SUSPENSION-FEEDING BY THE LOPHOPHORE-LIKE APPARATUS OF THE PTEROBRANCH HEMICHORDATE RHABDOPLEURA-NORMANI

Selective forces acting on feeding mechanisms have played an important role in the evolution of metazoans. To develop a more accurate understanding of the evolution of feeding within the deuterostome and lophophorate clades, I used video analysis and electron microscopy to examine suspension-feeding behavior and ciliation patterns in Rhabdopleura normani, a pterobranch hemichordate. In R. normani, local reversals of ciliary beat capture food particles, and the normal feeding activity does not involve the use of mucus to capture particles. Also, several different methods of particle rejection are identified. Lateral, frontal,and fronto lateral ciliary bands, which are composed of cilia 8-13 mu m in length, are present on the tentacles. In R. normani from Bermuda, ciliated perforations occur along the length of the arms, specifically between the bases of adjacent tentacles. These structures are previously undescribed in suspension-feeding organisms. My findings are consistent with the hypothesis that the tentaculated arms of pterobranchs are homologous to the lophophores of brachiopods, phoronids, and bryozoans. The similarities in the feeding biology of these groups are discussed.