The bivalve pump

A filter feeding bivalve, represented by the mussel Mytilus edulis, is modelled as a pump, consisting of the bands of lateral cilia along the gill filaments, and a system of afferent and efferent canals. The relation between pump pressure ΔHp and pumping rate V was approached through studies of the relations between system resistances and V. The most important resistances distinguished were the frictional resistance in the canal system ΔHf, the exit loss ΔHex, and the back pressure ΔH₁₂, imposed on the pump as a means to study the performance characteristics of the gill system. Removal of the laterofrontal cirri from the flow path by treatment with serotonin did not measurably affect the frictional resistance, but the cirri presumably constituted a flow-rate-independent resistance ΔHlf, arising from their beating against the flow. Thus, in steady state, ΔHp = ΔHf + ΔHex + ΔH₁₂ + ΔHlf. ΔHf and ΔH₁₂ were linear functions of V, whereas ΔHex was a quadratic function of V. At values of V up to the maximum flow rates, ΔHf and ΔH₁₂ dominated the system resistance, and the pump characteristic was therefore approximately linear. The interfilament canals and the exhalant siphon presumably constitute the major resistances, in an optimally pumping 'standard' 35 mm mussel, thus constituting head losses of about 0.4 mm and 0.5 mm H₂O, respectively. ΔHlf amounted to ≤0.4 mm H₂O. The operating point at zero back pressure was estimated at about 1.4 mm H₂O. Removal of the laterofrontal cirri from the flow path drastically decreased the efficiency with which ≤6 μm algal particles were retained by the gills, but it had no or little effect on the retention of 14 μm particles, supporting previous indications that the laterofrontal cirri do not act as sieves of the through current, but rather as modulators in the process of particle retention. Power output of the 35 mm mussel at zero back pressure was 14 μW, only slightly lower than the maximum of 15 μW. The work done by the pump constituted about 1.5 % of the total aerobic metabolic rate of the mussel.