Ecological divergence of closely related Diplostomum (Trematoda) parasites

Parasite life-cycles present intriguing model systems to study divergence in resource use and ecology between parasite taxa. In ecologically similar taxa, consistent selective forces may lead to convergence of life-history traits, but resource overlap and similarity of life-cycles may also promote divergence between the taxa in (1) use of host species or (2) specific niche within a host. We studied the life-history characteristics of 2 sympatric species of Diplostomum parasites, D. spathaceum and D. gasterostei, concentrating particularly on differences in intermediate host use and characteristics of the infective stages between the species. This group of trematodes is a notoriously difficult challenge for morphological taxonomy and therefore any information on the ecology of these species can also be helpful in resolving their taxonomy. We observed that these species indeed had diverged as they used mainly different snail and fish species as intermediate hosts and in controlled experiments infected different regions of the eye (lens and vitreous body) of a novel fish host. Interestingly, cercarial characteristics (activity and life-span) were similar between the species and the species were difficult to separate at the cercarial stage unless one observes their swimming behaviour. The release of cercaria from the snail hosts was higher in D. spathaceum, but when cercarial numbers were proportioned to the volume of the snail host, the production was higher in D. gasterostei suggesting differences in the rate of snail host exploitation between the parasite species. These results corroborate the prediction that closely related parasite taxa which are competing for the same resources should have diverged in their life-history characteristics and host use.