FEEDING, PARTICLE SELECTION AND ABSORPTION IN COCKLES CERASTODERMA-EDULE (L) EXPOSED TO VARIABLE CONDITIONS OF FOOD CONCENTRATION AND QUALITY

Cockles Cerastoderma edule (Linne) were exposed to different food suspensions consisting in mixtures of sediment and Tetraselmis suecica Butcher. Several parameters of feeding behaviour, including rates of filtration, pseudofaecal production, ingestion and absorption, were analysed as a function of both particle concentration and composition. We demonstrated particle selection at the preingestive level and resulting preferential organic ingestion. The relative energetic benefits derived from these processes were evaluated by computing different indices of sorting and food enrichment. Cockles fed organically rich diets were found to regulate rates of food ingestion mainly by adjusting clearance rates in order to maintain filtration of suspended matter virtually independent from changes in particle concentration. Conversely, with diets consisting of particles low in food value, clearance rates were kept nearly constant and rates of ingestion were regulated by increasing pseudofaeces production. Relative advantages derived from pseudofaeces formation have been evaluated for different dietary compositions by using a Benefit Ratio. Higher benefits were associated with preingestive food selection in the case of suspensions where particles of low organic content constituted the main dietary component, which may help to explain the maintenance of high rates of filtration coupled with extensive pseudofaecal deposition observed in cockles fed on low quality diets.