We describe an unusually high infection rate of Gyrodactylus salaris Malmberg in juvenile Atlantic salmon Salmo salar L. of Baltic Sea origin, which are generally believed to be more resistant to G. salaris than East Atlantic salmon populations. Based on analyses of mitochondrial (complete cytochrome oxidase 1 [CO1] gene, 1548 bp) and nuclear (ADNAM1, 435 bp; internal transcribed spacer [ITS] rDNA region, 1232 bp) DNA fragments, the closest relatives of the characterized Estonian G. salaris strain were parasites found off the Swedish west coast and in Raasakka hatchery, Iijoki (Baltic Sea, Finland). Analyses of 14 microsatellite loci of the host S. salar revealed that approximately 40% of studied fish were triploids. We subsequently identified triploid Atlantic salmon of Baltic origin as more susceptible to G. salaris infection than their diploid counterparts, possibly due to compromised complement-dependent immune pathways in triploid salmon. This is in accordance with earlier studies that have shown elevated susceptibility of triploids to various viral or bacterial pathogens, and represents one of the first reports of increased susceptibility of triploid salmonid fish to an ectoparasite. However, further experimental work is needed to determine whether triploid Atlantic salmon is generally more susceptible to G. salaris compared to their diploid counterparts, irrespective of the particular triploidization method and population of origin. KEY WORDS: Atlantic salmon · Gyrodactylus salaris · Pathogen susceptibility · Triploid · Microsatellites · Baltic Sea Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [129][130][131][132][133][134][135][136] 2010 tect significant differences in susceptibility between diploid and triploid fish. For example, both diploid and triploid rainbow trout exposed to Vibrio ordalii, Aeromonas salmonicida, or infectious hematopoietic necrosis virus demonstrated similar mortality rates (Yamamoto & Iida 1995). Similar mortality rates between diploid and triploid Atlantic salmon were also observed after experimental infection with Renibacterium salmoninarum (Bruno & Johnstone 1990).Gyrodactylosis is a parasitic disease of salmonid fishes caused by the viviparous ectoparasite monogenean flatworm Gyrodactylus salaris, which belongs to the G. wageneri species-group, of the subgenus Limnonephrotus (family Gyrodactylidae, Malmberg, 1957). Due to 'hyperviviparity' (also known as a 'Russian doll' style of reproduction), combined with a rapid generation time, it reproduces fast, and in a matter of weeks a single worm can produce thousands of progeny , Buchmann 2008. G. salaris damages fish not only by consuming mucus and epithelial cells of the host but also by piercing the fish epithelium with its hooklets, compromising its osmoregulatory function, and leaving it vulnerable to fungal and bacterial infections. Importantly, Atlantic salmon populations exhibit marked differences in susceptibility to G. salaris infection, with populations fr...

Päkk P., Hussar P., Paaver T. 2011. Alterations of club cell activity in epidermis of common carp, Cyprinus carpio (Actinopterygii: Cypriniformes: Cyprinidae), due to infection by Ichthyophthirius multifiliis (Protista: Ciliophora). Acta Ichthyol. Piscat. 41 (3): 185-192.Background. The abundance of club cells in epidermal tissue of fishes in the superorder Ostariophysi is a poorly understood phenomenon. Previous results have suggested that epidermal club cells have a generic role in response to injury and that they display intense phagocytotic activity, having an anti-parasitic function in the host. Earlier works suggested that club cells are usually located in the middle of the epidermis and that they do not communicate with the epidermal surface or do it only when the epidermis has been ruptured by predation. The presently reported study focused on the alterations of club cell activity in carp epidermis induced by ectoparasite, Ichthyophthirius multifiliis. We hoped that our observations would help to understand the function(s) of these cells. Materials and methods. This study was based on 200 four-month old common carp, Cyprinus carpio L., with mean body weight of 65 ± 5 g. The fish were experimentally infected with theronts of Ichthyophthirius multifiliis. In sequential days post infection, samples of fins and body skin were collected for histological and histochemical examination. The correlation between club cell densities and mucous cell densities was analysed using Pearson correlation analyses. Results. A local reduction of mucous cells occurred after theront invasion-induced proliferation, and increased club cell density around the parasite during the growth of trophonts. After parasites left the skin due to salt-water treatment, a decrease in the number of club cells was detected. During reinvasion the decrease in parasite activity in areas of club cells proliferation was not noted. It was found that giant mature club cells were opened on the surface. Conclusion. Club cells have no anti-parasitic function against I. multifiliis and these mature cells released their viscous secretion into water. The high density of club cells in the epidermis compensates an overall low density or absence of mucous cells. As it can be hardly concluded that the function of club cells is phagocytic removal of cell debris, an integrated research on mucosal immune mechanisms, as well as studies on epidermal tissue responses on product(s) released by club cells ("alarm substance cells") should be carried out in the future.