Evaporative Water Loss, Corporal Temperature and the Distribution of Sympatric Fiddler Crabs ( Uca) from South Texas

Abstract

Desiccation and thermal stress are among the primary factors limiting terrestriality in crustaceans. Water loss was estimated as weight change in five sympatric species of Uca from south Texas for periods up to 7 hr in dry air. Simultaneously, corporal temperature was measured with a thermocouple placed under the carapace. To estimate integumental permeability to water, 115 mm 2 portions of dorsal carapace were glued to U-shaped tubes containing a crab Ringer's solution. These were exposed to dry air and water permeability was estimated from weight change. In whole-animal studies, most rapid weight loss occurred in the first 5 min of exposure to dry air as the body temperature fell below ambient (25°C) in all species. The three most terrestrial species exhibited significant survival over more aquatic congeners after prolonged desiccation. The greatest rate of water loss was observed in Uca subcylindrica which lost 22.9 ± 3.0% body weight. Uca panacea and Uca spinicarpa lost 14.1 ± 1.6% and 18.5 ± 1.8%, respectively. Based on blood osmolarity changes, Uca longisignalis and Uca rapax were more resistant to water loss than Uca subcylindrica under these conditions. Water loss from sections of the dorsal carapace were highest in Uca spinicarpa (10.4 mg/hr/cm 2) and Uca longisignalis (8.9 mg/hr/cm 2). Uca subcylindrica and Uca panacea were intermediate (4.5 and 4.2 mg/hr/cm 2) while Uca rapax expressed the lowest value (2.9 mg/hr/cm 2). These observations support the notion that water loss can effectively lower body temperature in fiddler crabs. However, an inverse relationship between terrestriality and integumental permeability was not evident in these sympatric congeners. Ultimately a balance between physiological and behavioral mechanisms must be achieved for adaptation to the semi-arid habitats in south Texas.