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, evolutionary lines within the Gyrodactylidae and the origin of the genera Ooegyrodactylus, Swingleus, Gyrdicotylus and Polyclithrum are discussed. An excretory bladder (on both sides of the body) with separate openings for the main canals of the anterior and the posterior systems, a median junction 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 characters. A system similar to that in the acanthocotylid larva but with secondarily evolved lateral flames in the posterior systems may have given rise to a first protogyrodactylid system, in turn giving rise to the system of Macrogyrodactylus. In a second protogyrodactylid line, the main canal of the anterior system (on both sides of the body) was connected to that of the posterior system and lateral flames were developed also in the anterior system. From this system all other described gyrodactylid systems may arise: one main line gave rise to the system of the G. (Gyrodactylus) and another to a system of G. (Mesonephrothus) type. The systems of Swingleus and Gyrdicotylus are of the latter type. Through loss of all lateral flames, the system of G. (Metanephrotus) may have evolved. Isancistrum has a system of the G. (Metanephrotus) type. From this type of system, those of Polyclithrum, G. (Neonephrotus), G. (Paranephrotus) and G. (Limnonephrotus) can be derived. The fact that the system of Ooegyrodactylus is more closely related to that of G. (Gyrodactylus) than to that of Macrogyrodactylus prompts discussion regarding the evolution of the viviparity of the family Gyrodactylidae. According to the evolutionary system presented, primitive gyrodactylids parasitise primitive fish species and the most advanced gyrodactylids parasitise the most advanced fish species. However, Anguillidae and Clupeidae may have been secondarily infected by members of G. (Metanephrotus) and G. (Neonephrotus) respectively, and Cyprinidae, Salmonidae and Esocidae may similarly have been secondarily infected by members of G. (Limnonephrotus). Isancistrum may have infected squids (Loligidae) at a medium level in the evolution within the Gyrodactylidae, while progenitors of Gyrdicotylus may have been adapted to clawed toads (Xenopidae) earlier in evolution.