Crab Gill Intra-epithelial Carbonic Anhydrase Plays a Major Role in Haemolymph Co2 and Chloride Ion Regulation

The distribution and function of the enzyme carbonic anhydrase in the crab Pachygrapsus crassipes was investigated. Carbonic anhydrase was found in gill epithelial tissue but not in the haemolymph, muscle, heart, hepatopancreas or gonads of male crabs. Enzyme activity was completely inhibited in vitro by 2 x IO~M sodium acetazolamide (Diamox). Radiolabelled Diamox (2 x IO~M) in sea water was found to equilibrate with the intact crab's haemolymph within one hour. Haemolymph CO2 content, pH and chloride ion concentration were measured in crabs acclimated to different salinities and exposed to 2 x IO~M Diamox. Haemolymph CO2 content increased at all salinities, especially low salinities, while pH remained unchanged, except at low salinities where it increased. Diamox impaired the regulation of haemolymph chloride at low salinities, but had no effect on chloride regulation at high salinities. Measurement of O2 uptake (estimating CO2 production) in crabs before and after Diamox exposure confirmed that elevated haemolymph CO2 was not due to increases in tissue CO2 production.