Calcium balance in crustaceans: nutritional aspects of physiological regulation.

Calcium homeostasis in crustaceans is influenced by their natural molting cycle that periodically requires replacement of the calcified exoskeleton in order for growth to occur. Whole body Ca balance transitions from intermolt (zero net flux) to premolt (net efflux) and postmolt (net influx at the rate of 2 mmol kg(-1)h(-1)). As such, molting provides a convenient model to study up- and down-regulation of epithelial Ca transporting proteins (such as Ca pumps and exchangers), the genes that encode them, and the steroid hormone (ecdysone) that putatively regulates the genes. Species residing in either freshwater or in terrestrial environments are more limited in their Ca availability than are marine species. Further the advance towards terrestriality is accompanied by decreased reliance upon branchial Ca uptake and increased reliance upon digestive uptake. This review will correlate Ca handling strategies with environment in semi-terrestrial and terrestrial crabs through examining environmental sources of Ca uptake. Ca homeostasis will also be discussed at the whole animal level, cellular, subcellular and molecular levels of regulation.