The presently reported study was intended to describe the current range of an Atlanto–Mediterranean fish, the black-striped pipefish, Syngnathus abaster Risso, 1827, in Ukrainian waters and to analyze biological tags (size parameters and parasites) of its different populations. The parasitological survey was carried at five different localities, including one marine site, two deltaic zones, and two localities in the middle Dnipro basin. The study provides comprehensive new data on parasites of the black-striped pipefish in Ukraine, with supporting data on its newly acquired freshwater range. A total of 21 parasite species (taxa) were revealed. Several parasite species were recorded for the first time on this host, i.e., Trichodinella epizootica (Raabe, 1950); Trypanosoma sp.; Bothriocephalus scorpii (Müller, 1776); Progrillotia dasyatidis Beveridge, Neifar et Euzet, 2004; Ophiotaenia europaea Odening, 1963; Cryptocotyle jejuna (Nicoll, 1907); Metorchis xanthosomus (Creplin, 1846); Tylodelphys clavata (von Nordmann, 1832); Holostephanus luehei Szidat, 1936; Contracaecum rudolphii Hartwich, 1964; Mothocya epimerica Costa in Hope, 1851; and Unionidae gen. sp. Formation of the species’ parasite component community depends entirely on environmental factors, with local parasite community features forming due to 1) presence of “marine” unicellular parasite species (ciliates) in marine localities (10‰–17‰ salinity) only, the community forming as a refraction of relative stenohalinity (Trichodina rectuncinata Raabe, 1958), findings of “marine” ciliate species in freshwater locations representing examples of successful osmoconformation (Trichodina partidisci Lom, 1962); or 2) presence of multicellular parasites in localities with abiotic/biotic conditions that allow completion of complex life cycles, such as those of trematodes (freshwater/marine mollusks as obligate first hosts) or cestodes (freshwater/marine invertebrates as intermediate hosts or marine/freshwater vertebrates as definitive hosts).
