HOST SPECIFICITY AND DISPERSAL STRATEGY IN GYRODACTYLID MONOGENEANS, WITH PARTICULAR REFERENCE TO GYRODACTYLUS-SALARIS (PLATYHELMINTHES, MONOGENEA)

Gyrodactylus salaris Malmberg, 1957 is an important pathogen in Norwegian populations of Atlantic salmon Salmo salar. It can infect a wide range of salmonid host species, but on most the infections are probably ultimately limited by a host response. Generally, on Norwegian salmon stocks, infections grow unchecked until the host dies. On a Baltic salmon stock, originally from the Neva River, a host reaction is mounted, limiting parasite population growth on those fishes initially susceptible. Among rainbow trouts Oncorhynchus mykiss from the same stock and among full sib anadromous arctic char Salvelinus alpinus, both naturally resistant and susceptible individuals later mounting a host response can be observed. This is in contrast to an anadromous stock of brown trout Salmo trutta where only innately resistant individuals were found. A general feature of salmonid infections is the considerable variation of susceptibility between individual fish of the same stock, which appears genetic in origin. The parasite seems to be generally unable to reproduce on non-salmonids, and on cyprinids, individual behavioural mechanisms of the parasite may prevent infection. Transmission occurs directly through host contact, and by detached gyrodactylids and also from dead fishes. Relative importance of these routes and of different host species in the epidemiology of the disease is discussed with reference to laboratory experiments and existing knowledge concerning the host-parasite ecology.