New approaches to the systematics of the ancyrocephalid Monogenea from Nearctic freshwater fishes.

A total of 133 ancyrocephalid species found on Nearctic freshwater fishes have been named. Of these, 103 are now considered valid; 14 and 16 have been declared previously as synonyms or species inquirendae, respectively, and Urocleidus rarus Mizelle, 1940, is designated as a species inquirenda in the present work. A cladistic analysis, based on 13 binary and 4 multistate characters and 32 apomorphic character states was carried out. Generalized forms of Entobdella, Gyrodactylus, and Tetraonchus were used to generate a hypothetical outgroup taxon. Thirteen terminal taxa emerged. Of these, 3 (the articulating bar group, Ligictaluridus, and Onchocleidus sensu Wheeler and Beverley-Burton, 1989) have been recognized previously as monophyletic; 3 (Aethycteron, Lyrodiscus, and Salsuginus) were shown to be monophyletic in the present study; 3 (Leptocleidus, Macrohaptor, and Tetracleidus) are monotypic genera; and 4 were identified as unresolved assemblages that contain a number of species considered to be incertae sedis as well as those assigned to the following genera: Aristocleidus Mueller, 1936, Urocleidus sensu Suriano and Beverley-Burton, 1981, Cleidodiscoides Mayes and Miller, 1973, and Cleidodiscus sensu Beverley-Burton and Suriano, 1980. The resultant cladogram was 40 steps long with a consistency index of 0.80. The monophyly of the ingroup is supported by 5 synapomorphies. Comparison and mapping of the parasite cladogram with/onto a family-level cladogram of hosts indicated that the present data provided no unequivocal evidence of cospeciation. Some possible origins for the Nearctic ancyrocephalids are discussed, and the need for further taxonomic studies on the Holarctic ancyrocephalids to explore the possible sister-group relationship between the Nearctic and Eurasian faunas is emphasized.