[Microevolutionary aspects of morphological variability and specificity of cestodes by the example of Proteocephalus longicollis (Zeder, 1800) (Proteocephalidae), a parasite of Coregonids].

Morphological variability of the cestode Proteocephalus longicollis from the smelt Osmerus eperlanus L., which was spontaneously introduced into the Syamozero Lake, has been studied. It was established that P. longicollis became an abundant element of the smelt parasite fauna during last 20 years, thereby showing the absence of host specificity. It was revealed that morphological polymorphism of P. longicollis from smelt combines features of ecological forms characteristic for aboriginal host species, namely whitefish Coregonus lavaretus L. and vendace Coregonus albula L. Cestodes from smelt are more similar to ecological forms from whitefish by the characteristics of variability. It is concluded that colonization of a host species, which is new for a given water body, is accompanied by the forming of a new group of host, which contributes to the maintenance of the parasite number and to its population stability under changing conditions of freshwater ecosystems.