POTASSIUM AND SODIUM REGULATION IN AN INTERTIDAL CRAB

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Abstract
1. When Pachygrapsus is immersed in a stress medium its blood concentration is altered by a loss of ions to a hypotonic medium and a gain of ions from a hypertonic medium. Water exchanges are insignificant in magnitude. 2. The observed ratio, change in blood ions/change in medium ions yields values for "apparent volume of distribution" for the respective ions. Such values vary according to the treatment for sodium and in moderate stresses average 38.5% body weight, in extreme stresses 45.9% body weight. For potassium most values came to more than 100% body weight and do not vary with increased stress. The above ratios are the same for a hypotonic medium as for a hypertonic medium. 3. Varying values for "apparent volume of distribution" under different magnitudes of osmotic stress suggest the presence of salt pools which may represent incidental participation of the formed tissues, or may represent an adaptive mechanism which functions to assist in the ionic and osmotic regulatory mechanism. 4. In stress media blood potassium and sodium are regulated equally well, percentage-wise, but a source other than the blood participates in the exchanges of potassium between animal and medium. Thus the potassium change in the blood does not account for the total potassium change in the animal. 5. "Apparent volume of distribution" calculated from the increased blood concentration caused by a given water loss by evaporation averages 48.9% body weight for sodium and only 13% body weight for potassium. The blood potassium therefore, increases percentage-wise about four times more than blood sodium. This indicates that potassium leaves a pool (probably the intra-cellular space) to enter the blood. This appears to be a physiological failure rather than regulation, and may play a role in ecological limitations. 6. Potassium concentrations in the blood of normal crabs (Pachygrapsus) are less than those of sea water. When immersed in dilute sea water of lower potassium concentrations than found in the blood of normal animals, the crabs usually tend to lose potassium so that it remains less concentrated than the potassium of the medium.

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