Salinity tolerance of alien copepods Acartia tonsa and Oithona davisae in the Black Sea

Acartia tonsa and Oithona davisae are typically near-shore species which were transferred to the Black Sea with ship ballast water and established self-sustaining populations in estuarine ecosystems. In order to estimate the salinity tolerance range and osmotic response in A. tonsa and O. davisae from Sevastopol Bay (a salinity of 18), the effect of salinity changes on mortality and body mass density of females of both species was studied. More than 50% of A. tonsa survived for 3-7 days after the gradual salinity decrease and increase at a rate of 2-3 h −1 within a salinity range from 3 to 30 while separate individuals withstood the salinity decrease down to 0.5 and salinity increase of up to 70 at a rate of 2-5 day −1. In accordance with the LD50 index, the salinity tolerance range of O. davisae acclimated to a salinity of 18 amounted to 3-40. A. tonsa females maintained constant body mass density within the whole salinity tolerance range but hyper-regulated at the salinities higher than 30. In O. davisae females we observed a hysteretic response of body mass density within the salinity range of 12-40, probably, due to low osmotic water permeability of their integuments. At the lowest salinities (about 3) O. davisae hypo-regulated with body swelling, however, this species was iso-osmotic at the salinities higher than 40. Consequently, estuarine A. tonsa showed osmoregulatory capacities at low and intermediate salinities while salinity tolerance range of O. davisae was shifted to high salinities indicating marine origin of that species. Probably, an invasive success of A. tonsa and O. davisae in the brackish Black Sea is due to their osmoregulatory abilities.