WATER BALANCE IN ANOMURAN LAND CRABS ON A DRY ATOLL

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

1. Water balance in the anomuran crabs, Birgus latro, Coenobita perlatus and Coenobita brevimanus, was studied in the field at Eniwetok Atoll during the dry season.

2. Birgus and C. brevimanus usually are found in wooded areas in or near piles of rotten cocoanuts which are damp in their interiors. The osmotic concentration of the serum of Birgus and C. brevimanus from such conditions was relatively constant, averaging 74.7 and 80.3% sea water, respectively. Such animals do not depend on the sea for their source of water.

3. The osmotic concentration of serum from Birgus captured on the exposed dry islet of Jieroru averaged 91.6% sea water. Evidence suggests that these animals were either using the sea as their source of water or were tolerating slow desiccation in well-sealed burrows.

4. Coenobita perlatus is more common and less discriminating than the other two species in its choice of habitat. It is found in such extremes as exposed positions at the edge of the lagoon and protected conditions, such as the interior of piles of cocoanuts in wooded areas.

5. Serum osmotic concentrations for C. perlatus taken in the field were usually hypertonic to the available sea water and ranged from 102% sea water (cocoanut piles) to 150% sea water (active at night in forest).

6. Coenobita perlatus was observed to enter sea water and brackish water at night; this resulted in filling their adopted shells with water.

7. Neither Birgus nor C. brevimanus was observed in sea water in nature although both species will use sea water under laboratory conditions.

8. Shell water found in C. brevimanus taken from cocoanut piles was always hypotonic to sea water, but essentially isotonic to the serum. The source of shell water in this species was not found, but experiments demonstrated that such large volumes are not secreted.

9. C. brevimanus can thrive on a damp substrate in the absence of surface water for more than two weeks. The serum concentration following this treatment remains relatively low (mean: 86.4% sea water), but abnormal quantities of fresh water and sea water are used by the animal following treatment. This suggests that dehydration occurred and that salts also were lost to the damp substrate.

10. The osmotic concentration of urine in all three species is close to that of the blood, although it tends to be slightly hypotonic to the blood in C. brevimanus and Birgus. Therefore, there is no evidence that the antennary glands of these crabs are especially adaptive for terrestrial life.

11. Evidence presented suggests that the three subject species represent three steps from a marine life toward a terrestrial existence, Birgus being considered the most terrestrial and C. perlatus the closest to the sea.

12. Birgus, probably the most terrestrial of the anomuran crabs, is physiologically more marine in the adult stages than Gecarcinus lateralis, one of the most terrestrial brachyuran crabs.