[Polymorphism and intraspecific variability in the cestode Proteocephalus tetrastomus (Rudolphi, 1810) (Cestoda: Proteocephalidae), a specific parasite of smelt (Osmeridae)].

Morphological variability in the cestode Proteocephalus tetrastomus parasitizing smelt in Lake Ladoga (Baltic Sea basin) and Lake Goluboye (Pechora River basin) was studied. Two forms of P. tetrastomus differing in the proglottid structure, and two variations of the ovary were discriminated. Slightly craspedote individuals with a dumbbell-like ovary were included in the first group. The second group includes distinctly craspedote individuals. It is heterogeneous by the ovary shape and comprises cestodes with two variants of the latter, dumbbell-like and triangular. Both forms are present in the Lake Ladoga. The slightly craspedote form dominates, whereas the distinctly craspedote form is rare. In Lake Goluboye, P. tetrastomus is represented by the only form, distinctly craspedote with a triangular ovary. Differences in the morphometric parameters of the forms, as well as geographic and habitat distinctions, were determined. As compared to other studied cestode species of the genus Proteocephalus (P. torulosus, P. longicollis, P. percae), the smelt parasite P. tetrastomus has fewer polymorphic traits and their variations, and a much more restricted range of variability in the measurable, counting and relative characters. Analysis of literature (Willemse, 1969; Scholz and Hanzelova, 1998; Scholz et al., 2004) and original data suggest that the species is host-specific and relatively stable. It can be concluded, that P. tetrastomus is a polymorphic, complex-structured species.