Embryonic and larval development of Ruditapes decussatus (Bivalvia: Veneridae): a study of the shell differentiation process

The embryonic and larval development of Ruditapes decussatus, from fertilization to metamorphosis, is described using scanning electron microscopy (SEM). Shell formation during embryonic development is investigated using SEM and transmission electron microscopy (TEM). Released oocytes are about 67 µm in diameter. Gastrulation takes place by epiboly and starts 7 h after fertilization (T0 + 7 h). The early gastrula is characterized by the appearance of a large, open cavity posteriorly and a small, round blastopore anteriorly, which represent the shell field and the blastopore, respectively. In TEM views, the open cavity expands under and posterior to the developing prototrochal pad. The shell field comprises a few cells in which microvilli progressively regress; these cells will secrete the periostracum. After 13 h (T0 + 13 h) the late gastrula has differentiated into a typically pyriform and motile trochophore. The periostracum emerges from periostracum-secreting cells (T1) and spreads over shell-secretory cells (T3). Thus, the trochophore is laterally compressed and the periostracum inserts to the mantle edge. Typical straight-hinged D-shaped larvae develop from a trochophore by 26 h postfertilization (T0 + 26 h) and by then the valves completely enclose the soft body of the larva. At T0 + 39 h, the newly hatched veliger larva is already enclosed in the fully calcified prodissoconch I (PI) and the prototroch has transformed into the velum. Late D-larvae already have a developed digestive system and start exogenous feeding. At the same time, the prodissoconch II is newly secreted at the margin of the PI. Settlement occurs at 27 d postfertilization (T0 + 27 d), when the larvae are about 207 µm long. Once metamorphosis is completed, the mantle folds begin the secretion of the dissoconch shell and the postlarval stage is reached.