Paucivitellosus fragilis belongs to a small family of digenean trematodes call Bivesiculidae. They are parasitic in the gut of marine fishes and unlike most other families of digeneans, they do not have a second intermediate host, but infect the fish host directly when the cercaria larval stage is ingested by the fish. Upon emerging from the snail first intermediate host (where the cercarial stages are produced via cloning), a structure call a caudal vesicle folds itself over the cercaria body like a sock, acting as a kind of protective wetsuit. It then begin swimming actively with vigorous vibration of its tail. After a few hours, it sticks itself either to some floating debris or the surface of rocks, waiting to be eaten by an unwary grazing fish.
Photo came from:
Abdul-Salam, J. and Sreelatha, B.N.S. (1996) Light and scanning electron microscopic observations of the rediae and cercariae of Paucivitellosus fragilis (Digenea, Bivesiculidae). Acta Parasitologica 41:108-114.
Contributed by Tommy Leung.