CALCIUM AND MAGNESIUM BALANCE DURING MOLTING IN LAND CRABS

Calcium and magnesium contents were measured in the terrestrial crabs Cardisoma hirtipes, Gecarcoidea natalis, Geograpsus grayi, and Ocypode cordimana in relation to the intermolt cycle. Contents of these elements were of a similar order to those of marine decapod crustaceans with the exception of the Ca:Mg ratio in Geograpsus grayi (13:2) which was higher than in the other species. Regression equations relating Ca and Mg content to body size are supplied for each species. Behavioral adaptations allowed essentially all calcium and magnesium to be retained through the molt. Crabs molted in burrows (gecarcinids and Ocypode) or other safe retreats (Geograpsus) where they ate the cast exuviae and utilized the contained minerals before reemergence. Physiological adaptations for retention of Ca and Mg contribute significantly to storage in Gecarcoidea natalis, allowing retention of 14.2 +/- 4.4% Ca and 19.5 +/- 7.2% Mg by the soft crab of which 8.5 +/- 3.4% and 5.5 +/- 2.7%, respectively, were in the gastroliths. In Ocypode cordimana, storage occurred in the hemolymph and digestive gland. Physiological retention of calcium was much lower in Cardisoma hirtipes (4.5 +/- 1.36%) and Geograpsus grayi (5.1 +/- 2. 1 %), but all species retained relatively high proportions of their magnesium.