It was hypothesized that the capacity for extracellular ion regulation of marine crustaceans is a factor that determines their cold tolerance and biogeography in the Southern Ocean. Antarctic amphipods hyporegulated extracellular magnesium just as well as their temperate counterparts. Antarctic isopods exhibited similarly high haemolymph magnesium concentrations as the sub-Antarctic lithodids. Therefore, high haemolymph magnesium concentration does not represent a constraint for isopods to occur at water temperatures below 0°C. Low temperature and high magnesium concentration synergistically impede larval survival and development of P. granulosa. The temperature-dependent effect of magnesium was most prominent during forced swimming activity of the zoea I, but was hardly detectable in the resting juvenile. This may indicate that highly active larval stages, despite their higher haemolymph magnesium concentrations, are more susceptible to magnesium than the more advanced life stages. Neither magnesium concentration nor oxygen delivery set limits to cold tolerance in the adult stage of P. granulosa and it is doubtful that the high haemolymph magnesium concentration of this species plays a role in its geographic distribution. Other decapod crustaceans might however be constrained by low temperature, if their lifecycle includes planktotrophic larvae with a low capacity for magnesium regulation. In the shore crab C. maenas walking speed was not significantly affected by magnesium concentration. Food consumption remained significantly higher at reduced magnesium concentration than in natural conditions at low temperature. Acute cold exposure and online recording of arterial oxygen partial pressure, ventilation and heart beat frequency at rest revealed a significant effect of reduced magnesium concentration on ventilation and heart rates, but this translated only into an insignificant increase of arterial oxygen partial pressure at low temperatures. In this study there was no clear indication for an effect of magnesium concentration on cold tolerance of C. maenas. In addition to the originally planned studies we have tested our hypothesis that physiological performance at low temperatures depends on [Mg2+]e regulation with pelagic and sympagic Antarctic copepod species. The promising results, even if not related to magnesium regulation directly but to different ion regulatory mechanisms have led to a following DFG project.