Species Relationships in a Marine Gastropod-Trematode Ecological System

Individual snails (Ilyanassa obsoleta) on Cape Henlopen, Delaware, frequently are host to one or more trematode species. When different species occupy the same host, interactions might be expected. We investigated five species of parasites to determine whether their existence in different combinations would lead to altered within-host distributions or changed numbers of shed cercariae. Snails (32 samples, total = 379) were collected from June to August, in 1989, and microscopically examined. Parasite species and stages present in five sections through each snail were recorded. Before examination, 206 of these snails were held in individual chambers in the field. After two high tides (ca. 24 h), the chambers were checked for species and the numbers of cercariae shed. Overall, 22 trematode combinations in single hosts were observed. Analysis revealed that co-occurrence with other species had no significant effects on any trematode. Further, analyses of species richness of infecting assemblages over two distinct intervals failed to show that competition is important in determining assemblage richness. One pair of trematodes (Himasthla quissetensis and Lepocreadium setiferoides) has been reported not to co-occur. We observed co-occurrences, but so few that the apparent conflict between them could not be statistically demonstrated. We suggest that, in this system, parasites are adapted to the host only; they may interact, but they are not adapted to each other. Chances for a parasite to live free from other parasites seem too great for evolved (adapted) relationships to develop. The host, for similar reasons, is probably not adapted to the parasites.