Multi-clone infections and the impact of intraspecific competition on trematode colonies with a division of labour.

A division of labour occurs in colonies of the trematode Philophthalmus sp. within their first intermediate hosts. Two castes exist: one which reproduces and one which does not reproduce. It has been hypothesized that the benefit of the non-reproductive caste is in competitive interactions. Evidence for this from past experiments with Philophthalmus sp. colonies has been contradictory: the non-reproductive caste appears to benefit the colony in some way but not necessarily by combating interspecific competitors. The aims of this study were to consider intraspecific competition as a possible cause of the division of labour in Philophthalmus sp. colonies. Results show that mixed genotype infections occur in Philophthalmus sp. infected hosts and thus intraspecific competition is likely. Furthermore, the total number of individuals per colony is reduced in mixed genotype infections, indicating that intraspecific competition reduces colony fitness. However, the results do not indicate that the division of labour in Philophthalmus sp. plays a role in competitive interactions as the ratio of small, non-reproductive to large, reproductive individuals is unaffected by the presence of intraspecific competition. This is the first study to identify and quantify intraspecific competition in Philophthalmus sp., and to assess its selective role in this species' division of labour.