each isolated crab was offered a selected group of measured clams, usually five. When large clams were used this number was often re- duced. Observations were made at regular intervals to deter- mine the size of clams that had been consumed. These obser- vations were made usually every 30 minutes. Records were maintained of the size clams that were broken, cracked or shucked. In each case the crab was undisturbed for a minimum of 48 hours after feeding. If a clam was not utilized within 72 hours, I assumed, on the basis of observed feeding habits, that the crab was unable to handle clams of that particular size. Only crabs with all their appendages and with hard carapaces were selected for the initial study. Crabs lacking appendages, berried females, and crabs in various stages of moulting were kept for longer periods of time but also were fed given numbers and sizes of clams in an effort to deter- mine their feeding tendencies under these conditions. Information on food preferences was found by present- ing a crab or group of crabs with a variety of bivalves of relatively equal size. These included Mya,§;2n§21§,,fiéggmé balthiga, Mytilug ggulig, and'Venus mergenaria. A variety of other invertebrates was offered to observe whether they would be acceptable as food to the crabs. As the experiments progressed some data were obtained on the ability of green crabs to ingest shell and also the subsequent elimination of the shell. A rough measure of diges- tion rate and of the methods used in eliminating shell bits were obtained by observation, microscopic examination of the feces, and timing of periods between feeding and excretion of feces. 8 Approximately 300 green crabs were used in the various aspects of the study although the information on many indi- viduals was not complete. DISCUSSION Collecting Areas The original reason for using two collecting areas in obtaining samples of green crabs was to contrast the feeding habits of crabs from two different ecological situations. Unfortunately I was unable to get uniform crab samples from both sites of all the size classes with which I wanted to work. Also I could not get sufficient data for such a comp parative study in the time available to me for laboratory work during one summer. The sample sizes utilized were too small to derive any definite information on feeding varia- tions due to habitat differences alone. I therefore restric- ted the project to green crab feeding methods and effects as particularly related to shellfish. Since comparative samples were not necessary, my only requirement was to collect from areas where there was little chance of the crabs having fed on debris or having been influenced to any extent by human activity. Sam's Cove in Bremen, Maine, was convenient because the area is used for experimental work by both federal and state biologists and the flats there have been closed to commercial digging for some years. Powderhorn Island, in the Sheepscot River, was chosen when the original comparative study was outlined. Since some collecting had been started there, I continued using this location particularly to obtain the larger crabs. 10 ll Sam's Cove (Fig. 11) is an area of mud flats about four acres in extent. The east side of the cove is bordered by sloping ledge and large rocks while the west side is most- ly cobble. Rockweed, W n d , and M13 in. are the main plants along the intertidal zone. They cover the rocks and ledges all around the margins with the exception of the head of the cove where marsh grasses take over. The cove is completely flooded at high tide and drains dry at low water. Collecting here was all done by hand and most of the crabs were picked up on the east shore near the mouth of the cove. The majority of the smaller sized crabs used in the laboratory feeding experiments were collected here. Powderhorn Island, the second site, is small and ex- posed with a steep rocky shoreline around all but one end (Fig. 12). At the northern tip of the island is an area of mud flats that supports a small population of Ema. On the eastern shore is a small somewhat sheltered cove. Here some hand collecting was done but mainly I relied on a single crab trap set just off shore. Nbst of the crabs I used came from this location. As at Sam's Cove, the dominant algal cover on the rocks was 2333; and Agggphyllum. m alWandLLtsfiiam The green crab (Figs. 1-8) is characterized by having similar, rather stout legs with no swimming paddles on the last pair. All other portunids have the dactylus of the fourth or swimming leg modified into a paddle structure. 