Valve-gaping behavior of raft-cultivated mussels in the Ría de Arousa, Spain

We describe the valve-opening behavior of raft-cultivated mussels (Mytilus galloprovincialis) in the Ría de Arousa (Arousa estuary), Spain. Eight rope-grown mussels [mean ± standard error (SEM), shell length 61.6 ± 2.1 mm] were connected to a non-invasive valvometry apparatus that monitored (one measurement min−1) the magnitude of valve openness systematically over a 10 day period. It was found that valves were open 97.5 ± 1.3% percent of the time. Valve closures were not synchronized among the eight monitored mussels, suggesting that feeding cessation was physiologically-regulated rather than environmentally-mediated. The opening amplitudes that were most frequently observed were in the range of 60–90%, indicating that, when open, valves are usually opened relatively close to their maximum possible extent. The majority (7/8) of mussels displayed a circadian rhythm (τ = 24.0 h) in valve opening amplitude. They tended to exhibit maximum valve opening during nighttime and minimum opening during daytime. It is possible that the light:dark cycle represents an environmental zeitgeber entraining an endogenous gaping rhythm in this bivalve.