Mesochra rostrata Gurney, 1927 (Copepoda, Harpacticoida) in Sivash Bay (Sea of Azov): Is it a new alien species or a relict of Tethys?

Harpacticoid Mesochra rostrata Gurney, 1927 was found in Sivash Bay (Sea of Azov) for the first time in 2013. In 2015, it became the most common and abundant species of Harpacticoida in the bay. The morphological characteristics of males and females of M. rostrata found in the Sivash accord with the species description given in earlier publications. The bay population comprised adult males and females, naupliar and copepodite stages. Adult male size varied from 0.30 to 0.40 mm, and adult female size varied from 0.38 to 0.45 mm. Since 2014, a substantial increase in salinity has been observed in Sivash Bay. In August 2015, seven species of Harpacticoida were registered in the bay in total; M. rostrata dominated and was abundant in benthos (up to 56000 ind./m 2 ), in the floating mats of filamentous green algae (up to 336 400 ind./m 2 ), and in plankton (up to 580 ind./m 3 ) at water salinity of 60–75 g/L. Earlier, this species was registered in the Sinai Peninsula at the water salinity not exceeding 45 g/L. M. rostrata can be considered a new invader in the Azov-Black Sea Region; the resting stage of this crustacean was brought here by wind or birds. However, it can be assumed also that the species is an aboriginal one for the Azov-Black Sea Region from times of the Tethys Ocean. In the Black and Mediterranean seas, there are some relicts of Tethys. In normal conditions, this species is rare and, possibly, can succeed only in destabilized biotopes, such as Sivash Bay nowadays during the period of a sharp increase in the water salinity.