Many physiological processes in bivalves are influenced by the valve opening sate and thus the valve opening-closing response times of the experimental animals. Controlled laboratory studies using underwater video camera recording of valve-gape responses of mussels (Mytilus edulis) to presence or absence of algal cells in the ambient water have revealed that valve opening and closing responses are strongly influenced by the preceding feeding conditions. The observations indicate that the period during which mussels may “learn” or “forget” to respond to the presence or absence of algal cells can last for weeks in the laboratory. Further, the critical algal concentration below which mussels close their valves has been identified to be about 700 Rhodomonas sp. cells mL⁻¹, or 0.9 μg chl a L⁻¹. Such knowledge is essential for making allowance for possible deviation from natural-response times in mussels kept in the laboratory previous to experimental physiological studies. In situ simultaneous registrations of water flow, chl a and valve-opening degree of mussels were made within a dense mussel bed in a shallow, tidally dominated inlet to a Danish fjord. Phases of strong tidal flow brought ample supply of phytoplankton to the mussel bed, and it was observed that concentrations higher than about 1 μg chl a L⁻¹ in the near-bed layer stimulated the mussels to keep their valves wide open, whereas tidal slack with slow flow resulting in near-bed concentrations below 1 μg chl a L⁻¹ caused by grazing by the mussels, after some time led to shell closure, or reduced valve gapes of the mussels. The valve opening time from opening degree 50% to 100% and the valve closing time from opening degree 100% to 50% in response to variation in near-bed chl a concentration were on average 59 ± 22 and 50 ± 19 min, respectively, which is considerably faster than observed in unfed mussels in the laboratory.