Interspecific Antagonism and Species Coexistence in a Diverse Guild of Larval Trematode Parasites
Authors: Sousa Wayne P. 
Published: April 2006

The salt marsh snail Cerithidea californica is first intermediate host to a diverse guild of larval trematode parasites. In Bolinas Lagoon, in central California, the site of this study, at least 15 species of trematodes infect snail populations. This study investigated patterns of interspecific association and interaction among members of this parasite guild. Seven to 19 host subpopulations were sampled annually at each of two sites in the lagoon from 1981 to 1988. Mixed—species infections constituted only 2.5% of the 5025 infections examined in the study. A Monte Carlo simulation procedure demonstrated that the numbers of such infections were often less than would be expected by chance, especially when the overall prevalence of infection was high. Patterns of association between particular pairs of species depended on whether the species' life histories include redial or only sporocyst larvae. Species that develop as rediae were predominantly negatively associated with other redial species and with most species that develop only as sporocysts. There was weak evidence of positive interspecific association between a few redial and sporocyst—only species, while members of other such pairs were distributed independently. Associations between sporocyst—only species were either weakly positive or neutral. Snails carrying known infections were marked, released, and recaptured at both study sites. During their exposure in the field, some initial infections were invaded by another parasite species that often excluded the first parasite. The vulnerability of a parasite species to invasion and replacement by another differed among the tested species. Infections of the largest redial species, Parorchis acanthus, were especially resistant to replacement, while those of the smallest redial species, Euhaplorchis californiensis, were the most frequently excluded. Four other species were invaded or replaced at intermediate rates. The two largest redial species, P. acanthus and Himasthla rhigedana, were responsible for >90% of the invasions or exclusions. Direct observations showed that the rediae of these species prey on the larval stages of other species, as do the rediae of Echinoparyphium sp. This direct form of interspecific antagonism is probably the primary mechanism by which such species exclude others from host snails, as has been widely demonstrated in similar freshwater snail—trematode systems. While hierarchical, negative interactions prevent the coexistence of species at the level of the individual host, the mark—recapture study showed that rates of exclusion are low for most subordinate species, with the exception of Euhaplorchis californiensis. At the level of the host subpopulation, the assemblage of larval trematodes is diverse, and its composition is temporally and spatially variable. There is no trend toward dominance of the assemblage by large redial species as the level of infection rises within aging cohorts or subpopulations of hosts. These patterns of guild structure within host cohorts and subpopulations are consistent with the hypothesis that recruitment processes rather than interspecific interactions primarily determine the composition and relative abundance of species at this regional level. Several characteristics of snail—trematode systems that may promote regional coexistence of such a large number of potentially interacting parasite species are the isolated and subdivided nature of the host resource, the aggregated distributions of larval stages, and the differential exploitation of different—sized hosts. Many features of this system are consistent with Price's (1980) non—equilibrial view of parasite communities.