Origination of a separate form of Neoechinorhynchus salmonis Ching, 1984 (Acanthocephales: Neoechinorhynchidae) in severe environment of the Asian Arctic

摘要

Previous morphological research has shown that the species Neoechinorhynchus salmonis, first described from Canada, is also widespread in the northern Asia, in different climatic zones. Analysis of data from samples collected in various parts of its area of distribution leads to a conclusion about the existing two life forms of this species, namely temperate and polar ones. The specimens of the latter at the adult stage develop at far greater rate and reach maturity being smaller in size. It results in morphological difference: in the temperate form, the maximal body length of males is 7.8 mm; of females - 15.0 mm, while in the polar form corresponding values are 4.5 and 7.8 mm (irrespective of host species). In the temperate form, in the course of maturing and growth of the adult worms, no seasonality was found which means a continuous replenishment of infection in the definitive hosts throughout a year. In the polar form, seasonal behavior of the intermediate hosts, the ostracods Cypria kolymensis, secures a heavy infection of the definitive hosts at the very beginning of the warm period (late June). In late August, the majority of parasites reach maturity and there are solid grounds to believe that adult worms are eliminated during the nearing wintering. In our opinion, the traits of the polar form are an adaptation to severe climatic conditions.