SOME FUNCTIONS OF THE URINARY BLADDER IN A CRAB

¹ Department of Life Sciences, University of California, Riverside, California 92502

1. The concentration of urine Mg⁺⁺ in immersed specimens of Pachygrapsus is independent of the Mg⁺⁺ influx as well as the concentration of Mg⁺⁺ in the medium. It is, however, a function of the salinity of the medium.

2. Low U/B values for inulin indicate that water withdrawal has little effect in causing the high urine Mg⁺⁺ concentrations and Mg⁺⁺ U/B values observed in Pachygrapsus.

3. Repetitive samplings of urine from individual crabs immersed in 100% sea water reveal that the urine Mg⁺⁺ concentration fluctuates with time, varying as much as three-fold in a single crab. This is not believed to be due to fluctuations in the Mg⁺⁺ transport mechanism.

4. The wide range of urine Mg⁺⁺ concentrations observed in the field can be explained chiefly on the basis of fluctuating urine concentrations in individuals rather than on large variations in the ability to concentrate Mg⁺⁺.

5. There is evidence that the membranes of the bladder transport Mg⁺⁺ from blood to urine, and the concentration of Mg⁺⁺ attained in the urine of Pachygrapsus depends on the length of time that urine is held in the bladder. Thus, hypo-regulating crabs immersed in hypersaline water having a small water influx will hold urine in the bladder sufficiently long to build up the Mg⁺⁺ concentration. Hyper-regulating crabs in dilute sea water with a large water influx release urine too frequently to permit Mg⁺⁺ buildup. This explains how the urine Mg⁺⁺ concentration can be independent of the Mg⁺⁺ concentration in the medium, but is a function of the salinity of the external medium.

6. Fluctuating urine Mg⁺⁺ concentrations in crabs are believed to indicate periods of bladder evacuation, low Mg⁺⁺ following evacuation and high Mg⁺⁺ preceding evacuation.

7. There is evidencethat when Mg⁺⁺ is transported into the urine through the bladder wall, electro-chemical balance is achieved by direct exchange with Na⁺, but also by some movement of Cl^- with the Mg⁺⁺. Such a mechanism is compatible with the observed decreases in urine Na⁺ concentration accompanying increases in urine Mg⁺⁺ concentration.

8. Crabs treated with the Na⁺ transport inhibitor ouabain can concentrate Mg⁺⁺ in the urine. Thus, there is no evidence that Mg⁺⁺ transport is coupled to the active transport of Na⁺⁺.

9. Mg⁺⁺ transport from blood to urine is more rapid when the crab is immersed in high salinities than when immersed in low salinities. The mechanism controlling the rate of Mg⁺⁺ transport seems to be triggered directly by the Mg⁺⁺ concentrations in the blood and possibly by the blood osmotic concentration.

10. The concentration of Mg⁺⁺ attained in the urine of a crab does not necesarily indicate the relative ability to excrete Mg⁺⁺. It is suggested that permeability of the animal to water determines the rate of water turnover and therefore the rate of bladder evacuation. This, in turn, limits the period during which Mg⁺⁺ can be accumulated in a given volume of urine.

11. Direct catheterization of Pachygrapsus would be expected to deprive the bladder of its normal renal function, thus giving spurious values for urine flow and ion losses.