The physiological processes involved in the growth process of mussels are explained by the energy balance equation in which growth is the difference between the energy absorbed (absorption) and the energy used (metabolic rate). Feeding behavior of mussels is influenced by the variation in quantity and quality of the food (plankton, organic matter and minerals) available.
Mussels are filter-feeders that filter the surrounding water and select food for ingestion. The eco-physiological processes of food uptake (Figure 1) can be summarized in the following processes: flow of water through the mantle cavity; retention of particles on the gills; particle sorting (with the unassimilated material being returned to the water column as pseudofeces); uptake of particles in the gut (with feces being material that is shed before nutrients are absorbed); and nutrient uptake. These processes allow mussels to sort and absorb food from the water.
To read more about this see: https://schelpdieren.com/2023/10/30/voedselopname-door-mosselen-in-de-waddenzee/