ON THE EVOLUTION WITHIN THE FAMILY GYRODACTYLIDAE (MONOGENEA)

On the basis of six main types of protonephridial systems in the genus Gyrodactylus and results from further studies of the protonephridial systems in other gyrodactylid genera and other monogenean families, ev olutionary lines within the Gyrodactylidae and the origin of the gener a Ooegyrodactylus, Swingleus, and Polyclithrum are discussed. An excre tory bladder (on both sides of the body) with separate openings for th e main canals of the anterior and the posterior systems, a median junc tion between the two anterior systems and likewise a junction between the two posterior systems, many flame bulbs and an absence of lateral flames in the main canals were found to be primitive monogenean charac ters. A system similar to that in the acanthocotylid larva but with se condarily evolved lateral flames in the posterior systems may have giv en rise to a first protogyrodactylid system, in turn giving rise to th e system of Macrogyrodcatylus. In a second protogyrodactylid line, the main canal of the anterior system (on both sides of the body) was con nected to that of the posterior system and lateral flames were develop ed also in the anterior system. From this system all other described g yrodactylid systems may arise: one main line gave rise to the system o f the G. (Gyrodactylus) and another to a system of G. (Mesonephrothus) type. The systems of Swingleus and Gyradicotylus are of the latter ty pe. Through loss of all lateral flames, the system of G. (Metanephrotu s) may have evolved. Isancistrum has a system of the G. (Metanephrotus ) type. From this type of system, those of Polyclithrum, G. (Neonephro tus), G. (Paianephiotus) and G. (Limnonephrotus) can be derived. The f act that the system of Ooegyrodactylus is more closely related to that of G. (Gyrodactylus) than to that of Macrogyrodactylus prompts discus sion regarding the evolution of the viviparity of the family Gyrodacty lidae. According to the evolutionary system presented, primitive gyrod actylids parasitise primitive fish species and the most advanced gyrod actylids parasitise the most advanced fish species. However, Anguillid ae and Clupeidae may have been secondarily infected by members of G. ( Metanephrotus) and G. (Neonephrotus) respectively, and Cyprinidae, Sal monidae and Esocidae may similarly have been secondarily infected by m embers of G. (Limnonephrotus). Isancistrum may have infected squids (L oligidae) at a medium level in the evolution within the Gyrodactylidae , while progenitors of Gyrdicotylus may have been adapted to clawed to ads (Xenopidae) earlier in evolution. (C) 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd. All rights reserved.