Interspecific interactions among larval trematode parasites of freshwater and marine snails

Abstract

Freshwater and marine snails serve as intermediate hosts for numerous species of larval trematodes. Any particular population of snails may be infected by several species. It is commonly observed that mixed species infections are less frequent than expected by change in collections of host snails from natural populations. While several mechanisms might generate such negative associations, laboratory studies of freshwater snail-trematode associations have demonstrated the presence of strong antagonistic interactions between intramolluscan larval stages (rediae and sporocysts) of species that infect the same host individual. Both predatory and non-predatory antagonism has been observed, the former taking the form of predation by large, dominant redial forms on the sporocysts and rediae of subordinate species. These interactions are largely hierarchical, although in some systems priority effects have been observed, and in one case a sporocyst species replaced a redial species by strong non-predatory antagonism. Several instances of positive association between larval trematode species have also been observed. In such cases, interference with host defense mechanisms by the first parasite appears to enhance superinfection by the second. My own study of the larval trematode guild that infects the salt marsh snail, Cerithidea californica, has revealed patterns of association and interaction that are very similar to those demonstrated by laboratory studies of freshwater systems. Ultimately, the frequency of interactions among larval trematodes depends on the availability, relative to the numbers of susceptible snails, of infective eggs and miracidial larvae transmitted from definitive hosts. © 1992 by the American Society of Zoologists.