On the filter-feeding of Doliolum (Tunicata: Thaliacea)

Abstract Filter feeding in doliolids is re-described. Feeding involves the dorsal spiral volute of the peripharyngeal bands, which rotates the filter in the pharynx so that particles inhaled are trapped between two layers of the mucous filter. The process is more complex than in salps and is akin to that found in young stages of the aplousobrach ascidian Clavelina . Feeding proceeds normally after removal of the brain and is controlled by a small visceral nervous system (linked to the brain by a small posterior visceral nerve). This visceral nervous system controls the activity of the cilia of the gills, peripharyngeal bandsand oesophagus, and the production of the feeding filter by the endostyle. Analysis of video records has shown that in the two life cycle stages examined, maximum particle entry velocity to the filter was 600 μm s − . Lateral area of the filter was estimated at 1.15 mm 2 hence mean flow velocity across the filter was up to 170 μm s − 1 . Direct measurements of the filter mesh have not been obtained: calculations are discussed of volumes filtered daily, and of mesh size (based on the pressure drops across the filters of ascidian tunicates). It is concluded that there are wide variations in the filtration rates of doliolids.