Evidence is reviewed showing that biotic factors influencing the relative utilizations of absorbed protein and energy in marine mussels and other organisms may either be metabolic or dietary. Metabolic factors include genotype, size and nutritional/physiological state, which each influence the efficiencies with which absorbed ration is utilized for maintenance metabolism. Dietary factors comprise quantitative and qualitative variations in food intake, which influence the relative efficiencies with which absorbed protein and energy are utilized both for maintenance and for growth. Effects of metabolic factors on the relative utilizations of protein and energy for maintenance stem largely from their influence on whole-body protein turnover (WBPT), defined as the continuous breakdown and renewal of cellular proteins. WBPT incurs high energy losses due to the costs of protein synthesis, whereas up to 89% of the amino-N derived from protein breakdown may be recycled directly to protein synthesis. Consequences of different WBPT are therefore evidenced primarily in terms of maintenance requirements for energy, rather than protein. Further, regardless of dietary intake, significant WBPT results in consistent conservation of the absorbed protein relative to energy. On this basis, we suggest that mussels and other sedentary filter-feeders are more likely to be growth-limited by available energy than by protein. We also predict that genetically heterozygous individuals or populations characterized by reduced energy requirements for maintenance will be least affected by nutrient limitation, and will achieve faster growth on diets containing smaller proportions of non-protein energy. We suggest that metabolic requirements may vary according to genotype, dietary composition and nutritional/physiological status. This signifies that ranges of values, both in absolute and in qualitative terms, will prove most useful for defining nutrient requirements. Average protein requirements for maintenance in mussels (87.3 ± 29.06 mg protein N per kg 0.75 dry weight d −1) are nevertheless similar to those published for fish and mammals, so that absolute differences between the metabolic requirements of poikilothermic marine organisms and homeotherms appear restricted to energy.