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Mussels were fed four concentrations of seston (between 0-99 and 10-3 mg total seston I 1 ), comprising three levels of organic content (71 -9,63-6 and 40-8%), made up from natural silt and two species of cultured phytoplankton. Two of the seston concentrations were below, and two above, the threshold at which pseudofaeces were produced. Measurements of physiological traits (filtration rates, pseudofaeces production, selection efficiency, absorption efficiency, absorption rates and rates of oxygen consumption) were made after 2 days and, for two of the seston concentrations, also after 12 days. When fed at a high concentration of seston of low organic content, the mussels increased their filtration rate, rejected a higher proportion of filtered material as pseudofaeces, and increased the efficiency with which filtered matter of higher organic content was selected for ingestion; this resulted in a constancy of the relationship between ingestion rate and the concentration of particulate organic matter, regardless of differences in seston organic content. Between 2 and 12 d, the mussels increased absorption rates for organics, primarily by increasing absorption efficiency, both for total organics and for the carbohydrate component of the diet. We suggest that these responses to changes in the food environment comprise physiological adjustments which result in higher net rates of absorption than would be predicted from considerations only of the organic/inorganic ratio of the suspended particles and assumptions of a non-compensating feeding behaviour.