The ultrastructural features of embryonic and early larval development in Yesso scallop, Mizuhopecten yessoensis.

Fine structural features of Mizuhopecten yessoensis in different embryonic and larval developmental stages from oocyte to early veliger have been studied. Spermatozoa have a conical head, consisting of a short acrosome, nucleus, and 4 mitochondria in the middle piece. The flagellum is approximately 40 μm in length. Oocyte cytoplasm contains numerous yolk granules, which are gradually expended during the larval development and disappear at the early veliger stage. Gastrula has two invaginations: shell gland anlage on the dorsal side and ventral side archenteron. Prototroch of a trochophore consists of two ciliary rings. Telotroch is absent. Myoblasts containing bundles of myofilaments are found in the blastocoel of trochophore. Velum and shell begin forming at the late trochophore stage. Cells of gut epithelium form numerous microvilli, organized into a brush border structure on the surface of enterocytes. A suggestion is made that larvae at the late trochophore stage are capable of digesting food and feeding. The structure of a M. yessoensis veliger is typical for pectinids. The velar retractor muscles have cross striation with a period of 1 μm. The digestive system of an 8-day-old veliger consists of the esophagus, stomach (with subdivided gastric shield and style sac), and short intestine. The digestive gland (with no discernible loops at this stage) is a part of the stomach epithelium with two types of digestive enzyme-producing cells.