Aspects of the phytogeny of the Trematoda Rudolphi, 1808 (Platyhelminthes: Cercomeria)

Abstract

Systematic analyses of various familial- and suprafamilial -level groups of trematodes are evaluated using phylogenetic systematic methods, and the results are compared with previous phylogenetic systematic studies of the major trematode groups. Within the Aspidobothrea, the Aspidogastridae and Multicalycidae are sister-groups, the Stichocotylidae is the sister-group of those two, and the Rugogastridae is the sister-group of those three, based on a suite of 15 morphological characters with a consistency index of 84.6%. No autapomorphy is yet known for the Multicalycidae. Characters newly considered within the digeneans produced the following modifications in the phylogenetic systematic analysis presented by D. R. Brooks, R. T. O'Grady, and D. R. Glen (1985. Can. J. Zool. 63: 411–443): (i) the fellodistomes are a paraphyletic assemblage within the superfamily Gymnophalloidea, order Strigeiformes: (ii) the Campulidae is transferred to the suborder Acanthocolpata, order Plagiorchiformes, from the superfamily Fascioloidea, order Echinostomatiformes; (iii) the suborder Troglotrematata is the sister-group of the Renicolata plus the Plagiorchiata; (iv) the Plagiorchiata is divided into two infra-suborders: the possibly paraphyletic Plagiorchiatea plus the Opecoelatea, including the superfamilies Opecoeloidea and Microphalloidea; (v) the superfamily Telorchioidea includes the Ochetosomatidae and Telorchiidae but not the Urotrematidae. Characters used by authors in critiques of other portions of the phylogenetic tree support rather than conflict with the tree when analyzed phylogenetically. Aspidobothreans appear to have been associated primitively with chondrichthyans, with host-switching to teleosts and turtles in the Aspidogastridae. Digeneans appear to have been primitively associated with the sister-group of chondrichthyans, the stem group leading to all the rest of the gnathostomous vertebrates. The plesiomorphic life cycle pattern for trematodes appears to be the mollusc–vertebrate two-host cycle. Because complex life cycle ecology evolved before the complex developmental programs in digeneans the evolution of digeneans occurred under conditions of conservative ecology and innovative ontogeny.