Morphometric correlates of host specificity in Dactylogyrus species (Monogenea) parasites of European Cyprinid fish

We test the hypothesis that living on larger fish may impose constraints, i .e. the need to develop large attachment organs, related to the necessity t o remain attached on large gills. For this, we compiled data on body size a nd morphometric measurements of attachment organs of 44 Dactylogyrus specie s (ectoparasites with direct life-cycle) from 19 cyprinid species. Nineteen dactylogyrid species were considered as specialists (infecting only 1 host species) and 25 as generalists (infecting more than 1 species). The lack o f phylogenetic information lead us to perform comparative analyses using ra w values and independent contrasts obtained by random phylogenies. Our resu lts show that rich parasite communities are formed by specialists and gener alists whereas poor communities are composed mainly of generalist parasites . Moreover, specialists are found on larger hosts, which may reflect a spec ialization on a predictable resource, as larger fish live longer and offer large gills for parasite colonization. Parasite specialization is shown to be linked with adaptation of attachment organs to their fish hosts. Two mor phometric variables of the attachment organ, the total length of anchor and length of base of anchor, were positively correlated with host length for specialists.