Water lung and body wall contributions to respiration in an echiuran worm

The modified hindgut of the echiuran marine worm Urechis caupo functions as a water lung and has been assumed to be a much more important respiratory surface than its thick, muscular body wall. We tested this assumption by measuring whole animal O2 consumption, hindgut O2 uptake, and hindgut ventilation activity simultaneously in unrestrained worms in artificial burrows from 25 to 300 Torr O2. Under experimental conditions the contribution of the hindgut to total O2 uptake is variable and strongly correlated to hindgut ventilatory activity. Over a PO2 range simulating that encountered in the natural environment, the hindgut accounts for approximately half of total O2 uptake on average. Under progressive hypoxic exposure total O2 consumption decreased by 50%, yet O2 conductance and extraction increased. The results suggest that the water lung function of the modified hindgut supplements O2 uptake across the body wall, and may be especially important during periods of high activity such as may occur during feeding and burrowing.