Behavioral Regulation of Hemolymph Osmolarity Through Selective Drinking in Land Crabs, Birgus latro and Gecarcoidea lalandii.

Drinking behavior in Birgus latro and Gecarcoidea lalandii was videotaped under controlled laboratory conditions. B. latro displayed the drinking behavior typically observed in nature, spooning up water with the chelae (Lister, 1888; Gross, 1955). G. lalandii is documented for the first time displaying this same behavior; however it obtained water primarily through immersion. Under normal hydrated conditions (hemolymph osmolarities < 1050 mOsm) B. latro showed no preference for drinking fresh or seawater. When dehydrated (hemolymph osmolarities > 1050 mOsm) B. latro altered its drinking behavior and showed a distinct preference for freshwater. This strategy resulted in restoration of original hemolymph osmolarities and wet weights and was accomplished through periods of intensive drinking activity. Conversely, G. lalandii never experienced true dehydration; rather, the hemolymph became hyperosmotic compared with control animals. This species preferred freshwater both under normal and hemoconcentrated conditions. G. lalandii was also able to osmoregulate behaviorally and was able to restore hemolymph osmolarities to normal concentrations via immersion in freshwater following experimentally induced hemoconcentration. Possible physiological and ecological reasons for the differences in water uptake strategies and preferences are discussed.