12 There are five antero-lateral teeth on the margin of the carapace. The base color of the carapace is green with a heavy mottling of black, yellow and red. In females the base color is often orange. Color is extremely variable (Fig. 7). This species is quite cosmopolitan as it is found along both sides of the Atlantic and also along the coasts of North Africa, Suez Canal, Red Sea, Ceylon, Australia and the Hawai- ian Islands, (Rathbun, 1930). Workers in Great Britain and Europe have studied the green crab in some detail but little work has been done on the life history of this species in North America. A study of green crabs in the Netherlands (Broekhuysen, 1936) has par- ticularly advanced our knowledge of growth and deveIOpment in this species. Male green crabs are usually sexually mature by the end of the first year. Females require a somewhat longer per- iod for development to sexual maturity and most berried females are two or three years old. The shell of a female green crab must be soft for copulation to take place. During the mating period a male may carry a female around for several days until she moults and copulation occurs before the exoskeleton har- dens. She is usually held under the abdomen of the male by his walking legs. I have observed this carrying process several times in the laboratory but do not know whether this action is the usual proceedure. Following mating the sperm is stored in the copulatory pouches of the female and may remain viable for a number of months. Cases have been noted 13 in which two batches of eggs have been fertilized from a single copulation. Eggs are usually deposited from one to four months after mating occurs. The eggs become attached to the endOpodites of the pleopods and are carried from four to six months, depending upon the environmental conditions. A single large female may carry as many as 200,000 eggs. Berried females are present throughout the year but appear to be least abundant in late summer and early fall. As green crab larvae hatch from.the egg mass they are without protective spines and little development of the app- endages has taken place. Within a few hours after hatching, however, the first moult occurs and the larvae enter the zoea stage. At this point each larva is about 1.25 mm. in length. The helmet-shaped carapace becomes armed with both an anter- ior and a dorsal spine. The narrow flexible abdomen ends in a forked telson. Two pairs of swimming appendages and both antennules and antennae are developed at this stage. A func- tional pair of large compound eyes is set under the anterior portion of the carapace. During the zoea stage the crabs are at or near the surface of the sea and feed on both zooplankton and phytoplankton. Through a series of moults the size of the larvae increase and by the end of the zoea stage all the app- endages have begun development. As growth continues the dorsal spine of the carapace is lost, the appendages complete devel- opment, and the eyes become stalked and movable. In this stage the larvae are known as megalops. The abdomen is still stretched out behind the body but later during this period 14 the larvae settle to the bottom and the abdomen becomes tuck- ed under the body as in an adult crab. The free-swimming per- iod between hatching and the settling of the megalops prob— ably lasts at least a month. Lebour (1947) noted that green crab larvae were present for every month in plankton samples taken at Plymouth, England. After settling to the bottom the crabs continue their deveIOpment. Shen (1935) found that the period from megalops to the ninth instar was about 132 days. Food habits change with the mode of life. Planktonic food is replaced by minute benthic organisms and perhaps some detri- tus. The role played by bivalve larvae as crab food during this period is not known. Growth is rapid the first year. Crabs may develop a carapace width of 30 to 40 mm. during this time as a result of from 10 to 15 moults. Increase in size occurs only at moulting. The mean increase in size after each moult amounts to one-third to one-fifth of the original size. Orton (1936) presented some good data on size in relation to age for both sexes. Comparative data for growth of abdomen and carapace were given by Huxley and Richards (1931). The number of moults is reduced in each succeeding year and a mature crab may moult only twice or even once a year. The majority of green crabs die before they are four years old. Green crabs are not commonly found below four or five fathoms. They are mainly intertidal inhabitants and for this reason are more affected by climatic conditions than other species of crabs that are found in deeper water. Severe 15 winters inflict a high mortality rate on both eggs and adults. Green crabs seem to be affected adversely by salinities over 31 parts per thousand but may stand a low extreme of four parts per thousand. Feeding Methgdg Green crabs feed on bivalves in two ways. The shell is either cracked or crushed, or the meat is shucked