Developmental dynamics of myogenesis in Yesso Scallop Patinopecten yessoensis.

The development of bivalves has been extensively studied over the last 150 years. Despite this, the developmental dynamics of myogenesis in bivalves remains largely unknown, particularly at the early developmental stages. In the present study, we investigate the characteristics of muscle development of Yesso scallop Patinopecten yessoensis by phalloidin staining, light, electron and confocal microscopy. Myoblasts containing chaotic myofilaments are initially found in the blastocoel of trochophore, and become more organized during the transformation from trochophore into veliger. This is followed by a well-structured musculature including an anterior adductor, velum retractors and ventral retractors at the early veliger stage. With development into late veliger, larval muscle system is composed of the branched velum retractors and ventral retractors, anterior and posterior adductors. The most striking change for pediveliger is the development of foot retractor and mantle related muscles at this stage. During metamorphosis, the retractor muscles and anterior adductor undergo the irreversible shrink until vanishing completely towards the end of larval life, which coincide with the growth of foot retractor and mantle margin. All retractor muscles are found to be composed of striated fibres, whereas the adductor muscles have both smooth and striated components. The present study provides new evidences on the dynamic pattern of myogenesis during embryonic and larval development in scallops, which will greatly improve our understanding of scallop myogenesis and provide the basis for comparative analysis of muscle development in bivalves.