Microbial communities and parasites associated with Diporeia spp. (Amphipoda, Gammaridae) and their potential impacts on diporeia spp. health in the laurentian Great Lakes basin Winters, Andrew David Amphipoda--Ecology--Great Lakes (North America) Pathogenic bacteria--Great Lakes (North America) Lake ecology--Great Lakes (North America) Amphipoda--Ecology Lake ecology Pathogenic bacteria Fisheries and aquatic sciences Pathology Genetics Thesis Ph. D. Michigan State University. Fisheries and Wildlife 2013. Due to their unique position in the foodweb, <italic>Diporeia</italic> (Amphipoda, Gammaridae) are an important component in the Great Lakes ecosystem. Unfortunately, <italic>Diporeia</italic> abundances have been declining from the majority of their habitat throughout lakes Michigan, Huron, Erie, and Ontario. The hypothesis that pathogens are the probable disease agents behind <italic>Diporeia</italic> declines, whether due to the presence of invasive dreissenid mussels or not, was investigated. This was investigated through examination of the bacterial communities associated with <italic>Diporeia</italic> through high throughput molecular techniques and gene sequencing, and identification of pathogens and their lesions through light and electron microscopical studies as well as phylogenetic studies. Analysis of 16S rRNA genes revealed that the bacterial communities associated with sediments were dominated by Actinobacteria and Acidobacteria while those associated with <italic>Diporeia</italic> were dominated by <italic>Flavobacterium</italic> spp. (Bacteroidetes) and <italic>Pseudomonas</italic> spp. (Gammproteobacteria). The presence of a bacterium belonging to Rickettsiales, an order of Bacteria containing known pathogens for freshwater amphipods, in <italic>Diporeia</italic> was confirmed. A significant temporal shift in bacterial community diversity was observed for <italic>Diporeia</italic> samples collected from one site in Lake Superior; however, the ecological significance of the shift remains to be determined. Microscopical examination of <italic>Diporeia</italic> collected from multiple sites in the southern basin of Lake Michigan between 1980 and 2007 revealed that <italic>Diporeia</italic> were host to a total of eight different groups of uni- and multicellular pathogens including, amoeba, microsporidia, haplosporida, filamentous fungi, yeast, ciliates, acanthocephala, and cestodes. Spatio-temporal variability in parasitic infections was observed with prevalences often fluctuating by depth, sampling site, and life stage of <italic>Diporeia</italic>. Additionally, the presence of the fish-pathogenic rhabdovirus viral hemorrhagic septicemia virus (VHSV) genotype IVb was confirmed in <italic>Diporeia</italic> samples collected from lakes Ontario, Huron, and Michigan, illustrating the role macroinvertebrates may play in VHSV ecology. Pathologies associated with pathogenic parasitic infections ranged from inflammation to destruction of tissues vital for the ecological performance of the amphipod. No significant positive correlations were observed between any group of parasites and dreissenid densities. My prevailing hypothesis is that parasite species belonging to Microsporidia and Haplosporidia are associated with detrimental effects that may have impacted <italic>Diporeia</italic> populations. Microscopical and phylogenetic investigations revealed the presence of two novel parasite species infecting <italic>Diporeia</italic>. The first parasite is a <italic>Haplsporidium</italic> sp. (Haplosporidia) that is similar to <italic>H. nelsoni</italic>, the causative agent of MSX disease in the eastern oyster (<italic>Crassostrea virginica</italic>). The second parasite is a <italic>Dictyocoela</italic> sp. (Microsporidia), a group of vertically transmitted parasites that infect both ovarian tissue and adjacent muscle of their amphipod hosts and are often associated with sex-ratio distortion in amphipod populations. Both novel parasites elicited a host immune response and were associated with destruction of muscle tissue in <italic>Diporeia</italic>. The findings of these studies shed light on pathogens as potential causes of <italic>Diporeia</italic> declines in the Laurentian Great Lakes. Includes bibliographical references. Description based on online resource; title from PDF t.p. (viewed on Sept. 8, 2014) Faisal, Mohamed Brenden, Travis O Fitzgerald, Scott Marsh, Terence L 2013 text Electronic dissertations Academic theses application/pdf 1 online resource (xix, 263 pages) : illustrations (some color). isbn:9781303527890 isbn:1303527898 umi:3601500 local:Winters_grad.msu_0128D_12467 en In Copyright Ph.D. Doctoral Fisheries and Wildlife - Doctor of Philosophy Michigan State University