OSMOREGULATION, IONIC EXCHANGE, BLOOD CHEMISTRY, AND NITROGENOUS WASTE EXCRETION IN THE LAND CRAB CARDISOMA CARNIFEX: A FIELD AND LABORATORY STUDY

¹ Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4Kl
² Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 2A9

Cardisoma carnifex in Moorea, French Polynesia, were sampled in the field and after exposure in the laboratory to either fresh-or seawater under conditions which allowed the crabs to flush their branchial chambers with the medium but not to ventilate it. Relative to field data, ionic and osmotic status of the hemolymph was virtually unchanged by exposure to freshwater, but markedly disturbed by seawater. The crabs were capable of net Na⁺ and Cl^– uptake from freshwater. Water sampled from natural crab burrows was essentially freshwater. It is concluded that the population was "in equilibrium" with freshwater in the wild.

Net H⁺ uptake (= base excretion) occurred in both fresh-and seawater; in freshwater there was a 1:1 relationship between net H⁺ flux and strong cation minus anion flux (i.e., Na⁺ + Mg⁺⁺ + Ca⁺⁺ + K⁺ – Cl^–). Unidirectional Na⁺ and Cl^– exchanges, measured radioisotopically, were typical of euryhaline crabs in freshwater, but influxes were unusual in showing no increase in seawater. Mild dehydration caused complex alterations in these exchanges in both media, associated with small and quickly reversed changes in hemolymph composition in freshwater, but larger effects in seawater which were not reversed. High levels of ammonia in hemolymph occurred in the field but declined in the laboratory, while the level of urea was low in both situations. Both wastes were excreted into the water. Neither uric acid nor gaseous ammonia excretion were detected, and uric acid was generally not found in hemolymph. The results are discussed in relation to the ecology of this unusual animal.