The nature and evolution of the association among digeneans, molluscs and fishes

Patterns of association of digenean families and their mollusc and vertebra te hosts are assessed by way of a new database containing information on ov er 1000 species of digeneans for lift-cycles and over 5000 species from fis hes. Analysis of the distribution of digenean families in molluscs suggests that the group was associated primitively with gastropods and that infecti on of polychaetes, bivalves and scaphopods are all the results of host-swit ching. For the vertebrates. infections of agnathans and chondrichthyans are apparently the result of host-switching from teleosts. For digenean famili es the ratio of orders of fishes infected to superfamilies of molluscs infe cted ranges from 0.5 (Mesometridae) to 16 (Bivesiculidae) and has a mean of 5.6. Individual patterns of host association of 13 dipenean families and s uperfamilies are reviewed. Two, Bucephalidae and Sanguinicolidae. are excep tional in infecting a range of first intermediate hosts qualitatively as br oad as their range of definitive hosts. No well-studied taxon shows narrowe r association with vertebrate than with mollusc clades. The range of defini tive hosts of digeneans is characteristically defined by eco-physiological similarity rather than phylogenetic relationship. The range of associations of digenean families with mollusc taxa is generally much narrower. These d ata are considered in the light of ideas about the significance of differen t forms of host association. If Manter's Second Rule (the longer the associ ation with a host group, the mure pronounced the specificity exhibited by t he parasite group) is invoked, then the data may suggest that the Digenea f irst parasitised molluscs before adopting vertebrate hosts. This interpreta tion is consistent with most previous ideas about the evolution of the Dige nea but contrary to current interpretations based on the monophyly of the N eodermata. The basis of Manter's Second Rule is. however, considered too fl imsy for this interpretation to be robust. Problems of the inference of the evolution of patterns of parasitism in the Neodermata al-e discussed and c onsidered so intractable that the truth may be presently unknowable. (C) 20 01 Australian Society for Parasitology Inc. Published by Elsevier Science L td. All rights reserved.