Due to their unique position in the foodweb, Diporeia (Amphipoda, Gammaridae) are an important component in the Great Lakes ecosystem. Unfortunately, Diporeia 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 Diporeia 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 Diporeia 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 Diporeia were dominated by Flavobacterium spp. (Bacteroidetes) and Pseudomonas spp. (Gammproteobacteria). The presence of a bacterium belonging to Rickettsiales, an order of Bacteria containing known pathogens for freshwater amphipods, in Diporeia was confirmed. A significant temporal shift in bacterial community diversity was observed for Diporeia samples collected from one site in Lake Superior; however, the ecological significance of the shift remains to be determined. Microscopical examination of Diporeia collected from multiple sites in the southern basin of Lake Michigan between 1980 and 2007 revealed that Diporeia 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 Diporeia. Additionally, the presence of the fish-pathogenic rhabdovirus viral hemorrhagic septicemia virus (VHSV) genotype IVb was confirmed in Diporeia 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 Diporeia populations. Microscopical and phylogenetic investigations revealed the presence of two novel parasite species infecting Diporeia. The first parasite is a Haplsporidium sp. (Haplosporidia) that is similar to H. nelsoni, the causative agent of MSX disease in the eastern oyster (Crassostrea virginica). The second parasite is a Dictyocoela 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 Diporeia. The findings of these studies shed light on pathogens as potential causes of Diporeia declines in the Laurentian Great Lakes.
Read
