Infection success of different trematode genotypes in two alternative intermediate hosts: evidence for intraspecific specialization?

The evolution of host specificity and the potential trade-off between being a generalist and a specialist are central issues in the evolutionary ecology of parasites. Different species of parasites or even different populations of the same species often show different degrees of host specificity. However, less is known about intraspecific variation in host specificity within a population. We investigated intraspecific variation by experimentally exposing cercariae from different clones of the trematode Curtuteria australis to two species of second intermediate hosts, the New Zealand cockle Austrovenus stutchburyi and the wedge shell Macomona liliana. We found an overall difference in infection success between the two bivalve species, with A. stutchburyi being the more heavily infected host. However, the cercariae showed a consistent preference for encysting at the tip of the bivalve's foot, regardless of host species. Importantly, there were no significant differences among parasite clones in either relative infection success in the two hosts or preference for the host foot tip. This lack of intraspecific variation may be due to the life-history traits of both parasite and hosts in our system, which may limit opportunities for variation in performance and exploitation strategies in different hosts to evolve within the population.