Group-living and the richness of the parasite fauna in Canadian freshwater fishes.

An increased transmission of ectoparasites among individual animals is considered to be an inevitable cost of living in groups, since several kinds of ectoparasites require close proximity between large numbers of hosts for successful transmission. However, we do not know whether individuals belonging to group-living species incur a greater risk of ectoparasitism than individuals of solitary species. Here, using published data from 3 families (60 species) of Canadian freshwater fishes, I test the hypothesis that group-living species are host to more species of "contagious ectoparasites" (copepods and monogeneans) than solitary host species. As the different fish species have been studied with varying intensity, I used the mean number of parasite species recorded per study as a standard measure of parasite numbers. Multiple regression analyses were performed separately for each family to determine the effects of group-living and of 3 other variables (host size, age, and range) on the richness of the recorded parasite fauna. Once the effects of the other variables were removed, I found no significant effect of sociality on the richness of the parasite fauna per fish species, for contagious ectoparasites and other types of parasites. Neither of the other variables had any influence on the numbers of parasite species per fish species. These results suggest that a richer ectoparasite fauna is not a cost of group-living in fishes.