Variability of feeding processes in the cockle Cerastoderma edule (L.) in response to changes in seston concentration and composition

Abstract

Physiological processes controlling food acquisition by the filter feeding bivalve Cerastoderma edule (L.) were quantified under a broad range of seston concentrations and compositions. Experimental diets consisted of suspensions elaborated by adding variable amounts of microalgal cells of different species (or sediment particles in one case) to natural sea-water. Clearance rates exponentially decreased with seston concentration, but the rate of reduction was higher with suspensions of high organic content. Pseudofaeces production appeared as a positive function of the rate of particle filtration; however, for a given filtration rate, more pseudofaeces were rejected when filtered matter had a low organic content. As a consequence, after an initial elevation, ingestion rate remained almost constant across particle concentrations. Pre-ingestive food selection enhanced the rate of particulate organic matter ingestion and this organic enrichment of ingested matter became more pronounced for diets of low food value, where most filtered matter was being rejected as pseudofaeces. Selection of particles at the pre-ingestive level was more efficient in terms of chlorophyll, revealing preferential ingestion of algal particles compared with the whole organic matter. Stronger selection for algae, however, was not evident in terms of preferential nitrogen ingestion, as compared with carbon, which was probably due to similar low values of the C N ratios in all experimental conditions. Absorption efficiency depended on the organic content of ingested matter according to an exponential, saturating function. In general, feeding processes of cockles appear well adapted to cope with elevations in particle concentration and simultaneous reductions in the food value of available seston that occur when resuspended bottom sediments constitute a significant fraction of particulate materials of the water column. Under these conditions, high rates of seston filtration and pseudofaeces production, together with preferential organic ingestion act to compensate for the dilution of organic matter in suspension and its detrimental effect on the rate of food absorption. However, this compensatory behaviour is not so efficient as to make absorption rate independent of the organic value of available particles. The organic content of resuspensible sediments may thus become the main determinant of food acquisition in cockles.