Uptake of Interstitial Water from Soil: Mechanisms and Ecological Significance in the Ghost Crab Ocypode quadrata and Two Gecarcinid Land Crabs

Abstract

Mechanisms by which terrestrial crabs take up interstitial water from soil, limiting conditions and rates of uptake, and their ecological relevance in nature were examined in the ghost crab Ocypode quadrata. Comparisons were made with two species of land crabs (Gecarcinus lateralis and Cardisoma guanhumi). Rapid rehydration, by bulk uptake of water, occurs in desiccated individuals of all three species when sufficient moisture is present in soil, regardless of osmotic gradient. Slow rehydration from hypoosmotic soil moisture also occurs, particularly in G. lateralis, even when the water content is below the threshold for bulk uptake. Ocypode quadrata has the lowest threshold (<5% water) for bulk uptake from damp sand. Water is collected from soil spaces by capillary tufts of setae, drawn into the gill chamber by suction up to 76 mm Hg, and at least some of it passed to the mouth and swallowed. This mechanism provides water ad lib. anywhere the water table lies near the surface of sandy soil. The highly terrestrial land crab G. lateralis is less capable of producing suction and of extracting soil water in bulk. In this crab's natural habitat, burrow soil never contains sufficient water for bulk uptake to contribute significantly to water balance, although gradual "osmotic uptake" may be important. Cardisoma guanhumi, whose burrows contain a pool of water, presumably need not rely on uptake of soil water. Nevertheless, it possesses structures similar to, and soil-water extraction capabilities superior to, those of G. lateralis. The great terrestriality of the land crabs, particularly G. lateralis, cannot be attributed to highly developed mechanisms for taking up bulk water from soil. These species meet the challenge of water balance on land by relying on conservation, while ghost crabs emphasize water uptake.