SALT AND WATER ANATOMY, CONSTANCY AND REGULATION IN RELATED CRABS FROM MARINE AND TERRESTRIAL HABITATS

¹ Edward Martin Biological Laboratories, Swarthmore College, Swarthmore, Pennsylvania, and Bermuda Biological Station, St. George's West, Bermuda

1. Exposure to environmental salinities ranging from 120 to 720 mM. Cl/L. for 72 hours did not produce changes in fresh weights of the land crab (Gecarcinus lateralis), the ghost crab (Ocypode albicans) or the mangrove crab (Goniopsis cruentatus). There was an increase in weight of questionable significance after 24 hours in crabs exposed to distilled water. Only in distilled water was there any change in the blood specific gravity of ghost crabs. Even this change was of questionable significance.

2. The total body water content of ghost crabs is significantly larger than those of land crabs and mangrove crabs, which are similar. The fractions of total water content which are available for the dilution of thiocyanate and inulin are similar in the three species. The volumes available for the dilution of inulin are about two-third the volumes in which SCN appears to be diluted. This suggests the interesting possibility of a functionally closed, lumen flow, circulation.

3. The blood chloride concentration of mangrove crabs, although less than that of their environment, is significantly greater than those of the more terrestrial ghost crabs and land crabs, which are similar. The urine chloride concentration of mangrove crabs is identical to that of its environment and is more concentrated than that of ghost crabs.

4. Exposed to environmental fluids of 120 to 600 mM. Cl/L. sea water for 72 hours, land crabs show adequate regulation of blood chloride concentration over a limited hypotonic range, but little or no regulation in fluids hypertonic to its blood chloride. Blood chloride regulation in ghost crabs is adequate over this range, but with the production of a urine which wastes chloride in hypotonic fluids. Mangrove crabs show an adequate and closely held regulation of blood chloride concentration in this range and the production of a urine with chloride levels similar to those of the environment, but with some chloride leakage in hypotonic fluids. Blood chloride regulation failed in all three species when exposed to distilled water for 24 hours, and in land crabs and ghost crabs exposed to 720 mM. Cl/L. for about 24 hours. Mangrove crabs survived 72 hours in 720 mM. Cl/L. fluid with regulation intact, but could not survive 24 hours in air.

5. On dry sand, land crabs filter across the antennal gland a volume equal to their inulin space in 60 hours. It also re-absorbs most of the water of the urine thus formed. This is not true of ghost crabs in which the formation of urine appears to depend on water gained during brief nightly exposures to the surf. When exposed to 600 mM. Cl/L. sea water, their normal habitat, mangrove crabs filter their inulin volume in 24 hours. There is an apparent correlation between these filtration rates and the availability of water in the habitat.

6. Antennal gland filtration and re-absorption rates are adequate to account for the rate of chloride loss in mangrove crabs in distilled water. This is not true for ghost crabs and land crabs in which filtration rates are not much faster than those on sand. Electrolytes are escaping across some other membrane, supposedly gills and, perhaps, branchial epithelium. The loss of electrolyte by a route other than the antennal gland is also apparent in animals of all three species exposed to environmental fluids from 120 to 720 mM. Cl/L.

7. Re-absorption of chloride by the antennal gland of ghost crabs and mangrove crabs exposed to hypotonic fluids and of water in animals exposed to hypertonic fluids is apparent from the similarity between urine and environmental chloride concentrations. Similar re-absorptions can be inferred from data presented on land crabs.

8. The similarity of the mechanisms and thresholds involved in antennal gland function is indicated by (1) the approach of urine chloride concentrations to the blood chloride levels when ghost crabs and mangrove crabs are exposed to environmental fluid chloride levels 100 mM. Cl/L. less concentrated than the blood, and (2) the similarity in filtration rates in all three species when animals are exposed to environmental fluids which are near isotonic to their own blood chloride concentrations.

9. The blood concentrations of SCN absorbed from 120 to 720 mM. Cl/L. environmental fluids tend to plateau, due to equating of inbound and outbound ion passage, at a point roughly equal to the ratio between blood chloride and environmental chloride levels. The point of plateau is reached more slowly in hypotonic situations indicating the difficulty of handling the required volume of environmental fluid. The persistence of electrolyte loss, even in situations where ions must be rapidly absorbed to maintain constancy, is indicated by the SCN loss rate curves for the various environments.

10. The rates of turnover of water and electrolyte are as remarkable as the constancy of the regulation from which they result and for which they are responsible. The effectiveness of this regulation in mangrove crabs and the maintenance of a concentration gradient in land crabs can be related to the successful adaptation of these two species to totally different habitats.