Osmotic and ionic regulation in response to salinity variations and cold resistance in the Arctic under-ice amphipod Apherusa glacialis
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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 ≥ 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 ± 1.9°C. Several ions were specifically downregulated ([Mg2+], [SO4 2−]), or upregulated ([K+], [Ca2+]) in comparison to the medium. Strong downregulation of [Mg2+], is probably necessary to avoid an anaesthetic effect at low temperatures.
KeywordsSympagic Osmolality Ion composition Supercooling point ARK XXII/2
We thank captain and crew of RV “Polarstern” and chief scientist Ursula Schauer for their constant support during ARK XXII/2. We furthermore thank Franz-Josef Sartoris for help with the determination of ion concentrations and for general discussion and three anonymous reviewers for critical comments on the manuscript.
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