Marine brachyuran and anomuran crustaceans are completely absent from the extremely cold (-1.8°C) Antarctic continental shelf, but caridean shrimps are abundant. This has at least partly been attributed to low capacities for magnesium excretion in brachyuran and anomuran lithodid crabs ([Mg2+]HL=20-50mmol L-1) compared to caridean shrimp species ([Mg2+]HL=5-12mmol L-1). Magnesium has an anaesthetizing effect and reduces cold tolerance and activity of adult brachyuran crabs. We investigated whether the capacity for magnesium regulation is a factor that influences temperature-dependent activity of early ontogenetic stages of the Sub-Antarctic lithodid crab Paralomis granulosa. Ion composition (Na+, Mg2+, Ca2+, Cl-, SO42-) was measured in haemolymph withdrawn from larval stages, the first and second juvenile instars (crabs I and II) and adult males and females. Magnesium excretion improved during ontogeny, but haemolymph sulphate concentration was lowest in the zoeal stages. Neither haemolymph magnesium concentrations nor Ca2+:Mg2+ ratios paralleled activity levels of the life stages. Long-term (3 week) cold exposure of crab I to 1°C caused a significant rise of haemolymph sulphate concentration and a decrease in magnesium and calcium concentrations compared to control temperature (9°C). Spontaneous swimming activity of the zoeal stages was determined at 1, 4 and 9°C in natural sea water (NSW, [Mg2+]=51mmol L-1) and in sea water enriched with magnesium (NSW+Mg2+, [Mg2+]=97mmol L-1). It declined significantly with temperature but only insignificantly with increased magnesium concentration. Spontaneous velocities were low, reflecting the demersal life style of the zoeae. Heart rate, scaphognathite beat rate and forced swimming activity (maxilliped beat rate, zoea I) or antennule beat rate (crab I) were investigated in response to acute temperature change (9, 6, 3, 1, -1°C) in NSW or NSW+Mg2+. High magnesium concentration reduced heart rates in both stages. The temperature-frequency curve of the maxilliped beat (maximum: 9.6 beats s-1 at 6.6°C in NSW) of zoea I was depressed and shifted towards warmer temperatures by 2°C in NSW+Mg2+, but antennule beat rate of crab I was not affected. Magnesium may therefore influence cold tolerance of highly active larvae, but it remains questionable whether the slow-moving lithodid crabs with demersal larvae would benefit from an enhanced magnesium excretion in nature. © 2010 Elsevier B.V.