Phenotypic Diversity and Host Variability of Proteocephalus longicollis (Zeder 1800) (Cestoda, Proteocephalidae), a Parasite of Salmoniformis Fishes of Siberia

Intraspecific variability of the cestode P. longicollis was studied in three host species: the Nelma (Stenodus nelma), the Lenok (Brachymistax lenok), and the Malma (Salvelinus malma) of the fish order Salmoniformes. Host forms of helminths were found to be similar in the intrapopulation diversity and in the proportion of rare forms evaluated by polymorphous features of the scolex shape and sexually mature segments. Host differences in morphophysiological traits such as the attachment, trophics, and reproduction have been revealed. The parasites from Nelma are shown to be the most diverse phenotypically. These forms show the maximum heterogeneity of diversity and distribution frequencies. The conclusion is drawn that the helminths respond to changes in the habitat conditions in the host predominantly by plastic traits.