Diversity in the Monogenea and Digenea: does lifestyle matter?

If the cestodes are excluded, then the parasitic platyhelminths of fishes divide neatly into the external and monoxenous Monogenea and the internal and heteroxenous Digenea. Both groups have apparently had long associations of coevolution, host switching and adaptation with fishes and have become highly successful in their respective habitats. Current estimates of species richness for the two groups suggest that they may be remarkably similar. Here we consider the nature of the diversity of the Monogenea and Digenea of fishes in terms of richness of species and higher taxa to determine what processes may be responsible for observed differences. The Monogenea includes at least two super-genera (Dactylogyrus and Gyrodactylus) each of which has hundreds of species; no comparable genera are found in the Digenea. Possible reasons for this difference include the higher host specificity of monogeneans and their shorter generation time. If allowance is made for the vagaries of taxonomic 'lumping' and 'splitting', then there are probably comparable numbers of families of monogeneans and digeneans in fishes. However, the nature of the families differ profoundly. Richness in higher taxa (families) in the Digenea is explicable in terms of processes that appear to have been unimportant in the Monogenea. Readily identifiable sources of diversity in the Digenea are: recolonisation of fishes by taxa that arose in association with tetrapods; adoption of new sites within hosts; adoption of new diets and feeding mechanisms; adaptations relating to the exploitation of ecologically similar groups of fishes and second intermediate hosts; and adaptations relating to the exploitation of phylogenetic lineages of molluscs. In contrast, most higher- level monogenean diversity (other than that associated with the subclasses) relates principally to morphological specialisation for attachment by the haptor.