NATURAL SEDIMENT AS A FOOD SOURCE FOR THE COCKLE CERASTODERMA-EDULE (L) - EFFECT OF VARIABLE PARTICLE CONCENTRATION ON FEEDING, DIGESTION AND THE SCOPE FOR GROWTH

Cockles Cerastoderma edule (L.) from a mud-flat estuarine population were exposed to variable concentrations of suspended bottom sediment to determine the physiological components of the energy budget and scope for growth, as a function of particle concentration. Associated processes of feeding and digestion were given particular attention, involving quantification of selective rejection of material in pseudofaeces and estimations of gut passage time of food and gut contents. Although clearance rate declined with increasing particle concentration, regulation of ingestion rate at high particle load was mainly achieved by pseudofaeces production. Two mechanisms contributed to maintain absorption efficiency of filtered material approximately constant throughout changing food concentration: (a) Preingestive selection of organically rich particles, resulting in adjustments of organic content in the ingested ration; and (b) reduced effects of ingestion on gut passage time of food, since changing gut contents provide an important mechanism for the adjustment of variations in ingestion rate. Scope for growth increased to reach an asymptotic value at food concentrations which correspond to a maximum of ingestion. Constraints on feeding rates thus appear as the main limiting factor for growth.