Ion composition and osmolarity of Caspian Sea ctenophore, Mnemiopsis leidyi, in different salinities

The aim was to study osmotic and ionic regulation in alien ctenophore, Mnemiopsis leidyi (total length of 1.5-3 cm), acclimated gradually to artificial sea water of different salinities (8, 13, 18 and 23 ppt) in laboratory conditions (24 ± 2 °C) with the salt composition and proportion of the Caspian Sea. Results showed that this species is hyper-osmoconformer (maintaining internal osmolarity 2-22 mOsmol l^{− 1} above external) in non-lethal salinities ranging from 8 to 23 ppt. The results of ionic regulatory investigations revealed that this species can regulate Ca²⁺ and SO₄²⁻ so that the concentration of Ca²⁺ in the internal fluid and SO₄²⁻ in the external fluid were significantly more in 8 and 13 ppt, respectively. No regulation was observed about other ions such as Cl⁻, Mg²⁺ and K⁺ in the mentioned salinities.