Development of Lacuna pallidula (da Costa, 1778) from the White Sea (Caenogastropoda: Littorinimorpha) with emphasis on radula formation

The gastropod radula is highly diverse in structure in comparison with that of other mollusсs. The main radular types in the different phylogenetic groups of gastropods differ not only in the general morphology and configuration of the teeth but also in the mode of tooth synthesis and the ultrastructure of the formation zone. Previously, the formation and anlage of the radula in the ontogeny of radulae of all major phylogenetic gastropods groups have been studied, with the exception of the taenioglossan radula of Caenogastropoda. The data obtained in this study on the radular anlage and synthesis in one littorinimorph species Lacuna pallidula supplement the existing knowledge of diversity in gastropod radula formation. The radula is initially formed at the stage of the post-torsion veliger, with five teeth in each transverse row, and acquires the adult morphology before hatching from the egg mass. The larval radula is synthesized by a few morphologically uniform cells in the radular sac. Synthesis of the adult radula also occurs at the blind end of the radular sac, where groups of numerous odontoblasts each form one tooth, and membranoblasts, located on the ventral side, form the membrane. Characteristic features of the adult radular sac are an additional supporting cartilage-like structure at the radula curve in the middle of the radular sac, a well-defined proliferation zone in the zone of radula formation and the presence of an additional extracellular matrix around the teeth in the maturation zone.