Distribution patterns of decapod crustaceans in polar areas:: a result of magnesium regulation?

Nearly all decapod crustaceans found in Antarctic waters south of the Antarctic Convergence are caridean shrimps (Natantia) while the group of Reptantia is largely absent in this area. Progress in the development of a physiological hypothesis is reported, which explains this distribution pattern based on differences in the regulation of magnesium levels in the haemolymph ([Mg2+](HL)) and on the Mg2+ dependence of threshold temperatures below which cold-induced failure of cardiac and ventilatory performance occurs. Previous studies had shown that an increase in oxygen consumption and activity levels in the cold can be induced by experimental reduction of [Mg2+](HL) in different reptant decapod species. In the present study, we tested the potential of these experimental findings for predicting the effect of low [Mg2+](HL) in nature, and investigated temperature-induced changes in oxygen consumption in two species with low but different [Mg2+](HL) from southern Chile, Halicarcinus planatus and Acanthocyclus albatrossis ([Mg2+](HL)= 10.7 and 21.6 mmol 1(-1), respectively). In accordance with previous findings, low [Mg2+](HL) levels were associated with a reduction of thermal sensitivity and a higher metabolic rate in the cold. A model is developed which describes how [Mg2+](HL) reduction caused a threshold temperature (pejus temperature, Tp) to fall, which characterises the onset of cold-induced failure in oxygen supply to tissues. This threshold temperature is interpreted, not only to indicate the limits of cold tolerance, but also of geographical distribution. Tp is shifted towards lower temperatures in Natantia, which are efficient [Mg2+](HL) regulators. In contrast, Reptantia, which are poor [Mg2+](HL) regulators, appear unable to colonise the permanently cold water of the Antarctic due to insufficient capacity of cardiac performance and, therefore, largely reduced scope for activity at high [Mg2+](HL).