Table 2 Morphological and Anatomical Features of Crassostrea gigas Larval Development Stages

Early trochophore

20 hpf

Spheroidal form, slightly conical at the basal region with two obvious invaginations (shell gland and presumptive mouth opening) and prototroch (Fig. 1a). Long cilia form an apical tuft that is present at the apical pole.

Middle trochophore

24 hpf

Spheroidal form with one invagination (mouth). Shell gland everted (Fig. 1b).

Late trochophore

28 hpf

Spheroidal form with well-developed prototroch and telotroch. Shell gland begins to secrete a shell (Fig. 1c). Locomotory organ—prototroch.

Early veliger (D-hinge-stage veliger)

36 hpf—72 hpf

Two D-shaped shells surround the larval body. Digestive tract is complete and consists of mouth, esophagus, stomach, intestine, and anus. Elements of the muscular system (retractors) are well developed. Locomotory organ—velum (Fig. 1d).

Middle veliger

92 hpf-96 hpf

Shells increase in size and are slightly elongated. Retractors and two adductors are well developed (Fig. 1e).

Late veliger

after 5 dpf

Right and left shells first demonstrate asymmetry (Fig. 1f). Velum is markedly larger than in the previous stage.

Pediveliger (umbo stage)

after 28 dpf

Umbo is distinguished as highest and most prominent part of each valve of the larval shell. Velum is markedly larger than at the previous stage.