out and the two valves usually left intact. In the first case, after the shell has been cracked.by the cheliped claws the clam is held by one of the crab's claws while the other proceeds to tear off shreds of meat and to place the food in the mouth. When a clam is shucked, it is turned by the chelae until it is in such a position that the claw tips of the crab can be inserted between the valves. By tearing bits of tissue away the crab eventually is able to sever one of the adductor muscles. When this happens the clam has lost the ability to keep the valves tightly closed. From this point it takes but a short time for the crab to clean out all vestige of tissue. When fed Mya, the smaller crabs (less than 20 mm.) used in this study tended to shuck out the clams rather than break them up. The large crabs (above 70 mm.) also shucked out the clams in most instances. With the large crabs this type of feeding seemed to be particularly apparent with clams about equal to or larger than the carapace width of the crab. Both types of feeding were found with all the size classes of crabs that I studied, but in general the above observations hold l6 true. In the entire study no cases were noted-in which1Myg (or any other mollusk) were consumed whole. The smallest clams that I used in the study were four mm. in length. Few data were obtained on feeding in relation to these small clam sizes and such limited information is certainly not conclusive. That green crabs could eat certain shellfish whole is possible, but the crab would have to be fairly large and the clam quite small, since food is so well chewed and broken up by the mouth parts and in the gastric mill. The soft-shell clam, M12 apenaria, is particularly vul- nerable to crab predation for two reasons. First, the shell itself, especially the margins, is relatively thin and easily crushed. Secondly, the two valves do not touch around the ant- erior and posterior margins. This lack of ability to close both shells tightly makes it easier for a crab to gain access to the soft parts. In contrast, the hard-shell clam or quahog, Venus mercenaria, in addition to having a much heavier shell, is able to seal its valves tightly in times of danger. Cannabalism Cannabalism was common when a number of green crabs of varying sizes were kept together. If an individual lost a claw or walking leg, the disadvantage often resulted in the crab's ultimate death. Several crabs would attack the help- less victim and soon its walking legs and even the chelae would be pulled off. If the victim was small, its body was often crushed and eaten by a larger crab. A large crab that 17 had lost its appendages would soon starve in the tank, since it would be unable to compete for any food that was offered. The number of crabs in the tank and the length of time since the last feeding were the apparent controlling factors in respect to cannabalism. Excessive Feeding When.yya were presented to a starved crab in limited numbers, all of the clam body masses were consumed. In a number of instances, however, I supplied crabs with an excess number of clams and often in this situation only the soft visceral masses were eaten. The siphons and mantles were usually left untouched. Crabs in the laboratory broke up more clams than they could possibly devour when presented with an excess of these bivalves. The softer portions were picked out and eaten at the crab's leisure. The rate at which green crabs can consume shellfish is well illustrated by the following examples. On August 22 a 58 mm. male green crab was offered 20 Myg‘between 17 and 34 mm. in length. Within 24 hours all the clams were crushed and all but eight had been eaten. There was no evidence of shell ingestion. The remaining clams were consumed within two days. A.58 mm. female crab was able to consume eight my; in 35 minutes. These clams ranged in size from 12 to 20 mm. It took just 40 minutes for a 63 mm. male green crab to eat eight Myg between 15 and 20 mm. in length. On August 29 I attempted to determine the number of shellfish a crab might 18 be able to destroy in a 48 hour period. A 64 mm. male was offered 15 M13 between 21 and 45 mm., 15 Mytilus between 19 and 41 mm., and 5 Maggma 10 to 22 mm. in length. At the end of the time period 13 2mg” nine Mytilus, and two of the Macoma had been crushed. Most of the flesh from these bivalves had been eaten. On September 10 a 70 mm. male crab and one of 56 mm. were placed together in a large crock. Fifteen M12, eight to 42 mm. in length and 15 Mytilug, 25 to 64 mm., were offered as food. Twenty-four hours later the crabs had crushed and eaten all but two of the Myg and all but the largest five m. The destructive potential of a single green crab has been outlined in the preceding paragraph. The vast increase in the numbers of