ON THE PHARYNGEAL FEEDING FILTER OF THE SALP PEGEA-CONFOEDERATA (TUNICATA, THALIACEA)

In Pegea, scanning electron microscopy of what appear to be the least damaged portions of the filter shows that it has a regular rectangular mesh consisting of thick (100 nm) fibres at right angles to thinner (50 nm) fibres. The rectangular pores of the filter are around 3.3 x 0.57-mu-m. These measured values from filters that have suffered shrinkage (to an uncertain degree) during preparation are considered to indicate that the actual pore size in life is some 4.0 x 0.7-mu-m. The mucous-net feeding filter of salps differs from that of other tunicates since flow through it results from muscular activity. Calculations based on the estimated pore size and filtering rate suggest that during part of the filtering cycle, the pressure drop across the filter is considerably greater than that across other tunicate mucous-net filters.