Availability of salts is not a limiting factor for the land crab Gecarcinus lateralis (Freminville)

The land crab Gecarcinus lateralis (Freminville) is restricted to within ≈ 250 m of the shore on Bermuda. Previous work demonstrated that availability of water did not account for the exclusion of crabs from inland habitats. The crabs' inability to produce dilute urine led to the hypothesis that availability of salts becomes limiting away from the shore. In the laboratory, osmoregulatory abilities of land crabs were tested under several ecologically realistic conditions. Salts were provided in interstitial water of various salinities in damp-sand “burrows”, in “drinking water” of various salinities, or in natural and artificial foods. Crabs maintained hemolymph concentrations very well even when exposed to salinities < 1% seawater (SW), particularly when given a choice of how much contact to have with test salinities (“drinking-water” regime). Rates of ion loss were apparently extremely low, even when low-ion artificial food (flavored filter paper) was passing through the gut. In the field, potential sources of salts were compared with the crabs' osmoregulatory abilities. All available water (rain, dew) was very dilute, and there was no evident gradient in availability of salts at increasing distances from shore that could account for the limited distribution of crabs. Concentrations of ions in forage plants were also low, and also showed no gradient correlated with the crabs' range limit. Crabs that were ion-depleted and then fed vegetation from inland of the normal range showed significant recovery in hemolymph concentrations. We concluded that availability of salts is adequate beyond the existing range of the land crabs, and is not the range-limiting factor. This is paradoxical, given the limited osmoregulatory abilities of crab antennal glands, and raises the question of how land crabs accomplish such extraordinary conservation of ions; the physiological mechanism (extrarenal modification of urine) will be described elsewhere. Factors that limit how far inland land crabs live remain enigmatic; hypotheses for further work are presented.