Aspects of Osmotic Regulation in Crabs Showing the Terrestrial Habit

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Abstract
Terrestrial and semi-terrestrial crabs generally regulate osmotically in both concentrated and dilute sea water. Hypo-osmotic regulation as such in concentrated sea water is not believed to be directly adaptive for the terrestrial habit, rather it is the manifestation of other processes concerned with water conservation. Hyper-osmotic regulation in dilute sea water in all cases studied is found among crabs which are hypo-regulators, but its adaptive significance is in doubt. The shore crab, Pachygrapsus crassipes, in a desiccated condition is able to absorb water against a gradient. The respiratory rate of Pachygrapsus decreases with time when the animal is exposed to air, thus indicating a factor limiting the terrestrial life of this animal. The coconut crab, Birgus latro, can drink sea water or fresh water and thus control the concentration of its blood by selecting the proper amount of salt from water of the appropriate salinity. Birgus can moisten its respiratory membranes by placing water into its branchial cavity by means of its walking appendages.

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