these crabs in recent years along the coast of New England and eastern Canada creates a most serious prob- lem to the ecology of important shellfish. It was not at all unusual to catch 200 to 300 green crabs per trap per day in the Boothbay Harbor area. In many places along the coast these figures would have been even higher. With such a rapid in- crease in the population of a predator, much additional work must be done to learn enough of the natural history of Cape ginidgg maenas to establish controls in the field. EQQQ Ezgfgrengg Some information on the food preferences of green crabs in respect to shellfish was obtained during this study. Twelve crabs were isolated and used in testing preference toward 19 certain bivalves. The sex and size of each crab and the num- ber of specimens of each of the bivalves used are given in Table 1. Each crab was given the listed number of food items, all of relatively equal size, with samples ranging from 10 to 40 mm. In seven instances just Myg and Mytilug were compared. Six of these seven crabs selected M12 first indicating a pref- erence for that species. Maggma and the other two bivalves listed above were presented to five crabs, and in all but one of the five cases ranked third in food choice. In all but three instances all of the shellfish offered were eaten or at least broken open. A 56 mm. male crab, when presented with two of each of the above bivalves tested, had no difficulty in eating all six within an hour and a half. TABLE I DATA ON FOOD PREFERENCE EXPERIMENT Crab size (mm.) # of aye # of Mutilu§_ # of Maggma and sex Aprgsgnted preggntgd preggntgd 52 male 3* 3 O 56 male 3* 3 0 56 male 15* 15 0 56 male 2 2* 2 64 male 15* 15 5 70 male 4* 2 2 70 male 15 15 0 73 male 3 3* 3 43 female 2 2 2 50 female 3* 3 0 56 female 3* 3 0 59 female 6* 2 0 * preference shown for this species 20 The quahog,'Egpu§,m§zg§nar1a, an additional species that is sometimes fed upon, is a common clam throughout much of the range of the green crab. The shell of this mollusk is considerably harder and thicker than that of the above bi- valves, and when the quahog reaches about 30 mm. in length it is relatively safe from predation by green crabs. A dozen quahogs of varying sizes were placed in a tank with ten un- sexed crabs. All of the crabs were over 50 mm. in width. Only four clams were broken and eaten and they were all under 30 mm. Crabs readily ate the shucked quahogs offered them and if a quahog shell is cracked or chipped a crab can often work this advantage into a meal. From these few laboratory studies and from numerous field observations throughout the summer it seems that the important factor in food selection by green crabs is not the species, but the abundance or availability of the food. When a crab was given a choice between a 20 mm. M12 or a Maggma of the same size, it usually took the M.a, apparently because the shell was thinner and easier to crush. When crabs were given the flesh from shucked bivalves of a variety of species, no definite preference was noted. Thus, if a crab population inhabits an area near a mussel bed, then these mollusks will form.the main food supply. On flats that support a heavy Maggma population, and a few large My; which are deeply buried, the crabs will certainly utilize the Maggma, since they are at or near the surface of the flat and much easier to obtain. Conversely, when presented with these two species 21 At various times during the summer the following food items were found acceptable to green crabs; crushed barnacles, Balanus,§p., cod and pollack fillets and entrails, mummichogs, Fundulug gp., squid, small pieces of kelp, Laminaria pp., and thatch grass, Spartina pp. A variety of shellfish were pre- sented and all were readily taken as food by the crabs. These food items were given in small numbers throughout the summer, whenever I was able to collect or otherwise obtain a few Spec- imens. The mollusks were all crushed before being given to the crabs in the large holding tank. Some of these shellfish would ordinarily be impossible for a green crab to obtain or utilize as food, but in this case I was interested in whether or not the meats would be consumed and not in the ability of a crab to crush the shells. These species offered and accepted as food included Cragsostrga Elggjpigg, Agtgpgglca ta a, ____inPect Hand 3, ____di_Yol a 11....2Lmat 9. Kamila s .. W silly-.19. Messiaen. Wmdi lu . Mamie.leass W, W (21.3.8.2) morrh ans, 42$ isu__l_a_ solidissima, lawns 211121.122. Gm sea-m. _.sl_E-‘n s dime 1: . _8s._xicava arctica, Lgttgrina litgrga and Littggina gbtugata. Green crabs appear to be omnivorous and, although they do act somewhat as scav- engers, they seem to prefer fresh food. Frozen fish were read- ily taken, but decayed putrid fish were usually left