For the tirst time for any benthic filter-feeder, this study documents each component process of nutrient acquisition through natural tidal variations of food availabillty. The organic content of seston available during both neap and spring tides in the bay of Marennes-Oléron. France, decreased from 28 to 8 % with mcreasing seston concentration from 10 to about 90 mg total particulate mass 1(1). Throughout this tidal variation, the blue mussel Mylilus edulis L. cleared more water of particles as seston availability increased. Rejection of fillered material as pseudofaeces prior to ingestion remained a constant fraction of about 0 .93 x the mass of filtered material, so that ingestion rate showed no signs of stabilising at even the highest food availabillties. We confirm that M. edulis may preferentially reject inorganic matter with pseudofaeces. More significant was the novel observation that the net selection efficiency with which filtered organics were selectively retained for ingestion increased rapidly with the rate at which seston was filtered, this increase being faster for seston of higher organic content. The result was that the organic content of ingested matter was enriched by up to 5 times the organic content of filtered particles. Further, net absorption efficiency for ingested organics varied in strong positive relation with the organic content of ingested material. Therefore, rates of organic absorption increased with seston filtration rate, and net energy balance increased despite the decreasing organic content of particles available at higher concentrations. These collective find ings demonstrate continuous interrelated changes in feeding physiology that help to maintain rates of nutrient acquisition independent of short-term fluctuations in seston composition.