Viscosity of seawater controls beat frequency of water-pumping cilia and filtration rate of mussels Mytilus edulis

The small-scale movements of cilia make the temperature-dependent viscosity of water a key physical/mechanical factor. In mussels and other suspension-feeding bivalves, bands of water-pumping lateral cilia produce the feeding current through the interfilamentary canals of the enlarged gill (which mainly serves as a water-pumping and food-particle collecting organ). Because the viscosity of seawater is inversely related to temperature, the linear increase of filtration rate with temperature in mussels cannot be explained solely by increased biological activity. To resolve the question of whether change in filtration rate with temperature in mussels Mytilus edulis is under mainly biological/physiological or physical/mechanical control, we conducted laboratory experiments on mussel-gill preparations stimulated with serotonin, and on intact mussels. In gill-preparations, the beat frequency of the water-pumping lateral cilia was measured both as a function of temperature and as a function of kinematic viscosity in dextran- and PVP-manipulated seawater at constant temperature. Data on kinematic viscosity of seawater and viscosity-manipulated seawater were converted to 'temperature equivalents' of seawater by measurement of the viscosities, and this showed that the effect of temperature on lateral cilia activity in gill-preparations is purely mechanical, controlled by the viscosity of the ambient seawater. Further, the ciliary beat frequency was found to depend on kinematic viscosity to the power -3/2 (f approximate to v(-3/2)), a relation we used to develop a mussel-pump model for the filtration rate of intact mussels vs. viscosity. The model is in agreement with filtration rates measured in intact mussels, suggesting that viscosity of seawater is the controlling factor for the beat frequency of lateral pump-cilia and filtration rate in mussels. According to the model, the mussel pump yields a constant power within the temperature-tolerance interval.