The gastropod Ilyanassa obsoleta as a resource: utilization by larval trematodes in a low-prevalence system

The snail Ilyanassa obsoleta is native in estuaries along the eastern seaboard of North America. In Delaware, most studies on this snail and its larval trematode parasites have been done in snail populations where prevalence (percent infected) is high. Here, data were obtained from a lower-prevalence population in southeastern Rehoboth Bay, Delaware to study how the resident trematodes exploit the snail resource among the salt marsh islands. Snails were sieved from 3 plots (1 m each) at each of 21 locations in the approximately 1.5-km2 study area. Snails were counted and all, or a random selection, were measured for size and dissected to reveal sex and trematode infections. Snails in the area were distributed heterogeneously with respect to density, size, and parasite prevalence. Overall, utilization of the snail resource by trematodes was low. Prevalence of 6 trematode species (in 11 combinations) was 13.9% among 6862 snails collected. Most snails in the area had remained uninfected for many years. All snail infections except 1 were at least twice as aggregated in their spatial distributions as uninfected snails. In a test for species associations, trematodes were independent in locations where they infected snails. Parasite assemblages at species-poor locations were nonrandom subsets in species-rich locations; i.e., they were nested, suggesting that snails at different locations are exposed to different, and somewhat ordered, sets of infective larvae (miracidia). Low utilization of the snail resource could result from limited visitation by infected definitive hosts and/or unsuccessful miracidia. I propose it is mainly the latter. Miracidia are microscopic, swimming, and short-lived larvae. Tidal currents in the narrow waterways would sweep them along and make it difficult to stop and infect snails.