High parasite diversity in the amphipod Gammarus lacustris in a subarctic lake.

Amphipods are often key species in aquatic food webs due to their functional roles in the ecosystem and as intermediate hosts for trophically transmitted parasites. Amphipods can also host many parasite species, yet few studies address the entire parasite community of a gammarid population, precluding a more dynamic understanding of the food web. We set out to identify and quantify the parasite community ofGammarus lacustristo understand the contributions of the amphipod and its parasites to the Takvatn food web. We identified seven parasite taxa: a direct life cycle gregarine,Rotundulasp., and larval stages of two digenean trematode genera, two cestodes, one nematode, and one acanthocephalan. The larval parasites use either birds or fishes as final hosts. Bird parasites predominated, with trematodePlagiorchissp. having the highest prevalence (69%) and mean abundance (2.7). Fish parasites were also common, including trematodesCrepidostomumspp., nematodeCystidicola farionis, and cestodeCyathocephalus truncatus(prevalences 13, 6, and 3%, respectively). Five parasites depend entirely onG. lacustristo complete their life cycle. At least 11.4% of the overall parasite diversity in the lake was dependent onG. lacustris, and 16% of the helminth diversity required or used the amphipod in their life cycles. These dependencies reveal that in addition to being a key prey item in subarctic lakes,G. lacustrisis also an important host for maintaining parasite diversity in such ecosystems.