Osmotic and ionic regulation in response to salinity variations and cold resistance in the Arctic under-ice amphipod Apherusa glacialis

Amphipods living at the underside of Arctic sea ice are exposed to varying salinities due to freezing and melting, and have to cope with the resulting osmotic stress. Extracellular osmotic and ionic regulation at different salinities, thermal hysteresis, and supercooling points (SCPs) were studied in the under-ice amphipod Apherusa glacialis. The species is euryhaline, capable to regulate hyperosmotically at salinities S (R) < 30 g/kg, and osmoconforms at salinities S (R) a parts per thousand yen 30 g/kg. Hyperosmotic regulation is an adaptation to thrive in low-salinity meltwater below the ice. Conforming to the ambient salinity during freezing reduces the risk of internal ice formation. Thermal hysteresis was not observed in the haemolymph of A. glacialis. The SCP of the species was -7.8 +/- A 1.9A degrees C. Several ions were specifically downregulated ([Mg(2+)], [SO(4) (2-)]), or upregulated ([K(+)], [Ca(2+)]) in comparison to the medium. Strong downregulation of [Mg(2+)], is probably necessary to avoid an anaesthetic effect at low temperatures.