Effects of interspecific competition on asexual proliferation and clonal genetic diversity in larval trematode infections of snails

Interactions among different parasite species within hosts can be important factors shaping the evolution of parasite and host populations. Within snail hosts, antagonistic interactions among trematode species, such as competition and predation, can influence parasite abundance and diversity. In the present study we examined the strength of antagonistic interactions between 2 marine trematodes ('Maritrema novaezealandensis' and 'Philophthalmus' sp.) in naturally infected 'Zeacumantus subcarinatus' snails. We found approximately the same number of snails harbouring both species as would be expected by chance given the prevalence of each. However, snails infected with only 'M. novaezealandensis' and snails with 'M. novaezealandensis' and 'Philophthalmus' sp. co-occurring were smaller than snails harbouring only 'Philophthalmus' sp. In addition, the number of 'Philophthalmus' sp. rediae was not affected by the presence of 'M. novaezealandensis' sporocysts and the within-host clonal diversity of 'M. novaezealandensis' was not influenced by the presence of 'Philophthalmus' sp. Our results suggest that antagonistic interactions may not be a major force influencing the evolution of these trematodes and that characteristics such as host size and parasite infection longevity are shaping their abundance and population dynamics.