Urastoma cyprinae is a turbellarian that infects the gills of numerous species. It has been reported as free-living organism in marine mud and on algae. Urastoma cyprinae is reported as an opportunistic mantle inhabitant on the gills of various bivalve species, including the clams Tridacna maxima and Tridacna gigas, and the mussels Mytilus edulis and Mytilus galloprovincialis. They are also found throughout the gill surface of C. virginica and is attracted by mucus that coats the gills of oysters. However, the nature of the host-parasite relationship remains unknown.