Nature of relation between ventilation and oxygen consumption in filter feeders

Abstract

Correlations between rates of ventilation and oxygen consumption in filter feeders are usually interpreted in terms of causal relations, but authors disagree on whether ventilation is regulated to meet respiratory needs or whether oxygen consumption reflects energetic costs of water transport. It seems, however, that neither causality applies, the correlations being determined by physical characteristics of the ventilatory systems. The ventilatory currents both in a tube dwelling worm, Urechis caupo, and in bivalves, such as Mytilus edulis, are laminar, and oxygen in the water is available to the animals solely by diffusion. Various estimates of the diffusive boundary layer 6, developing at the interface between the ventilatory current and the body surface of U. caupo and mantle cavity in M. edulis, were in good agreement. The estimates of 6 amounted to about 0.1 mm, or a small fraction of the thickness of the ventilatory current, indicating that the larger part of the dissolved oxygen in the current is unavailable to the animals. At low rates of ventilation oxygen uptake is governed by the condition of diffusion in the ambient water. Oxygen consumption increases steeply with increasing ventilation until threshold levels are reached, and processes within the body become limiting for the rate of oxygen consumption. Oxygen consumption approaches independency of ventilation at ventilatory rates amounting to only one-fifth or less of the rates of the undisturbedly filtering animal. Regulation of respiration by means of ventilatory regulation thus seems incompatible with filter feeding in habitats that require high rates of water processing. The energetic costs of ventilation are insignificant. Presumably, in most habitats, water processing at low costs is a prerequi- site for a life based on filter feeding.