Physiological Responses to Air Exposure: Acid-Base Balance and the Role of Branchial Water Stores

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Abstract
SYNOPSIS. Intertidal organisms usually exhibit one of two behaviors when they are air exposed. They either isolate themselves from the aerial environment or they interact with the aerial environment. Among the animals using the first behavior, body fluid acid-base balance is partially maintained by dissolution of the calcium carbonate shell in the bivalve molluscs to buffer the metabolic acids produced anaerobically. Calcium ions compensate for the acidosis by increasing the strong ion difference. The release of carbonate from the shell causes an increase in molecular CO2 which offsets the compensation somewhat, but this effect is minimized by distributing the CO2 among the other fluids within the shell and/or by venting the shell to the air. In animals which have a fairly high concentration of a respiratory pigment, such as a lugworm, an anaerobically induced acidosis is minimized by a large Haldane effect. Among the animals which interact with the aerial environment, the decapod crustaceans by and large maintain their metabolism aerobically, although it may be greatly reduced. A respiratory acidosis due to elevated hemolymph Pco2 may be either fully compensated or not at all. Compensation involves an increase in hemolymph calcium, probably from the calcium carbonate exoskeleton. Compensatory mechanisms may also include branchial water stores which accumulate a titratable base. It is suggested that the alkalinization of the branchial water maintains a steeper Pco2 gradient across the gill and reduces the magnitude of the acidosis for a short period of time. The ability to use branchial water stores in this way may be tied to the ability of the animal to osmoregulate.

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