The life history of Plagioporus shawi (McIntosh 1939) (Trematoda:Opecoelida), an intestinal parasite of salmonid fishes.

Podocotyle shawi is transferred to the genus Plagioporus. The miracidium develops and hatches in 11 to 14 days at 22 to 25 C. The operculate snail, Lithoglyphus virens, serves as first intermediate host. Mother and daughter sporocysts develop, the latter producing cotylomicrocercous cercariae within 90 to 130 days. Cercariae encyst in the amphipod, Hyalella azteca; stonefly nymphs of the genus Arcynopteryx; mayfly nymphs of the genera Heptagenia and Paraleptophlebia; chironomid larvae of the genera Chironomus, Polypedilum, Phaenopsectra, Ablabesmyia, and Psectrocladius and caddis larvae of the genera Brachycentrus, Limnephilus, and Hydropsyche. Metacercariae 60 to 100 days old were fed to young steelhead trout, Salmo gairdneri, and partially developed specimens of P. shawi were recovered 26 days later. Stonefly nymphs are believed to be the most important second intermediate host in the natural environment. The parasite seems to be restricted to the Pacific Northwest of the United States and has been reported from the silver salmon, Oncorhynchus kisutch; kokanee, O. nerka; cutthroat trout, Salmo clarki; steelhead trout, S. gairdneri; and the mountain whitefish, Prosopium williamsoni.