THE EFFECT OF OSMOTIC STRESS ON THE IONIC EXCHANGE OF A SHORE CRAB

¹ Division of Life Sciences, University of California, Riverside, California

1. The effects of osmotic stress on the ion concentration in the blood of the crab, Pachygrapsus crassipes, were investigated. Stresses imposed were 50% sea water, 150% sea water and desiccation to a water loss of about 7% body weight.

2. The observed ratios, blood change (mEq./l.)/medium change (mEq./l.), for sodium, potassium, calcium and magnesium after the crab was transferred from normal sea water to 50% or 150% sea water yielded values for "apparent volume of distribution." The average value for sodium was 38.5% body weight, but for the other three ions was at least 100% body weight.

3. The large values for "apparent volume of distribution" in the cases of potassium, calcium and magnesium indicate that these ions are contained in extra-vascular pools in greater concentrations than they are in the blood and that these pools participate in ion exchanges between animal and medium. Thus, a concentration change can occur in the blood without being reflected in the medium or vice versa.

4. Calcium is lost to the medium by Pachygrapsus when it is immersed in normal sea water. Blood calcium increases when a crab is transferred from normal sea water to dilute sea water.

5. When Pachygrapsus is desiccated, the blood concentrations of potassium, calcium and magnesium average greater increases than does the sodium concentration. This suggests that potassium, calcium and possibly magnesium shift from an extra-vascular pool into the blood space. The phenomenon is interpreted as a physiological failure and a factor which may limit the terrestrial life of this species.

6. The ratio, urine concentration (mEq./l.)/blood concentration (mEq./l.), for the respective ions suggests the role of the antennary glands as ion regulators under the various stress conditions. Thus the antennary glands were found to be relatively ineffective as regulators of sodium, potassium and calcium for all conditions studied. The U/B ratio for magnesium averaged 5.62 when the crab was immersed in 50% sea water; 13.6 for normal sea water; 15.4 for 150% sea water and 23.6 when the crab was desiccated. These high ratios suggest that a principal role of the antennary glands is magnesium regulation.

7. The volumes of urine production which have not been measured must be known before the effectiveness of the antennary glands as magnesium regulators can be determined.