Mulinia edulis and Mytilus chilensis are commercially important filter feeding bivalves, which occur in intertidal benthic habitats of southern Chile. These environments are subject to large fluctuations in quantity and quality of suspended particulate matter (seston), due mainly to the resuspension of benthic sediments by the action of winds and tides. In spite of variations in the availability of suspended food, these species demonstrate rapid growth rates during most of the year. Laboratory tests feeding were carried out with different artificially prepared diets in order to understand the physiological compensations effected by these bivalves in response to fluctuations of suspended particulate matter. A total of 48 different combinations of microalgae ( Isochrysis galbana) and natural sediment collected from the intertidal zone were tested, having concentrations ranging from 1.9 to 203 mg l −1, and containing 18–91% organic matter. Absorption, oxygen consumption and excretion were measured in both species for each experimental diet in order to calculate the physiological index scope for growth. Temperature was maintained constant at 14 °C and salinity at 30‰. Both species demonstrated a high degree of physiological plasticity, which allowed them to maintain a positive scope for growth under most dietary conditions. The epifaunal species M. chilensis was better adapted to an environment having intermediate quantities of seston (50–150 mg l −1 and 40–60% organic matter); its scope for growth was high due to its high rate of energy absorption. The infaunal M. edulis was not well adapted with very low concentrations and quality of food (<3 mg l −1 and <25% organic material), and had a negative scope for growth under these conditions due to a low absorption rate and high energy losses through oxygen consumption and excretion. However, this species was well adapted to high concentrations and/or high quality of seston (>100 mg l −1 and 60% organic matter) where it showed a higher scope for growth due to an increase in absorption rate. M. chilensis showed better scope for growth in comparison with M. edulis, explainable by its greater feeding capacity, suggesting that the epifaunal species has an adaptive advantage through being able to better compete for available food, especially when there are low concentrations of seston in the water.