Molecular and morphological characterisation of Gyrodactylus alviga (Monogenea: Gyrodactylidae) from the Black Sea, and its synonymisation with G. pterygialis from the North European seas.

Sequences of the ITS1-5.8S-ITS2 rDNA of Gyrodactylus alviga Dmitrieva & Gerasev, 2000 from Merlangius merlangus L. (Gadiformes: Gadidae) in the Black Sea were obtained for the first time. Gyrodactylus alviga is 0.2% distinct from G. pterygialis Bychowsky & Polyansky, 1953 parasitising the gadid fish Pollachius virens L. in the Norwegian Sea and Gyrodactylus sp. from Microgadus tomcod Walbaum of the same fish family in the Northwest Atlantic, based on the genetic variability of the ITS region. The most species-specific ITS1 region was identical in both species. The differences in the ITS2 secondary structure and compensatory base changes in its hairpins between G. alviga and G. pterygialis were not observed. Morphometric comparison of G. alviga and G. pterygialis also showed no significant differences. On this basis, G. alviga is synonymised with G. pterygialis and a redescription of the latter is presented, including G. alviga new syn. Findings of this species in the White and Bering Seas, and possibly off the northeastern coast of North America, require confirmation based on both morphological and molecular data. The results of this study show that G. pterygialis has a wider distribution than previously known. The good concordance of the secondary structure of the first ITS2 hairpin with the phylogenetic reconstruction of Gyrodactylus species based on the whole ITS region was revealed, which is of interest for further studies on the phylogenetic systematics of Gyrodactylus.