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 lim~ted 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, limltlng parasite population growth on those fishes initially susceptible. Among rainbow trouts Oncorhynchus mykiss from the sam.e stock and among full sib anadromous arctic char Salvelinus alpjnus, 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.

Gyrodactylus turnbulli Harris, 1986 infected the caudal peduncle (42% of all flukes) and caudal fin (40%), of guppies (Poecilia reticulata Peters, 1859), occurring less frequently on the pectoral (8%), dorsal (5%), pelvic (2%), and anal (1%) fins. This distribution of parasites on the fish and their crowding varied with infection age. On newly infected guppies, G. turnbulli occurred less frequently on the peduncle, and mean nearest-neighbour distance was not correlated with population size. Over the first 4 days of the infection the proportion of parasites on the peduncle increased to 60%, and mean nearest-neighbour distance became correlated with population size. In declining infections (day 6 onwards) the proportion on the peduncle declined, and mean nearest-neighbour distance increased independent of population size. This pattern suggests random colonization of the fins followed by migration to the peduncle, from which dispersal occurred during the declining phase of the infection.

Species of the viviparous monogenean genus Gyrodactylus are difficult to identify morphologically and relationships within the genus are unclear. Partial or complete sequences were obtained for the internal transcribed spacers (ITS-1 and ITS-2) and 5.8S ribosomal DNA (rDNA) of 11 Gyrodactylus species (G. kobayashii, G. gurleyi, G. pungitii, G. rogatensis, G. gasterostei, G. derjavini, G. salaris, G. rarus, G. turnbulli, G. bullatarudis, and G. arcuatus) and the related Gyrdicotylus gallieni. Phylogenetic analyses (parsimony, distance, and maximum likelihood) separated species of the Gyrodactylus wageneri species-group (with a long ITS-1) from those of the Gyrodactylus eucaliae and G. arcuatus species-groups (with a short ITS-1). This subdivision into species with a long (610-630 base pairs (bp)) or short (300-500 bp) ITS-1 was also apparent with 5.8S rDNA and ITS-2 data. No support could be found for the hypothesis that G. salaris is a member of a separate species group, as this species clustered within the G. wageneri group. The morphologically homogeneous G. wageneri group was subdivided by our molecular data into five species that infect cottids, gasterosteids, and salmonids and two species that infect cyprinid fishes. Overall, the ITS-2, combined with the 5.8S rDNA (an alignment of 668 bp), appeared to be the most informative indicator of phylogenetic relationships within the genus Gyrodactylus. The potential importance of this technique for future gyrodactylid systematics is discussed.

A method is presented for the isolation and analysis of hamuli, marginal hooks, and bars from individual gyrodactylid monogeneans using scanning electron microscopy (SEM), while simultaneously processing parasites for rDNA analysis using the polymerase chain reaction (PCR). The haptors of ethanol-fixed gyrodactylids were protease digested to liberate hooks for SEM, whereas DNA extracted from the bodies was used for PCR. The method resulted in hooks and hamuli being prepared from more than 90% of Gyrodactylus turnbulli individuals, a significant improvement on previously published digestion-based SEM techniques. PCR on the same parasites was less successful, but sequence data were obtained from 50% of individuals. Amplification of rDNA internal-transcribed spacer regions from individual worms used for SEM gave PCR products consistent with those predicted from our previous sequence analysis. This method allows the correlation of morphology and DNA sequence from the same individual and can be applied to ethanol-fixed material, such as field collected and museum specimens.