Three Christmas Island land crabs do not alter extracellular magnesium in response to acute reduced temperature

Magnesium is essential to numerous biological processes in many species. Including protecting intracellular pH and ATP concentrations during temperature and anoxia stress. Extracellular [Mg²⁺] is tightly regulated in many, but not all crustaceans. There is evidence that extracellular [Mg²⁺] content may be open to environmental influence such as temperature. The alteration of extracellular [Mg²⁺] in decapod crustaceans in response to temperature has been proposed, with individuals exposed to low temperature often displaying an increase in extracellular [Mg²⁺]. However, data, supporting or refuting this hypothesis, are sparse and drawn mainly from temperate and sub-polar studies. Therefore, the aim of this study was to determine if extracellular [Mg²⁺] increased at low temperature, indicating a disruption of [Mg²⁺] regulation for tropical crab species that do not typically experience large temperature fluctuations. Three species of Christmas Island land crabs were exposed to environmental (25 °C) and reduced (6 °C) temperatures for 60-90 min (depending on species). Haemolymph was sampled every 30 min and concentrations of Mg²⁺, and other key ions (Ca²⁺, K⁺, Cu²⁺, and Na⁺) quantified. Contrary to our prediction, all species displayed extracellular [Mg²⁺] resilience to temperature, exhibiting either the absence of evolved physiological mechanisms, or presence of tight regulatory abilities. There were, however, small but significant changes in concentrations of some of the other measured ions. We conclude that the alteration of extracellular [Mg²⁺] in response to acute reduced temperature is not a ubiquitous function in crustaceans as there is little support from the three tropical Christmas Island land crabs investigated.