A Physiological Comparison of Bivalve Mollusc Cerebro-visceral Connectives With and Without Neurohemoglobin. III. Oxygen Demand

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Abstract
Several bivalve mollusc species possess neurohemoglobin in their nervous systems whereas most species do not. The cerebro-visceral connectives of Tellina alternata and Spisula solidissima with neurohemoglobin and Tagelus plebeius and Geukensia demissa without neurohemoglobin exhibit similar electrical characteristics dictated mostly by axon size (0.3-0.4 µm mean axon diameter, Kraus et al., 1988). Action potential conduction is sensitive to a depletion of both ambient and neurohemoglobin-bound oxygen. Connectives without neurohemoglobin and connectives with carbon monoxide neurohemoglobin ceased to conduct action potentials within 5-10 minutes after exposure to anoxic conditions. Connectives with neurohemoglobin conducted action potentials throughout the time course of neurohemoglobin deoxygenation, lasting 20-30 mintues.Connectives without neurohemoglobin exhibited an approximate five-fold elevation in oxygen consumption rate during action potential conduction, as predicted by axon diameter. However, connectives with neurohemoglobin consumed only ⅓ this amount of oxygen during electrical activity. The mechanism for this increased efficiency in action potential conduction is unknown, but the rate of oxygen consumption nearly matches the rate of neurohemoglobin oxygen unloading in situ. An operational aerobic nervous system might enable animals to maintain neuromuscular activity during hypoxic or anoxic conditions.

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