Bimodal breathing in the estuarine crab Chasmagnathus granulatus Dana 1851- physiological and morphological studies

Chasmagnathus granulatus is an estuarine crab which actively moves from sub tidal to supratidal areas. To elucidate the possible existence of extrabran chial sites for aerial gas exchange, we measured respiratory and acid-base variables in animals with and without branchial water (controls and experim ental crabs. respectively) during air exposure. An histological study of th e branchiostegite was also performed. Throughout 4 h of emergence C. granul atus did not suffer venous hypoxia, even without branchial water. The rate of oxygen uptake (M-O2) was similar in both groups. The rate of carbon diox ide excretion (M-CO2) and the gas exchange ratio (R) significantly decrease d during emergence in both groups, with R significantly lower for experimen tal crabs. Consequently, CO2 was accumulated in the hemolymph. This variabl e stabilized after 90 min in control animals. but experimental crabs contin ued accumulating CO2. Histological study of the branchiostegites demonstrat ed the presence of an attenuated and greatly perfused epithelium facing the branchial chamber lumen, with a shortest diffusion distance of 0.5 mu m. S imple folds and lobulated projections increase the respiratory surface area . These results suggest that C. granulatus is a bimodal breathing crab, act ive both in water and air. When emerged, this species extract oxygen direct ly from air through branchiostegal lungs, but relies on branchial exchange to eliminate carbon dioxide. (C) 2000 Elsevier Science Inc. All rights rese rved.