Crabs on the Run: Migration to Molecules - Understanding the Ecophysiology of the Terrestrial Christmas Island Red Crab
Steve Morris, University of Bristol
13 Jan 2006
The transition by decapod crustaceans from an ancestral aquatic lifestyle to terrestrial existence is accompanied by morphological adaptations to accommodate the change in respiratory media and the much reduced availability of water for osmotic and ionic regulation. These morphological adjustments include elaboration of gills as gas exchange organs, the evolutionary development of lungs and reorganization of post-renal urinary fluid flow as part of a kidney analogue. The management of respiratory, acid-base, ion and water balance is fundamentally different for air-breathing species and the morphological changes are accompanied by important physiological and biochemical adjustments in respiratory pigment function, muscle physiology and renal regulation. The decapod crustaceans appear to have adopted solutions more reminiscent of the vertebrates than of uniramian or chelicerate species. Christmas Island (Indian Ocean) is home to brachyuran and anomuran species which aside from a short larval phase are totally terrestrial. These are exemplified by Gecarcinids, the red crab Gecarcoidea natalis and the blue crab Discoplax hirtipes and by the robber crab Birgus latro. These species show seasonal behaviours and physiology, including the popularized breeding migration, dictated by the dry-wet season cycle. The seasonality in ecology is supported by differences in physiology underlain by changes in gene expression. Examples from these species serve to demonstrate both specific and general features that can be suggested as adaptive, illustrate parallel and convergent evolution to life on land, illuminate the presence of sophisticated control mechanisms, and provide some pointers as the antecedents of some the features.