SUSPENSION FEEDING IN THE POLYCHAETE NEREIS-DIVERSICOLOR

Properties of the pump and filter system were studied in the occasionally suspension-feeding polychaete Nereis diversicolor. Feeding behaviour was observed on worms in glass tubes immersed in seawater. When algal cells were added to the surrounding water the worm made a funnel-shaped net-bag through which water was pumped by means of vigorously undulating movements of the body. Particles suspended in the inhalant water were retained by the net and later swallowed by the worm. The times spent on net-spinning, water-pumping through the complete net bag, and net-eating were: 97 +/- 35, 191 +/- 58, and 16 +/- 4 s, respectively. Stereotyped suspension-feeding behaviour was continuously recorded for 10 h by means of a computer-aided monitoring system. The volume of water cleared of algal cells (Rhodomonas baltica, diameter = 6.3-mu-m) per unit time (clearance) was measured in worms of different size. Clearance (C, mu-l s-1) as a function of body dry wt (W, mg) was: C = 3.52 + 2.46 W (17-degrees-C). The clearance of 6.3-mu-m algal cells was 86% of directly measured pumping rate, while the clearance rates of 7.5 and 13.1-mu-m particles were identical with the directly measured pumping rate, thus indicating 100% retention efficiency of particles > 7.5-mu-m. For smaller particles the retention efficiency rapidly decreased to about 30% for 3-mu-m particles. In order to characterize the Nereis pump the relation between imposed hydrostatic back pressure and pumping rate was measured in a number of worms. In several cases the pumping rate was not influenced by the imposed back pressure up to 3 mm H2O; but at higher back pressures the pumping rate was reduced simultaneously with a reduction in the undulation frequency. The maximum pressure head was around 8 mm H2O. It was found that N. diversicolor pumps about 401 of water per ml oxygen consumed, and the worm thus fulfills conditions for subsisting exclusively as a suspension feeder. The population pumping rate at the collecting site (2400 ind. m-2) was estimated to 9.8 m3 d-1 or 10 times the whole water column daily. This indicates that N. diversicolor is a hitherto undervalued key organism in the control of phytoplankton production in shallow brackish water areas.