Gas Exchange, Hemolymph Acid-Base Status, and the Role of Branchial Water Stores during Air Exposure in Three Littoral Crab Species

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Abstract
O₂ uptake and hemolymph acid-base status, together with branchial water volume, CO₂ content, and titratable alkalinity, were measured in three species of intertidal crabs. In Pachygrapsus crassipes, a grapsid crab that actively moves between air and water, O₂ uptake increased on emersion. In Eurytium albidigitum, a mud-burrowing xanthid crab that is air exposed by tidal action, O₂ uptake declines dramatically on emersion. There is no significant lactate production by either crab following emersion. An emersion-induced respiratory acidosis was fully compensated in P. crassipes and another grapsid, Hemigrapsus nudus, but uncompensated in E. albidigitum. Branchial water volume 10 min after emersion was 0.013 ml/g crab weight in P. crassipes and 0.072 ml/g in E. albidigitum. The CO₂ content of branchial water in P. crassipes increased rapidly during air exposure and was accompanied by an increase in titratable alkalinity (TA). The CO₂ content of branchial water in E. albidigitum remained constant for at least 4 h and increased slightly after 8 h. TA remained unchanged for up to 8 h. We suggest that the ability of the crabs to compensate for the respiratory acidosis and to increase branchial water TA is correlated with osmoregulation in P. crassipes and H. nudus. On the other hand, E. albidigitum is an osmoconformer and neither compensates for the respiratory acidosis nor changes its TA during air exposure. Possible adaptive advantages of the two different strategies may be related to the relatively short duration of emersion and active habits of P. crassipes and the longer periods of air exposure and inactivity of E. albidigitum.