Functional morphology of the larval tentacles of Phragmatopoma californica (Polychaeta: Sabellariidae): composite larval and adult organs of multifunctional significance

Abstract

Larvae of the sabellariid polychaete Phragmatopoma californica (Fewkes), which were collected in San Diego, California and were competent to metamorphose after 18 to 30 d of development were observed in vivo by videoequipped light microscopy, and the fine structure of the larval tentacles was examined by transmission and scanning electron microscopy. Each tentacle has tufts of at least two types of immotile cilia arranged in dorsolateral and ventrolateral rows, and a ventral groove covered by two types of motile cilia that beat independently of each other. The epidermis is regionalized into glandular, sensory, locomotory, and support cell types and contains four longitudinal bundles of basiepithelial nerve fibers. A layer of connective tissue separates the epidermis and the nerve tracts from obliquely striated muscles that occur within the peritoneum that lines the central coelomic cavities. The peritoneum forms an intact coelomic epithelium that overlies and interdigitates with the muscle cells, with no intervening basal lamina. The muscle cells are considered to be intraperitoneal because they are located above the basal lamina and they lack intercellular junctions with the peritoneal cells. Specialized peritoneal cells form a striated myoepithelial blood vessel that partitions the coelom into medial and lateral cavities. No neuromuscular junctions were found, but both muscular and ciliary movement seem to be under neuronal control. The basiepithelial nerve terminals appear to synapse into the connective tissue layer toward the intraperitoneal muscle. Several similarities in tissue organization are noted between the larval tentacles of P. californica and the tube feet of echinoderms. Observations on the ontogeny, morphology, and behavior of the tentacles suggest that they are multifunctional organs involved in feeding, construction of the juvenile sand tube, locomotion, attachment, and sensory perception during larval and adult lives.