untouched, even by crabs that had been starved several days. Bergigg Fgmaleg Eight berried females (Figs. 3-4) were kept in beakers 22 and observed daily for varying lengths of time throughout the summer. The minimum length of time that any of these crabs were kept in the laboratory was three weeks and in most cases the period of observation was about five weeks. The beakers were supplied with a continual flow of sea water. Seven of the crabs were fed two or three M15 ranging in size between 10 and 30 mm. during the morning of each day. These berried females ate regularly during the entire period of laboratory observation. There was never any delay in their acceptance of the clams. The eighth individual, caught on July 15, was fed four or five My; ranging in size between 10 and 20 mm. every other day. The feeding was continued for this female until August 12, when most of the larvae and left the egg mass. This crab also consumed all the food presented to it. The above observations on all these berried females were made during the last few weeks of their egg-carrying periods. Observations on the food habits of a female green crab from the time the eggs are deposited on the abdomen until the larvae break away were not possible during this study. From my data on the above mentioned specimens, however, I feel certain that female green crabs feed regularly throughout the period they are in a berried condition. From field observations and the small numbers of berr- ied females caught in traps during the summer I found that female crabs were much less active while carrying eggs. This information has been generally known and accepted for some time. The berried crabs tended to stay in protected areas 23 and did not move about on the flats to any great extent. The laboratory observations indicate that feeding does not stop while a female crab is in a berried condition. Any predatory action on bivalves by female crabs during this period, how- ever, is undoubtedly reduced to a considerable degree. The type of food taken in may change. Perhaps an increase in the percentage of thatch, Spartina sp., or other plant material used as food might be noted. Studies concerning food habits during particular periods of the green crab life cycle would be most helpful. W and Food Habitg One can determine approximately when a green crab is going to moult by an examination of the posterior and ventral margins of the carapace. The edge of the carapace becomes soft along the submarginal fissure and also where it meets the abdomen. A split finally occurs along this lateral and posterior line and the crab "backs" out of its old shell. Observations were made on 15 crabs that appeared ready to moult (Table II). These individuals were placed in separate beakers and each one was given five Egg as food. The clams offered were all within the size limits that the particular crab was known to be able to consume. Thus food was available for each crab throughout the duration of the moulting period. When the crab moulted, the cast shell was removed from the beaker and the crab was left for the exoskeleton to harden. Twelve of the crabs refused to eat during this period. One 24 TABLE II OBSERVATIONS ON MOULTING GREEN CRABS Date crab isolated Date Sex Original New Food accepted and given food MOulted Width Width within seven (mm mm da s of moult 7/17 7/17 40 52 No 7/17 7/18 31 41 No 7/23 7/23 41 51 No 7/23 7/25 M so 61 Yes- 5 mg, taken on /1 7/23 7/25 M 47 57 Yes- 3 ma taken on 7/23 7/25 7/26 F 12 16 No 7/29 7/29 F 12 15 No 7/29 8/1 M 48 58 No 8/1 8/3 F 21 28 No 8/1 8/3 M 20 27 No 8/1 8/3 M l3 17 No 8/1 8/12 F 36 41 Yes- ate 3 M1; first two days 9/4 9/4 F 28 37 No >9/4 9/4 M 18 22 No 9/4 9/5 F 28 35 No 25 male specimen ate all five clams seven days after the moult had occured. This was the shortest length of time that I re- corded between a moult and the subsequent acceptance of food. Moulting from the time of capture varied with each crab stud- ied. The periods ranged from moults occurring the same day the crab was isolated to a moult occurring 12 days after capture. One male consumed two yy§,two days before a moult and a female ate three yyg nine days before moulting. These’ were the only two observed records of food acceptance prior to a moult once the moult was in progress. Green crabs are probably capable of feeding on bivalves up until the time of a moult. Whereas they may be able to consume food the ten- dency for reduced movement and activity in the field is app- arent and food intake is lowered considerably as the time of the actual moult approaches. "Soft" crabs are extremely vul- :nerable to predation by gulls, and by other crabs on a tidal flat, hence they seek shelter in a thatch bank or under a Irock during this stage. The time when feeding is resumed appears to vary with crab size and the physiological pro- <3esses involved in shell hardening. During these experiments 31 noted that the distal ends of the chelae are one of the Ifirst portions of the new shell to harden. This fact is Sparticularly interesting, since the cheliped is the grasping Portion of the "claw". The chelae would have to be hard in ‘IPder to break or shuck a bivalve. I have often observed ) r ? Fig. 3. Comparison of the dorsal aspects of a male green crab (top) and a female crab. The female is carrying eggs. s x Q ; LJQL ; V, \ f‘ ' " - '- ' " “ \~ -~ “-7 _ * \\«\ \ \~- \ ‘1 , x \r\/\ ,. 