The effects of velocity and seston concentration on the exhalant siphon area, valve gape and filtration rate of the mussel Mytilus edulis

With the inhalant siphon facing into the flow, and with adequate seston levels, water velocity has a significant negative linear effect on mussel exhalant siphon area, but no significant effect on valve gape. Mussel filtration rates of polystyrene beads, measured by ingestion, were three times less at 30 cm s −1 than at 10 cm s −1, and they mirrored the trends observed with the exhalant siphon area. However, with the inhalant siphon oriented downstream of flow direction at the two higher flow speeds (20 and 30 cm s −1), there was no significant effect of velocity on exhalant siphon area. There was also a significant positive effect of particle concentration on mussel exhalant siphon area. In contrast to the effects of velocity, mussel valve gape responses to increasing particle concentrations mirrored the responses of the exhalant siphon aperture. The thresholds for the initiation of active pumping, opening the valve gape, extending the mantle and opening the exhalant siphon are at minimum seston levels of about 4×10 3 particles ml −1, or particle volumes of about 1.0 mm 3 l −1. Thus, the closure of valves in the relatively non-turbid waters of Maine indicates insufficient food particle concentrations for feeding. Increases in exhalant siphon area caused by particle volume increases from 1 to 5 mm 3 l −1, or particle concentrations from 6×10 3 to 4×10 4 particles ml −1, indicate that blue mussels respond to increasing ambient food concentrations by increasing their pumping rates. These results demonstrate that exhalant siphon area has potential for the quantitative remote sensing of feeding rate, and that valve gape is a more indirect measure of activity with respect to minimum concentrations for the initiation of feeding. Maintaining an open valve gape with partial or full closure of the exhalant siphon under high flow conditions is similar to the response of blue mussels to changes in salinity, and is interpreted with respect to increased oxygen diffusion for respiration.