Suppression of egg production in Potamopyrgus antipodarum (Gastropoda: Hydrobiidae) by larval trematodes

Of 7100 Potamopyrgus antipodarum examined for the presence of larval trematodes, the 4% parasitised by 9 different species showed a significant reduction in egg production, culminating in ‘parasitic castration’. Seven species of trematode—a gymnocephalous species, 4 xiphidiocercariae, an amphistome, and a monostome—infected the host's gonad. Two species of furcocercariae, a longifurcate and a lophocercous type, were found in the hepatopancreas. ‘Castration’ occurred directly when the gonad was penetrated, or indirectly when the hepatopancreas was infected. Indirect castration appeared to result from deprivation of nutrients supplied by the hepatopancreas, or pressure exerted upon the gonad. Some encapsulated cercariae were found, indicating a host immune response. The suitability of P. antipodarum as a host for larval trematodes is thought to be related to its wide distribution and salinity tolerance, which make it more available to miracidia; also, the grazing behaviour of the snail affords an opportunity for trematode eggs to be ingested. The fact that Hydrobiidae are an old family suggests that they may be well adapted to tolerating such a parasite fauna.