Water Balance in the Land Crab, Gecarcinus lateralis, During the Intermolt Cycle

Gecarcinus lateralis can take moisture from a clamp substratum in amounts adequate for the needs of the entire intermolt cycle. It can also rehydrate in this way, even after severe dehydration. This crab is able to survive for many months when free water of a wide range of salinities (0.30‰; ‰ = parts per thousand) is made available in a shallow dish. The crab dies within seven weeks when the salinity of this water is 35‰. During proecdysis the pericardial sacs of eyestalkless crabs become most swollen when the salinity of the available water is 15‰ or 23‰, and survival during and after ecdysis is greatest with water of 15‰. A crab in proecdysis shows no increase in the rate at which water enters following dehydration. Yet large amounts of water are retained, particularly at the intermediate salinities. Maximal swelling of the pericardial sacs just prior to ecdysis is essentially equivalent in crabs with eyestalks, in eyestalkless crabs, and in eyestalkless crabs that have received an implant of central nervous tissue. Hence, we conclude that a hormone causing the retention of water exists, but not in the eyestalks, in the brain, or in the thoracic ganglionic mass. At ecdysis eyestalkless crabs show large increases in the dimensions of the carapace, while crabs with eyestalks and eyestalkless crabs that have received an implant of certain central nervous tissues show much less increase and may even show a decrease. Thus, we conclude that a hormone causing a release of water at ecdysis is produced in the central nervous system. The advantages to the crab of a dual hormonal control of its water balance are discussed.