Do parasites adopt different strategies in different intermediate hosts? Host size, not host species, influencesCoitocaecum parvum(Trematoda) life history strategy, size and egg production

Abstract

SUMMARYHost exploitation induces host defence responses and competition between parasites, resulting in individual parasites facing highly variable environments. Alternative life strategies may thus be expressed in context-dependent ways, depending on which host species is used and intra-host competition between parasites.Coitocaecum parvum(Trematode) can use facultative progenesis in amphipod intermediate hosts,Paracalliope fluviatilis, to abbreviate its life cycle in response to such environmental factors.Coitocaecum parvumalso uses another amphipod host,Paracorophium excavatum, a species widely different in size and ecology fromP. fluviatilis. In this study, parasite infection levels and strategies in the two amphipod species were compared to determine whether the adoption of progenesis byC. parvumvaried between these two hosts. Potential differences in size and/or egg production betweenC. parvumindividuals according to amphipod host species were also investigated. Results show thatC. parvumlife strategy was not influenced by host species. In contrast, host size significantly affectedC. parvumstrategy, size and egg production. Since intra-host interactions between co-infecting parasites also influencedC. parvumstrategy, size and fecundity, it is highly likely that within-host resource limitations affectC. parvumlife strategy and overall fitness regardless of host species.