A Self-Cleaning Mechanism in the Operculum of Serpula Vermicularis L. (Polychaeta: Serpulidae)

Mucus-secreting cells are described in the opercular plate epithelium of Serpula vermicularis. At the light-microscope level the cells are characterized by large granular inclusions identified at the electron-microscope level as electron-dense vesicles. Histochemical analysis of the mucous cells gives positive reactions for both neutral glycoproteins and non-sulfated acid mucopolysaccharides. The presence of mucous cells in the opercular plate of S. vermicularis is assumed to reflect the homology of the opercular filament with the branchial filaments from which they evolved. It is suggested that mucus is secreted across the substantial opercular plate cuticle to form a thin investing layer. The significance of a mucous layer on the opercular plate is discussed and both mechanical and antibiotic functions are implicated in a self-cleaning (antifouling) role. It is further suggested that “advanced” serpulids have abandoned mucus secretion as a protective mechanism in favor of increasing sclerotization/keratinization and eventual calcification. It is suggested that opercular mucous cells could be used as an additional tool in resolving serpulid phylogeny.