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

Nearly all decapod crustaceans found in Antarctic waters south of the Antar ctic Convergence are caridean shrimps (Natantia) while the group of Reptant ia is largely absent in this area. Progress in the development of a physiol ogical hypothesis is reported, which explains this distribution pattern bas ed on differences in the regulation of magnesium levels in the haemolymph ( [Mg2+](HL)) and on the Mg2+ dependence of threshold temperatures below whic h cold-induced failure of cardiac and ventilatory performance occurs. Previ ous studies had shown that an increase in oxygen consumption and activity l evels in the cold can be induced by experimental reduction of [Mg2+](HL) in different reptant decapod species. In the present study, we tested the pot ential of these experimental findings for predicting the effect of low [Mg2 +](HL) in nature, and investigated temperature-induced changes in oxygen co nsumption in two species with low but different [Mg2+](HL) from southern Ch ile, Halicarcinus planatus and Acanthocyclus albatrossis ([Mg2+](HL)= 10.7 and 21.6 mmol 1(-1), respectively). In accordance with previous findings, l ow [Mg2+](HL) levels were associated with a reduction of thermal sensitivit y and a higher metabolic rate in the cold. A model is developed which descr ibes how [Mg2+](HL) reduction caused a threshold temperature (pejus tempera ture, Tp) to fall, which characterises the onset of cold-induced failure in oxygen supply to tissues. This threshold temperature is interpreted, not o nly to indicate the limits of cold tolerance, but also of geographical dist ribution. Tp is shifted towards lower temperatures in Natantia, which are e fficient [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, ther efore, largely reduced scope for activity at high [Mg2+](HL).