<p>The continuous decline in size (age)-specific growth rate over the life span is an important feature of the life history of many animal populations. Differential size scaling of physiological rates has been invoked to account for such negative allometry in most currently available growth models. In this study, both feeding rates (for energy gain) and metabolic rates (for energy expenditure) were determined in two congeneric clam species using specimens spanning a wide size range covering the ontogenetic growth phases following spat settlement. Mass-scaling allometric functions were fitted to these measurements, both for interspecific comparisons and for intraspecific comparisons between age groups, and the results were incorporated into Scope for Growth models. Overall, growth simulations show nearly isometric trends in the juvenile (seed) phase, which become clearly allometric in the adult, while growth of the grooved carpet shell (GCS) clam appears to be more size constrained than that of the Manila clam, especially in the seed phase. The GCS clam was a more efficient filter feeder, exhibiting higher gill pumping performance (clearance rates per unit gill area) compared to the Manila clam, which instead had a more suitable gill to labial palp ratio, allowing for better pre-ingestive selection of high food value particles.</p>