No abstract

1. This article compares generalist (parasite species found on two or more host species) and specialist (found on only one host species) monogenean parasite species of fish. The reduction of the host range – that is an increase in host specificity – may correspond with a better adaptation of the parasite to a more predictable host environment. A more predictable environment may allow the parasite species to develop specific adaptations. 2. We assume that the more predictable host environment can be evaluated by host body size, since numerous life‐traits, such as longevity, are positively correlated with size. 3. We found that specialist parasites parasitize larger hosts species than generalist parasites. We also found a good relationship between host body size and parasite body size for specialist parasite species. 4. An adaptation to the mechanical problems encountered in the host's gill chamber may lead to an increase in parasite body size. The infection of a larger part of the host population in order to decrease the chances of local extinction due to fluctuations of host abundance may be another adaptive mechanism. 5. We found a negative correlation between parasite body size and prevalence for generalist parasite species. This relationship disappeared when using the comparative method controlling for phylogeny, which proved that it was a phylogenetic effect.