THE INFLUENCE OF SALINITY ON THE MAGNESIUM AND WATER FLUXES OF A CRAB

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

1. The concentration of Mg in the urine of Pachygrapsus is dictated by the salinity of the external medium and not by the Mg concentration in that medium or by the rate of Mg influx from the medium into the animal. Thus, during brief periods of immersion in 50%, 100% or 150% sea water the urine Mg concentration will reflect the salinity of the medium, irrespective of whether Mg is absent or in abnormally high concentrations.

2. The Na concentration in both blood and urine is not drastically altered by abnormal Mg levels in the external medium of any salinity tested.

3. After immersion in Mg-free 100% and 150% sea water the urine K of Pachygrapsus is higher than it is after immersion in the respective concentrations of natural sea water. Urine K is not influenced by the Mg concentration of 50% artificial sea water or by abnormally high Mg in 100% sea water. Blood K concentrations are affected by varying concentrations of Mg in the external medium of both dilute and concentrated salinities, but there is no definite trend.

4. The concentration of Ca in the urine of Pachygrapsus is unaffected by the Mg levels of all salinities tested. Blood Ca was not observed to be altered by abnormally high or low Mg levels in all media tested except in 100% artificial sea water with high Mg (156 meq./l.), where the blood Ca was significantly higher than for animals from 100% natural sea water.

5. While the concentration of urine Mg is not determined immediately by the influx of this ion into the animal, the blood Mg concentration is lowered when the crab is immersed in a Mg-free medium and raised when the medium Mg is abnormally high. The concentration of urine Mg is relatively independent of the levels of Mg in the blood.

6. Pachygrapsus excretes more Mg in 50% sea water than in 100% sea water and perhaps less in 150% sea water than in 100% sea water, even though the concentration of Mg in the urine is in the reverse order (i.e., 150% > 100% > 50%).

7. Calculated rates of urine production for Pachygrapsus completely immersed in different salinities follow: in 150% sea water, 1.5% body weight/day; in 100% sea water, 3.9% body weight/day; in 50% sea water, 58% body weight/day. The observed rate for crabs immersed in 50% sea water is not believed to be sustained for long, as suggested by the high mortality rate. When removed from water, the volume of urine excreted by Pachygrapsus is insignificant.

8. The concentration of urine Mg in Pachygrapsus thus is inversely related to the rate at which urine is produced by the antennary glands and this is dependent on the magnitude and direction of the water flux, imposed by the physical gradient between the crab and its external medium.

9. The volume for muscle tissue in Pachygrapsus increases when the crab is transferred from normal sea water to dilute sea water and decreases when it is transferred to concentrated sea water. Such volume changes take place at the expense of the blood space.

10. It is suggested that alterations in the volume of the blood space caused by osmotic stress likely would reduce the efficiency of the vascular system which in turn would impose further ecological limitations on this species.