The cement apparatus of the stalked barnacle Pollicipes pollicipes

Abstract

Barnacles are sessile marine crustaceans that produce a proteinaceous cement to attach themselves to the underwater surfaces of natural or man-made structures. This work was carried out to characterize the cement apparatus and localize this structure in the peduncle of the stalked barnacle Pollicipes pollicipes. Light and transmission electron microscopy techniques were used for these purposes. Sections of the peduncle revealed a central core encircled by a thick layer of muscular tissue. The cement apparatus, formed by clusters of adhesive-secreting cells and ducts, was located in the central core embedded in an apical layer of connective tissue with a thickness of a few mm, just beneath the capitulum. The ovary was also found in the peduncular core, but most of it was located below the region containing the cement apparatus. The cytoplasm of adhesive-secreting cells was very rich in rough endoplasmic reticulum and also contained small electron-dense secretory vesicles. Histochemical methods applied to semithin sections showed that these vesicles were filled with proteins, being also PAS positive. The voluminous nuclei of these cells presented diverse and irregular shapes, typically with very thin and long extensions, containing large nucleoli and a great amount of heterochromatin. Electron-dense inclusions with a concentric lamellar structure were also seen in these nuclei, resembling nuclear inclusions reported in fish oocytes. These cells presented the features of very active protein-secreting cells, being similar to the adhesive-secreting cells of other species of stalked barnacles.