Magnesium and calcium metabolism during molting in the freshwater prawn Macrobrachium rosenbergii

Magnesium and calcium concentrations were recorded in the extracellular fluid, exoskeleton, and hepatopancreas of the Malaysian freshwater prawn Macrobrachium rosenbergii throughout the molting cycle. Sixty male prawns distributed among five molt states (intermolt (C), early premolt [Formula: see text], late premolt [Formula: see text], early postmolt (A), and late postmolt (B)) were dissected for analysis of body components. Magnesium concentration in the whole bodies of 14 prawns was established. Magnesium levels in the extracellular fluid, exoskeleton, and hepatopancreas increased over the intermolt concentrations during premolt stages [Formula: see text], from 0.42 to 1.56 mmol∙L−1 in the extracellular fluid, from 0.09 to 0.47 mmol∙g dry weight−1 in the exoskeleton, and from 0.015 to 0.094 mmol∙g dry weight−1 in the hepatopancreas. A sevenfold increase in whole body magnesium, from 0.108 to 0.770 mmol∙g dry weight−1, occurred during premolt. During postmolt, magnesium concentrations of the whole body and of all body compartments measured declined steadily. Calcium levels in extracellular fluid, exoskeleton, and hepatopancreas also increased during premolt. In early postmolt stage A, however, exoskeleton calcium increased significantly while extracellular fluid and hepatopancreas calcium concentrations were depleted and during late postmolt stage B began to rise toward intermolt (C) levels. Whereas calcium changes during molting reflect the importance of calcium in the exoskeleton, changes in magnesium concentration are independent of any role this ion has in skeletal hardening.