TRENDS IN WATER AND SALT REGULATION AMONG AQUATIC AND AMPHIBIOUS CRABS

Abstract

The following seven species of crabs were studied with respect to their salt and water balance in aqueous media the aquatic Cancer antennarius, Hemigrapsus oregonensis, the semi-terrestrial, Pachygrapsus crassipes, Grapsus grapsus, Ocypode ceratophthalma, Uca crenulata, and the terrestrial Gecarcinus lateralis. All crabs examined except Cancer showed some degree of hypo- and hyperosmotic regulation, the more terrestrial crabs tending to be stronger hyporegulators than the more aquatic crabs. The antennary glands of none of these species are osmoregulatory in function but they are capable of regulating ions. With the exception of Gecarcinus, the more terrestrial crabs concentrate higher Mg in the urine than the more aquatic crabs. The concentration of Mg in the urine in increasing ambient salinities results in decreasing urine Na concentrations in all examined species except Gecarcinus which cannot concentrate urine Mg to any degree. The ability of a crab to concentrate Mg in the urine is not necessarily correlated with its ability to maintain low blood Mg. Thus, Uca showed the strongest ability to concentrate urine Mg, but its blood Mg concentration was as high as that of Cancer which showed weak ability to concentrate its urine Mg. There were no apparent trends with increasing terrestrialness for K regulation, but higher blood Ca was found in the more terrestrial crabs than in the more aquatic crabs. The response of Gecarcinus with respect to Na and Mg regulation which differs from other species studied suggests a fundamentally different mechanism in the antennary glands and a pathway to terrestrialness which differs from that of the other crabs.