Invited review Diversity in the Monogenea and Digenea: does lifestyle matter? Thomas H. Cribb a, * , Leslie A. Chisholm a , Rodney A. Bray b a Department of Microbiology and Parasitology, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Qld 4072, Australia b Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Received 6 June 2001; received in revised form 3 October 2001; accepted 12 October 2001 Abstract 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. q 2002 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Monogenea; Digenea; Diversity; Host specificity; Fish; Parasite 1. Introduction The platyhelminth parasites of fishes comprise the Trema- toda (Digenea and Aspidogastrea), the Monogenea and the Cestoda (tapeworms and allies). The site and host distribu- tion of these parasite groups is relatively neat and non-over- lapping. Cestodes are the dominant endohelminths of elasmobranchs and have only a few significant radiations in bony fishes (Caryophyllidea, Proteocephalidea and Pseudo- phyllidea). Infections in bony fishes are generally dominated by monogeneans and trematodes. The Monogenea are exter- nal parasites on all groups of fishes but have by far their greatest diversity on bony fishes; only eight families are reported regularly from chondrichthyans (Acanthocotylidae, Amphibdellatidae, Capsalidae, Chimaericolidae, Hexabo- thriidae, Loimoidae, Microbothriidae and Monocotylidae). Similarly, the Digenea (the overwhelmingly biggest trema- tode group), which are essentially internal parasites, have their greatest diversity in the teleosts. In a striking parallel to the Monogenea only five families of digeneans (Azygii- dae, Gorgoderidae, Ptychogonimidae, Sanguinicolidae and Syncoeliidae) are regular parasites of chondrichthyans. Thus, the dominant external platyhelminth parasites of bony fishes are monogeneans and their dominant internal parasites are digeneans. Aspidogastreans, the smaller sister group of the Digenea, occur in both chondrichthyans and bony fishes. From their first recognition (van Beneden, 1858) until the second half of the 20th century, the Aspidogastrea, Digenea and Monogenea were generally considered together as ‘trematodes’. The Monogenea was raised to class level by Bychowsky (1937) although this view took time to gain popularity; Yamaguti (1963) considered monogeneans to comprise no more than an order and as late as 1970, Dubois (1970) argued that they should be considered a subclass of the Trematoda. The group is now universally considered a class in its own right with affinities to cestodes rather than trematodes. As is frequently the case in paradigm shifts such as this, in retrospect the distinctions seem obvious. Recog- International Journal for Parasitology 32 (2002) 321–328 0020-7519/02/$20.00 q 2002 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S0020-7519(01)00333-2 www.parasitology-online.com * Corresponding author. Tel.: 161-7-3365-2581; fax: 161-7-3365-4620. E-mail address: t.cribb@mailbox.uq.edu.au (T.H. Cribb).