5‘ _. Fig. 4. Comparison of the ventral surface of a male green crab (top) and a female crab. Note the egg mass under the abdomen of the female. 43 Fig. 5. Ventral view of adult male green crab. crab . Fig. 7. Female green crabs showing some color variation. Fig. 8. A close-up of the mouth parts of a green crab. Fig. 9. The type of crab trap used in this study. Fig. 10. The soft-shell clam, 21E arggaria L. Fig. 11. Sam's Cove, Bremen, Maine, at low tide. Crabs were collected outside the ex- perimental crab fence (foreground). Fig. 12. Powderhorn Island, located in the Sheepscot River. The white buoy in the center of the picture is a marker for a crab trap. LITERATURE CITED Bateman, J.B. 1933. Osmotic and ionic regulation in the shore crab, gszgigus ma a , with notes on the blood concentrations of W locusts and Qgia W. Jour. Exp. Biol., 19: 355-371. Borradaile, L.A. 1922. On the mouth-parts of the shore crab. Jour. Linn. Soc. Zool. London., 35: 115-142. Broekhuysen, G.J. 1936. On development, growth, and distrib- ution of r n e lmagggs (L.). Arch. Neerland. de 2001. , g3 57-399e Dow, R.L., and D.E. wallace. 1952. Observations on green crabs (Q. msgpgs) in Maine. Maine Dept. Sea and Shore Fish., Fish. Ciro. No. 8: 11-15. Glude, J.B. 1954. The effects of temperature and predators on the abundance of the soft-shell clam, a gaggggia, x‘ in New England. Trans. Amer. Fish. Soc., : l -2 . . 1955. Report of Clam Investigations for fiscal year 1955. U.S. Fish and Wildlife Service. Presented to the Atlantic States Marine Fisheries Commission, November 15. 1955. Goucher, F.E. 1951. Commentary of a clam digger. unpublished manuscript, 1951. Harvey, L.A. 1929. The oogenesis of gagginns‘msgnas Penn. with special reference to yolk formation. Trans. Roy. Soc. mm., 56: 157—174. Huxley, J.S., and O.M. Richards. 1931. Relative growth of the abdomen and the carapace of the shore-crab Ca magg s. Jour. Marine Biol. Assoc., 11: 1001-1015. Lebour, M;V. 1928. The larval stages of the Plymouth Brachyura. Proc. Zool. Soc. London., Part 2: 473—560. . 1944. The larval stages of Egrtumnug (Crustacea brachyura) with notes on some other genera. Jour. Marine Biol. Assoc., 26: 7-15. . 1947. Notes on the inshore plankton of Plymbuth. Jour. Marine Biol. Assoc., 2.5,: 527-547. Leidy, J. 1855. Contributions towards a knowledge of marine invertebrate fauna of the coasts of Rhode Island and New Jersey. Jour. Acad. Nat. Sci. Phila. (Second Series), 2: 135-152. 45 47 Lim, R.K.S. 1918. Experiments on the respiratory mechanism of the shore-crab (Caggigus‘magngfi). Proc. Roy. Soc. Edinb., 28: 48-56. MacPhail, J.S., E.I. Lord, and L.M. Dickie. 1955. The green crab - a new clam enemy. Fish. Res. Bd. of Canada, Progress Reports of the Atlantic Coast Stations, No. 63: 3-12. Nedcof, J.C., and L.M. Dickie. 1955. Watch for the green crab - a new clam enemy. Fish. Res. Bd. of Canada., Gen. Series Circ., No. 26: l p. Morse, M. 1909. The Harpswell laboratory. Popular Sci. Monthly, g; 159-183. Orton, J.H. 1936. Experiments in the sea on rate of growth of some Crustacea Decapoda. Jour. Marine Biol. Assoc., 29: 673-689 0 Punnett, R.C. 1904. Note on the proportion of sexes in Cargigus magngs. Proc. Camb. Phil. Soc., 12: 293-296. Rathbun, M.J. 1905. 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Experimental farming of the soft-shell clam, Myg arenagia, in Mass- achusetts, 1949-1953. Comm. Fish. Review., 11,(6): l-l6. Smith, S.I. 1879. The stalk-eyed crustaceans of the Atlantic Coast of North.America north of Cape Cod. Trans. Conn. Acad. Sci., 5: 27-138. 48 Spaulding, J.F. 1942. The nature and formation of the sper- matOphore and sperm plug in Gagginus maepus. Quart. Jour. Microsc. Sci., 83: 399- 22. Webb, D.A. 1940. Ionic regulation in Ca ‘mggnus. Proc. Roy. Soc. London., 129 (B854): 107-13 . Williamson, H.C. 1903. On the larval and early young stages, and rate of growth of the shore-crab ggrgigug mae (L.). 21st Rep. Fish. Bd. for Scotland.,.3: 136-179. Vles, F. 1909. Mbnographie sommaire de la Nye (Nye arinagia Idnne 1767). ,Mem. Soc. Zool. France., gs; 90-1 2. English translation by U.S. Fish and Wildlife Service, 1954. _ Date Due , . ._ ', ; 5 a! as i "L Demco-293 1111117 11111717111111 '7“