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Keywords:

  • mollusca;
  • bivalvia;
  • ontogenesis;
  • shell differentiation

Abstract. The embryonic development in Chione cancellata, from fertilization to straight-hinge D-stage veliger larva, occurs in 24 h at 25°C. Transmission (TEM) and scanning (SEM) electron microscopy show that morphogenetic processes result in a gastrula with two depressions, 4 h after fertilization (T0+4 h). Two hours later, one depression, located at the animal pole, develops into an open cave, the floor of which becomes the shell field, characterized by scarce microvilli, located below the lower face of the prototrochal pad. The invagination located at the vegetal pole is characterized by regular microvilli and constitutes the digestive tract. At T0+8 h, the late gastrula differentiates into a typical motile trochophore, possessing a well-developed prototroch delimiting anterior and posterior embryonic regions. At T0+9 h, the shell field, located between the prototroch and the telotroch, appears as a saddle-shaped region with a wrinkled surface extending on both sides of the embryo, establishing bilateral symmetry. From TEM observations, the apical surface of the underlying cells possesses a modified glycocalyx featuring a dense pellicle overlapping the cells of the shell field. At T0+12 h, the prototroch is displaced toward the anterior region by outgrowth of the shell material. TEM views show that the wrinkled surface observed with SEM corresponds to the periostracum, attached dorsally to the hinge and ventrally to the cells of the future mantle edge. At T0+15 h, an alveolar matrix of proteinaceous material, which could contain calcite crystals, appears between the periostracum and the mantle. At T0+24 h, prodissoconch I formation is completed and the D-stage larvae possess a calcified shell. At this stage of development, only one mantle fold is present, representing the outer fold. The periostracum-secreting cells are in contact with the marginal cells of the velum. This functional velum is composed of four bands of cilia. Our TEM observations confirm previous descriptions of the velum ciliary bands obtained only from SEM analyses, such as described in other bivalves. The apical sense organ and the digestive tract are poorly developed. From observations at the ultrastructural level of shell differentiation in C. cancellata, we propose a new interpretation of shell differentiation, including hinge and ligament for bivalves as opposed to that described for gastropods.