A BEHAVIORAL MECHANISM FOR OSMOTIC REGULATION IN A SEMI-TERRESTRIAL CRAB

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Abstract
1. The shore crab Pachygrapsus crassipes prefers 100% sea water to 50, 75, 125 and 150% sea water. 2. This preference could not be altered by first desiccating the animal or by acclimating the animal for several days in 50% sea water. The preference could be altered somewhat by acclimating to 150% sea water. 3. The preference for normal sea water by Pachygrapsus suggests a mechanism which tends to restrict this crab to the intertidal and subtidal zones of the sea. 4. Pachygrapsus under the experimental conditions of the present investigation spends about 12 hours per day visiting water. This compares to one hour per day for the land crab Birgus. 5. Pachygrapsus tends to maintain normal sodium and potassium blood concentrations when given free choice of salinities, including 100% sea water. Normal concentrations are generally achieved under the same conditions even when the blood has been forced away from normal by acclimation in 50 or 150% sea water. However, animals previously desiccated may over-compensate when offered a choice of media varying in salinity and consequently achieve blood sodium concentrations below the normal range.

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