Properties and energy cost of the muscular piston pump in the suspension feeding polychaete Chaetopterus variopedatus

Abstract

The energetics of the muscular piston pump were studied for the filter-feeding polychaete Chaetopterus variopedatus. Agreement between clearance rates and directly measured pumping rates showed that worms transferred to glass tubes may filter at rates comparable to those of the worm in its natural tube. The respiration rate (R, .mu.l O2 h-1) as a function of dry weight (W, mg) was: R = 1.90 W0.59. Water-processing capacities of 25 to 50 l of water filtered per ml O2 consumed were found for a ''standard'' 50 mg dry weight worm pumping 150 to 300 .mu.l water s-1. The relation between imposed hydrostatic back pressure, .DELTA.H12, and pumping rate, P, (the back pressure-pumping rate characteristic) was measured, and the maximum pressure head, .DELTA.H120, varied between 5 and 8.6 mm H2O. Video recordings were used for analysis of the pump which operated by a ''positive displacement pump mechanism'', i.e.: P = A f Ls, where A is the effective piston area, f = stroke frequency of parapods, Ls = stroke length of parapods. The stroke volume of the parapods and f decreased with increasing back pressures, and usually the worms responded to the imposed back pressure by reversing themselves in the glass tube. The pump pressure was expressed as: .DELTA.Hp = .DELTA.Hs (system resistance) = .DELTA.Hm (pressure loss in mucous net-bag) + .DELTA.Hk (loss of kinetic energy in terminal constrictions of the tube) + .DELTA.Hf (frictional resistance in tube system) + .DELTA.H12 (back pressure). These components were examined and the pump characteristic was approximated by the expression: .DELTA.Hp = Cf1P(f)/P0(f) + CkP(f)2 + Cf2P(f) + .DELTA.H120 (1 - [P(f)/P0(f)]2), where P0(f) = A f Ls = (f/fmax)P0(fmax), fmax = maximal stroke frequency, P0 (fmax) = pumping rate capacity, Cf1 = mucous bag resistance constant, Ck = kinetic loss constant, Cf2 = frictional coefficient. In a maximally pumping ''standard'' worm, the total head loss from inlet to outlet was: 0.72 .DELTA.Hm + 0.64 .DELTA.Hk + 0.07 .DELTA.Hf = 1.43 mm H2O. The mechanical work done by the pump was 4.3 .mu.W (as compared to a total metabolic energy expenditure of R = 107 .mu.W). The maximal pressure rise and the total head loss in C. variopedatus are about 2 times higher than in bivalves and about 5 times higher than in ascidians. The pressure drop across the mucous net of the muscular pump of C. variopedatus is several times higher than across the mucous nets in ciliary pumps. The high total head loss allows C. variopedatus to maintain a relatively high flow velocity through a relatively small filter area.