Journal of Helminthology (1998) 72, 101-107 101 Some histochemical characteristics of the mucous microenvironment in four salmonids with different susceptibilities to gyrodactylid infections K. Buchmann* Section of Fish Diseases, Department of Veterinary Microbiology, Royal Veterinary and Agricultural University, 13 Biilowsvej, DK-1870 Frederiksberg C, Denmark Abstract Skin mucous cells and mucus from four salmonids (rainbow trout, brown trout, Conon salmon, Iijoki salmon) with different susceptibilities to infection with Gyrodactylus derjavini and G. salaris were partly characterized by cytochemistry, immunoblotting and immunocytochemistry. Mucous cell den- sities in various fin types were partly correlated with resistance to infection with G. derjavini but not with G. salaris. Lectin binding assays indicated slight differences in carbohydrate composition of mucus from the four salmonids but serum antigens specific for salmonids were found in mucus from all of them. Antisera against salmon immunoglobulin and rainbow trout complement factor C3 reacted with mucus from all of the salmonids but not with mucus from phylogenetically unrelated fish. Antisera raised against ACTH reacted with mucus components from rainbow trout, brown trout and Conon salmon but not with mucus from Iijoki salmon. These findings are discussed in relation to the host specificity of ectoparasites and susceptibility to infection. Introduction Closely related salmonids exhibit remarkable differ- ences in susceptibility to infection with regard to the ectoparasitic monogeneans Gyrodactylus salaris and G. derjavini. Thus, the Norwegian and the Scottish Conon and Shinn strains of Salmo salar are extremely susceptible to infection with G. salaris (Bakke et ah, 1990; Bakke & MacKenzie, 1993), whereas the Baltic Neva and Iijoki strains of the same host species are relatively resistant to this parasite species (Bakke et al., 1990; Rintamaki- Kinnunen & Valtonen, 1996). In addition, rainbow trout and brown trout differ significantly in their vulnerability to this parasite (Bakke et al, 1991; Jansen & Bakke, 1995). Likewise, rainbow trout, brown trout, Conon salmon and Iijoki salmon differ in their susceptibility to the congener G. derjavini (Buchmann & Uldal, 1997). Corresponding •Fax: 45 35 28 27 57 E-mail: vetmi@kvl.dk host specificities are well known in monogenean biology and have been treated in the literature for decades (Bychowsky, 1957; Malmberg, 1970; Kearn, 1976, 1994; Bakke, 1991). However, the causative mechanisms are nonetheless still unknown. The microhabitats of skin monogeneans on salmonids are dominated by mucous cells (Pickering, 1974; Wells & Cone, 1990; Buchmann & Bresciani, 1998; Buchmann & Uldal, 1997). Indeed, due to the high density in the salmonid skin of these specialized cells, several hundreds of them are in the reach of a single attached gyrodactylid. Thus, the content of these cells are in intimate contact with the parasite and are likely to play a role in host specificity. This was indicated by Nigrelli (1937) by the finding that Benedenia (Epibdella) melleni exhibited different survival in mucus from various host species with different susceptibilities. However, host specificity may involve several factors. First, the attrac- tion of the parasite to the host by more or less specific substances is indispensable if host-parasite contact should not merely be based on chance meetings. In