Morphology and seasonal incidence of infection ofProctoeces maculatus(Looss, 1901) Odhner, 1911 (Trematoda) inMytilus edulisL.

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Abstract
The morphology of adult Proctoeces maculatus from the mussel, Mytilus edulis, is described and compared to previous descriptions of the worm from fish and the bivalve, Scrobicularia plana. Stages in the mussel include two generations of sporocysts, cercariae and progenetic adults. Both sporocysts and adults exhibited seasonal variation in the incidence of infection during one year of sampling at two sites. Sporocysts were most common in late summer and the incidence of infection increased with increase in the size class of the host. The incidence of adult trematode infection peaked in early winter and rapidly declined with no worms being recovered by late May. Sporocysts appeared to persist until host death whereas adults matured, released eggs, and died during late winter. The mussel appears to represent an alternative final host of this trematode, perhaps replacing tropical fish hosts in temperate waters.