OSMOTIC TOLERANCE AND REGULATION IN CRABS FROM A HYPERSALINE LAGOON

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

1. The crabs Pachygrapsus crassipes and Hemigrapsus oregonsis were found thriving in an hypersaline lagoon which had been isolated from the sea for months and had attained salinities in excess of 175% of normal.

2. Contrary to previous reports Hemigrapsus was found to be a strong hypo-osmotic regulator in concentrations as high as 175% sea water. The discrepancy between previous laboratory findings and the field results of the present investigation are attributed to the prolonged acclimatization period in lagoon water of gradually increasing salinity and to severe selection permitting only hypo-regulators to be sampled.

3. No living Hemigrapsus were found in the lagoon when the salinities had exceeded 180% sea water.

4. Pachygrapsus, as was expected from previous studies, was regulating osmotically in the lagoon when the concentrations were above 185% sea water.

5. Both Pachygrapsus and Hemigrapsus can tolerate blood concentrations in excess of 160% sea water and remain active.

6. Pachygrapsus is a stronger hypo-osmotic regulator than Hemigrapsus. Since it maintains its blood Mg considerably lower than does Hemigrapsus in concentrated sea water, the stronger ability to hypo-regulate may be made possible by the greater capacity on the part of Pachygrapsus to excrete Mg.

7. The urine Mg for Hemigrapsus is about equal in concentration to that of Pachygrapsus when the crabs are immersed in 175% sea water, even though the blood Mg concentration is more than three times higher than that of Pachygrapsus.

8. Hemigrapsus is a stronger regulator of Na in hypersaline water than is Pachygrapsus.

9. The antennary glands in both Pachygrapsus and Hemigrapsus are ineffective as organs of hypo-osmotic regulation.

10. When Pachygrapsus is transferred from normal to concentrated sea water the volume of its muscle tissue, as indicated by water content, becomes reduced. This reduction in volume persists in hypersaline waters even after months of immersion. Thus, there is no evidence of volume control for muscle tissue.

11. After months of isolation in a hypersaline lagoon Pachygrapsus shows a preference for normal sea water when offered a choice of salinities and will avoid the hypersaline water from which it has been captured.

12. Evidence is produced showing that coastal lagoons present selective pressures favoring: (1) hyper-osmotic regulation, (2) hypo-osmotic regulation, and (3) aerial respiration, three physiological characteristics common to terrestrial and semi-terrestrial crabs. Coastal lagoons are suggested as ideal sites for the evolution of land crabs.