The periostracum of Digitaria digitaria (Bivalvia: Astartidae): formation and structure

The formation, structure and growth of the periostracum of Digitaria digitaria (L., 1758), a small species of the family Astartidae from the temperate eastern Atlantic and Mediterranean, was studied. The periostracum originates from a row of basal cells, located at the bottom of the periostracal groove. It is initially secreted in the intercellular space between the basal cell row and the first cell row of the outer and middle folds; at this point this periostracum consists of a thin membrane, which comprises two layers (granular and dark homogeneous). The inner epithelial cells of the outer fold are responsible for the secretion and thickening of the third, translucent layer. Along and across the periostracal groove, the periostracum adheres to the outer epithelial cells of the middle mantle fold and develops an undulating form. We observed that the dark layer of the periostracum thickens across the periostracal groove. We could not confirm the involvement of the middle mantle fold in this thickening; a tanning process of the translucent layer appears to be a more likely alternative. During the free periostracum stage the translucent layer is no longer secreted and it becomes transformed into dark layer through a tanning process, which implies a noticeable reduction in periostracal thickness. The outer layer of the periostracum is pitted when newly formed and later the pits become filled by mucus and bacteria-like microorganisms. The first cell row of the outer epithelium of the middle fold serves as template for the large periostracal pits, while the small pits correspond to minor projections of the cells. The mode of formation of pits implies that, rather than being continuous, the periostracal growth proceeds in pulses. The microvilli of the inner epithelial cells of the outer fold show a linear border, while the microvilli of the outer epithelial cells from the apical part of the middle fold show an undulating form. These large undulations of the microvilli in the distal part of the middle fold loosely attach to the periostracum and ease its extrusion from the periostracal groove. This study has also revealed the presence of sensory organs in the distal edge of the middle mantle fold.