Bivalve molluscs have a highly plastic feeding and growth physiology. The increasing availability of families artificially selected for faster growth has enabled physiological experiments to investigate the genetic basis for variable rates of growth. Fast growth is achieved by a combination of increased rates of feeding, reduced metabolic rates and lower metabolic costs of growth. In at least one species there is a trade-off between growth in protein and the storage of lipids that are utilized in gametogenesis. Energy requirements for maintenance are also higher in slow-growing individuals. Reduced costs of growth are due in part to increased efficiencies of protein turnover. Nevertheless, high protein turnover (and therefore high metabolic cost) may benefit fitness in the later stages of gametogenesis. Faster feeding rates do not impair flexibility in feeding behavior which compensates for changes in the food environment. Both inter- and intra-species differences in feeding behavior are evident and suggest possible constraints imposed by faster feeding on the efficiency of selection between food particles of different nutritional value.