Aspects of Osmotic Regulation in Crabs Showing the Terrestrial Habit

1. Terrestrial and semi-terrestrial crabs generally demonstrate the ability to regulate osmotically in dilute and concentrated sea water. 2. Osmotic regulation in terrestrial and semi-terrestrial crabs is not adaptive for aquatic life since osmotic stresses such as dilute and concentrated sea water are rarely experienced by these animals. Besides many such animals drown when completely immersed. 3. The branchial chamber of Pachygrapsus contains a volume of fluid equal to a maximum of about 3 per cent of the body volume. Thus, regulation against branchial fluid made concentrated by evaporation would not be adaptive as proposed by Jones because the total salt therein could not elevate the blood concentration above concentrations normally experienced in nature. 4. The respiratory rate of Pachygrapsus decreases when the crab is desiccated. This explains why the terrestrial behavior of Pachygrapsus is limited to brief periods. 5. Pachygrapsus can absorb water against a gradient when it has been desiccated. 6. The ability to regulate in concentrated sea water (hypo-osmotic regulation) is believed to be a manifestation of a mechanism for the conservation of water, namely, elimination of salts. 7. The mechanism involved in hyper-osmotic regulation seems to be necessary to the mechanism of hypo-osmotic regulation but apparently has no adaptive consequence such as salt elimination. 8. The coconut crab, Birgus latro, shows special adaptations to terrestrial life. a. It can drink water from small puddles. b. It can keep its respiratory membranes moist by placing water into the gill chamber without becoming immersed. c. It can control the concentration of its body fluids by selecting water of the appropriate salinity.