Filter-net structure and pumping activity in the polychaete Nereis diversicolor: effects of temperature and pump-modelling

Electron micrographs of the filter-net structure in the facultatively suspension-feeding polychaete Nereis diversicolor O. F. Müller showed that the net is composed of an irregular mesh-work made up of long, relatively thick filaments (up to 300 nm) interconnected with a variety of shorter and thinner filaments. The thinner filaments range in diameter from 5 to >25 nm. The average size of the meshes, measured directly on the micrographs, lies between 0.5 and 1.0 μm, but due to shrinkage the values represent only about 75 % of the actual dimension of the intact net. The effects of temperature on water processing were measured as clearance of suspended algal cells or measured directly. Pumping activity (undulating body movements of worms kept in glass tubes) was monitored using an infrared phototransducer technique. In the temperature interval from 5 to about 15 °C there was a linear increase in clearance, and a doubling of the temperature was followed by a doubling in clearance. Direct measurements of pumping rate showed that high stroke frequency was correlated with high pumping rate. A doubling of temperature from 13 to 23-29 °C led to a doubling of the stroke frequency and a halving of the net cycle length. At low temperatures a tendency towards an extended pause between pumping periods was noticed. The N. diversicolor pump was modelled as a positive displacement leaking unit, and earlier data on back pressure characteristics were examined in light of the proposed model. The pump model prediction of temperature dependence showed reasonably good agreement with experimental data. The consequences of leaving out the effect of viscosity or stroke frequency in the pump model were evaluated. Clearly, the temperature effect as related to viscosity is negligibly small in the N. diversicolor muscular positive displacement pump (unlike for viscous ciliary pumps).