Osmotic and ionic haemolymph concentrations in the Baltic Sea amphipod Gammarus oceanicus in relation to water salinity

The aim of this work was to determine the osmotic and ionic (Na +, K +, Ca 2+, Mg 2+ and Cl −) haemolymph concentrations in Gammarus oceanicus at different salinity levels. Being a species of marine origin it inhabits brackish waters of the Baltic Sea. G. oceanicus specimens were collected in January 2003 from the Gulf of Gdansk (salinity 7 psu). The animals were gradually acclimated to eight different salinity levels (5, 7, 14, 20, 25, 30, 35 and 41 psu) at a temperature of 5 °C and 100% oxygen saturation. The haemolymph osmolalities correlated positively with external salinity, from 545.4 ± 17.3 mOsm in 5 psu to 1185.9 ± 34.6 mOsm in 41 psu. G. oceanicus hyperregulated within the 5–31.5 psu range; above 31.5 psu it hyporegulated its body fluids in comparison to the external medium. At 31.5 psu (1017 mOsm) the haemolymph concentration of G. oceanicus was isoosmotic with the habitat. The haemolymph concentrations of all the studied ions, except K +, correlated positively with their concentrations at the various salinity.