Aspects of feeding, including estimates of gut residence time, in three mytilid species (Bivalvia, Mollusca) at two contrasting sites in the Cape Peninsula, South Africa.

Individuals of three mytilid species (Choromytilus meridionalis; Perna perna; Aulacomya ater) from two sites characterised by different qualities of ration available to these suspension feeders showed different rates in some components of the physiological energy budget. These differences included higher feeding (=clearance) and respiration rates, but lower absorption efficiencies, in individuals from the site with the higher quality ration. A novel technique was employed to estimate the residence time of food particles in the digestive gland. Individuals showing higher feeding rates had shorter residence times than those feeding more slowly and a significant positive correlation was demonstrated between residence time and absorption efficiency. These relationships, together with an exponential increase in rates of respiratory heat loss with an increase in ingested ration, are suggested to provide these animals with a physiological flexibility to compensate for reduced food quality in a way consistent with some theoretical predictions. Such compensations are made more effective if the total gut capacity can also change in response to the quality of the ration.