Physiological responses to salinity changes of the isopod Idotea chelipes from the Baltic brackish waters

Survival, haemolymph osmolality and the specific metabolic rate of the isopod Idotea chelipes from the brackish waters of the southern Baltic Sea (salinity 7 psu) were investigated after acclimation to different salinities in the range of 3–30 psu keeping other parameters constant (T = 15 °C, full air saturation). Haemolymph osmolality of I. chelipes increased from 379 ± 33 mmol kg− 1 at 3 psu to 935 ± 12 mmol kg− 1 at 30 psu. In the salinity range of 3–11 psu I. chelipes exhibited a hyperosmotic pattern of haemolymph regulation, whereas from 14 to 25 it behaved like an osmoconformer. Isoosmosis occurred at 28.5 psu (920 mmol kg− 1). The specific metabolic rate of I. chelipes was significantly (P < 0.05) related to salinity, with maximum (2.4 ± 0.8 J g− 1 wm h− 1) and minimum values (0.6 ± 0.3 J g− 1 wm h− 1) at 7 and 20 psu, respectively. The changes in the specific metabolic rate and in the difference between the osmolality of haemolymph and external medium were significantly correlated for I. chelipes (P < 0.05). Rates of change per salinity unit were, however, unequal varying according to salinity transition. These differences indicate that osmotic adjustment may be a more or less costly in terms of energy according to salinity.