Host specificity of Argulus coregoni (Crustacea: Branchiura) increases at maturation

We tested the hypothesis that host specificity in ectoparasites does not depend exclusivelly on the features of the hos but also on sorrounding habitats, using 2 fish ectoparasites. Argulus coregoni and A. foliaceus (Crustacea: branchiura), occuring sympatrically in Finnish lakes. Although these parasites are considered to be of low specificity, we found that the larger of the 2 species. A. coregoni developed a pronounced preference for salmonid hosts at the beginning of maturation (defined by the presence of copulating specimens). Argulus foliaceus infects a much wider range of fish hosts. We showed that specialization of A. caregoni on salmonids does not necessarily result from incompatibility with other fishes, but could instead reflect higher sensitivity of oxygen depletion compared with A. foliaceus. Adult A. Coregoni may meet these demands by attaching to samonids, the typical inhabitants of well-aerated waters. Young parasites of both species showed little host specificity and attached mainly to fishes with higher body reflectivity. In host choice experimnets, A. coregoni of 4-5 mm length preferred salmonids (rainbow trout) to cyprinids (roach) irrespective of the type of fish host, on which it had been previously grown in the laboratory. We suggest that such an innate ontogenetic shift in host preference maintain the major part of the parasite population on its principal host, ensuring successful reproduction within suitable habitats.