HLA-C and infection: The presence of a higher number of MHC allelic variants in C. nasus specimens compared to the number of MHC allelic variants in P. toxostoma specimens could result from co-evolutionary interactions between host and parasite genotypes and should explain the presence and high abundance of some monogenean parasite species in C. nasus and the low intensity of infection (or absence of some monogenean species) in P. toxostoma.