Development, morphology and ultrastructure of the branchial crown of Fabricia stellaris (Müller, 1774) (Polychaeta: Sabellida: Fabriciinae)

Randel, N. and Bick, A. 2011. Development, morphology and ultrastructure of the branchial crown of Fabricia stellaris (Muller, 1774) (Polychaeta: Sabellida: Fabriciinae). Acta Zoologica (Stockholm) 93: 409421. Sabellidae and Serpulidae are well-known tube-building polychaetes with a distinctive and often spectacularly colourful branchial crown. Morphological investigations suggest that these taxa form the monophyletic clade Sabellida, with the adelphotaxa Sabellidae and Serpulidae, but the relationship between these taxa remains ambiguous. Molecular investigations have indicated that the Fabriciinae, major taxon of Sabellidae, belongs to Serpulidae, thereby making Sabellidae paraphyletic; however, morphological characters are absent to support this result. We investigate the development, anatomy and ultrastructure of the branchial crown of Fabricia stellaris (Muller, 1774), describing morphological characteristics useful not only for constructing morphological phylogenies but also for understanding the evolution of the branchial crown. The morphology of the radioles and pinnules does not differ from each other. The supporting tissue of the branchial crown consists of myoepithelial cells and a solid extracellular matrix (ECM). Both ciliated and non-ciliated cells form the epidermal layer; ciliated cells shape the food groove. Most important is the result that radioles and pinnules within Sabellida may not be homologous, because the morphology and the branching of radioles and pinnules are completely different between Sabellinae, Fabriciinae and Serpulidae. The terms primary branch for radioles and secondary branch for pinnules are proposed for Fabriciinae. The phylogeny of the Sabellida is discussed.