Trematodes frequently have complex life cycles involving many hosts and Crepidostomum cooperi is no exception. The adults parasitize fish and lay eggs into the gut, where they pass out with the host's feces. The miracidia then invade tiny little pea- or fingernail clams (Pisidium spp.) The next stage goes on to infect mayflies, which then get eaten by fish to start the whole life cycle over again. A few years ago, I co-authored a paper with University of Colorado at Boulder biologists Rob Guralnick and Gene Hall where we looked at the relationship of the size of the molluscan hosts to parasitism by C. cooperi and other allocreadiid trematodes. Larger clam species (and this is relative - all of these clams are tiny!) were 12 times more likely to be infected with the trematodes than smaller species. We argued that these changes in body size over evolutionary time were a trade-off to balance the fact that the trematodes sterilize these clams.
The photo is actually of a related species, Crepidostomum opeongoensis and comes from this paper.
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