Dispersal of invading trematodes Nicolla skrjabini (Iwanitzky, 1928) and Plagioporus skrjabini Kowal, 1951 (Trematoda: Opecoelidae) in the Volga

Abstract

The species compositions of fish trematodes in the neighboring Volga and Don basins are similar in many ways, however, there are some differences. One difference is that trematodes of the genera Nicolla and Plagioporus were not previously recorded in the Volga basin. Their absence was accounted for by the fact that their initial hosts, snails of the genera Lithoglyphus (Gastropoda: Lithoglyphidae) and Theodoxus (Gastropoda: Neritidae), did not occur in the Volga until recently. At present, these snails, trematodes, and some other parasites have successfully naturalized in the Volga and actively expand their ranges in its basin (Zhokhov and Pugacheva, 2001). Here, we present new data on the dispersal of these invaders in the Volga basin. The distribution of the trematode Nicolla skrjabini Iwanitzki, 1928 is limited to the rivers of the Azov‐ Black Sea and Baltic basins. That of the trematode Plagioporus skrjabini Kowal, 1951 is confined to rivers of the Azov‐Black Sea basin alone. The life cycles of these parasites are known. For N. skrjabini , the first intermediate host is the snail Lithoglyphus natocoides Pfeiffer, and the second intermediate host (according to experimental data) is Gammarus balcanicus Schaferna (Sten’ko, 1976). This trematode has a wide range of definitive hosts that comprises 27 fish species. In P. skrjabini , the snail Theodoxus fluviatilis and six gammarid species are involved in the life cycle (Chernogorenko et al., 1978). The definitive hosts of P. skrjabini are Cobitis taenia L., Neogobius fluviatilis Pall., and Mesogobius batrachocephalus Pall. In 1952, the Volga‐Don connected the two river basins, and the snails with associated trematodes found an opportunity to invade the Volga. The snails of the genus Lithoglyphus appeared in the lower Volga in the late 1960s (Belyavslaya and Vyushkova, 1971). In 1971, they were already numerous in the Volga delta (Pirogov, 1972). At present, these snails occur almost all over the Volga delta and form very dense aggregations (up to 11 180 ind./m 2 ) in the upper part of the delta and in arms of its western part (Biserova, 1990, 1996;