Response to salinity and temperature changes in the alien Asian copepod Pseudodiaptomus marinus introduced in the Black Sea

The salinity tolerance and the effect of temperature were studied on the behavior and motor activity of the nonindigenous Indo-Pacific calanoid copepod Pseudodiaptomus marinus, first found in Sevastopol Bay (Black Sea) in autumn 2016. According to the index of median lethal salinity (LS50 ), the salinity tolerance range of adult P. marinus collected at 18.0 psu in November 2016 and subsequently reared in the laboratory amounted to 5.0-44.0 psu, independently of the acclimation regime. Females of P. marinus collected in December 2016 at 12.0°C became torpid at 8.0°C, a value typical of winter-spring Black Sea coastal areas. An increase in temperature from 8.0°C to 27.0°C led to an increase in the beat frequency of mouth appendages, swimming speed, and time spent cruising. However, at the same high temperature, the mean cruising speed in the feeding-current feeder P. marinus was 2-fold lower than that of the native, similarly sized cruise feeder Pseudocalanus elongatus. On the contrary, mouthpart beat frequency while cruising was 2-fold higher reaching 80 Hz, due to the creation of feeding currents in P. marinus. The results of our experiments confirm the euryhaline character of P. marinus, and point to an apparent ability to survive cold temperatures in a torpid state. This suggests the possibility of entering an overwintering stage to survive the adverse cold winter-spring environmental conditions of the Black Sea, similarly to the recent thermophilic Indo-Pacific invader Oithona davisae which established a successful population in the same area.