The saxitoxin-producing dinoflagellate Alexandrium minutum is a well-known microalga that causes paralytic shellfish poisoning (PSP) in many coastal regions of the world. In this study, we measured the valve movements of cultivated mussels Mytilus galloprovincialis feeding on toxic A. minutum (n = 29 mussels, shell length = 67.1 +/- 3.2 mm, (x) over bar +/- SD) or a morphologically-similar, but toxin-free phytoplankton, Heterocapsa triquetra (n = 24 mussels, shell length = 68.3 +/- 2.9 mm). Phytoplankton inoculations were conducted in three sequential "pulses" intended to increase microalgal cell concentrations in a stepwise manner up to similar to 5000 cells 1(-1) over a 9-h period. M. galloprovincialis was connected to a non-invasive valvometry apparatus that concurrently measured the magnitude of valve openness once every 0.1 s. It was found that M. galloprovincialis tended to keep its valves open over the course of the experiment, regardless of the phytoplankton species present in water. Standard valve opening metrics, such as the opening duration and opening amplitude, were not significantly affected by the species of phytoplankton. However, the frequency of brief and partial valve closures (microclosures) was significantly influenced by phytoplankton species (P < .01). M. galloprovincialis subjected to toxic A. minutum exhibited 20.3 +/- 0.4 (<(x)over bar> +/- SEM) microclosures per 3-h pulse period, whereas those exposed to the control H. triquetra exhibited 7.9 +/- 0.4 ((x) over bar +/- SEM) microclosures. This response was detectable over the 3 h following the first inoculation pulse that provided a phytoplankton concentration of 1000 cells 1(-1). Our findings are consistent with growing evidence that bivalves are sensitive to very low concentrations of harmful microalgae. Deploying in situ valvometry sensors with real-time monitoring capabilities may provide an early warning of harmful algal blooms.