Differential absorption of biochemical components in the diet of the cockle Cerastoderma edule: Enzymatic responses to variations in seston composition

Cockles (Cerastoderma edule (L.)) were fed different diets composed of freshly collected natural sediment and cells of Tetraselmis suecica mixed in different proportions. The experimental diets were designed to reproduce a simultaneous increase in both food quantity and organic content such as that brought about by algal blooms in either phytoplankton or microphytobenthos. Clearance, ingestion, and absorption rates, absorption efficiencies of specific biochemical components, and amylase, cellulase, laminarinase, and protease activities of the digestive gland were measured after 3 days of exposure to the diets. As food availability rises, net absorption rates are maximized by means of two mechanisms: (i) feeding rates are adjusted, with resulting regulation of the total amount of food entering the digestive tube, and (ii) rates of digestive investment contributing to digestion are varied according to food quality. The mass of the digestive gland, as well as specific and total cellulase activities, show positive correlation with the organic content of the food, which results in improved absorption of carbohydrates from high-quality diets. The costs incurred would be mainly in the form of increased metabolic faecal losses, which may explain the recorded negative effects on net absorption of lipids.