ENERGETICS OF THE VENTILATORY PISTON PUMP OF THE LUGWORM, A DEPOSIT-FEEDING POLYCHAETE LIVING IN A BURROW

The aim of this study was to tentatively estimate the energy cost of b reathing in the lugworm, Arenicola marina (L.), a gallery-dwelling, pi ston-pump breather that moves water in a tail-to-head direction. Each tested lugworm was placed in a horizontal glass tube. The caudal end o f the tube was connected to a well-aerated seawater reservoir at 20 de grees C, and the cephalic end attached to a drop meter through a tube resistance. At the exit of the cephalic chamber the O-2 tension was re corded via an in situ O-2 electrode, and the hydrostatic pressure of t he exhaled water was also recorded. Water now rate, total O-2 uptake r ate M(O2)(TOT), O-2 extraction coefficient, and the mechanical power n ecessary to pump water through the resistive anterior exit of the appa ratus (W-MEC), were computed. The basal metabolic rate of each animal (M(O2)(CONF)) was separately estimated by the confinement method. M(O2 )(CONF) subtracted from M(O2)(TOT) approximates M(O2)(CB), the O-2 upt ake rate necessary to activate the piston-pump breathing mechanism and to ensure the corresponding mechanical work rate, W-MEC. The results show that the energy cost of breathing, M(O2)(CB), of the piston-pump- breathing Arenicola is very high, with mean values approximating 47% o f the M(O2)(TOT) value; that the mechanical power we measured,W-MEC, i s very low; and that the mechanical-to-metabolic efficiency, the ratio W-MEC/M(O2)(CB), does not exceed 1%. These observations are compared to those obtained in other piston-pump breathers, such as Chaetopterus variopedatus and Urechis caupo, and in ciliary filter feeders includi ng polychaetes, bivalves, and ascidians.