History of formation and peculiarities of Ponto-Caspian fish myxosporean fauna

History of formation of Ponto-Caspian basin fish myxosporean fauna is examined. This work is based on our own material on myxosporean parasites of fish from the Black Sea (collected in 1987–2018) and the Sea of Azov (1997–2016). Totally, we have investigated 15 368 specimens of 87 species of fish (14 297 specimens of 80 species of fish in the Black Sea and 1071 specimens of 19 species of fish in the Sea of Azov). The material was collected by the method of incomplete parasitological dissections and treated by generally accepted methods. Also, all available literary sources on the myxosporean parasites of fish from the Black Sea, the Sea of Azov, and the Caspian Sea are analyzed (references list contains the most significant publications). A comparative analysis of fish myxosporean fauna in the Black Sea, the Sea of Azov, and the Caspian Sea is performed. It is stated that 108 parasite species are known in the Black Sea, 42 – in the Sea of Azov, and 68 – in the Caspian Sea. Number of myxosporean parasite species common for the Sea of Azov and the Black Sea is 32, for the Caspian and Black seas – 32, and for the Sea of Azov and the Caspian Sea – 20. Totally 16 species of myxosporean parasites are registered in all the mentioned seas. To date, of 108 myxosporean species of Black Sea region, 29 are registered only in freshwater fish in the estuaries with considerable brackish water. For 79 myxosporean species, marine fish species serve as hosts; they are registered mainly in full-salt sea part, and 17 of them are of freshwater origin: 7 species are registered in freshwater fish and in euryhaline mullets; 1 parasitizes on freshwater and marine salmon fishes; the only hosts for 9 species are mullets or other marine fish species. Marine forms are represented by Pontic (22 species), Ponto-Azov (3), and Ponto-Caspian (2) endemics, as well as by Mediterranean invaders (35). Among freshwater myxosporeans, only 1 species is Black Sea endemic; 1 species is Ponto-Azov endemic, and most other species are widely represented in freshwater reservoirs. Among Mediterranean invaders, 23 species found in the Mediterranean Sea should be noted; 12 species still have not been registered in the Mediterranean Sea, mainly due to very few studies on this group of parasites. The fauna of Sea of Azov fish parasites includes 42 myxosporean species; 32 of them are found in the Black Sea, 20 – in the Caspian Sea. Totally 19 species belong to freshwater ones, and they are parasitic only in freshwater fish species. Ten species are of freshwater origin but can be registered in marine fish species (mostly in mullets, and one – in gobies). Of the freshwater species, one is Ponto-Azov endemic. Totally 13 myxosporean species are marine ones: 2 species are Sea of Azov endemics; 3 species are Ponto-Azov endemics; 8 species are Mediterranean invaders. Caspian Sea fauna includes 68 species of myxosporeans: 8 are marine ones (1 is ancient marine species; 1 is Ponto-Caspian endemic; 6 are brackish-water myxosporeans), and 60 are freshwater species. Five endemics of the Caspian Sea are known (2 of marine origin and 3 of freshwater origin). When moving from west to east (from the Black Sea to the Caspian Sea), a gradual impoverishment of marine myxosporean fauna and its replacement by freshwater myxosporean species are observed. Impoverishment of Myxosporea species composition of the Sea of Azov and the Caspian Sea in comparison with the Black Sea one is also found.