Muscle development in the Japanese flounder, Paralichthys olivaceus, with special reference to some of the larval-specific muscles.

We investigated muscle development in the Japanese flounder Paralichthys olivaceus, focusing primarily on the cranial muscles, using a whole mount immunohistochemical staining method. It is well established that during the very early stages of morphogenesis, until 4 days post hatching (dph), muscles required for feeding develop. Later, between 8 and 16 dph, the muscle composition in the dorsal branchial arches changes to the adult form. We discovered the presence of larval-specific muscles in this ontogenetic period, termed the larval branchial levators 2 and 3, located in the dorsal branchial arches. The larval branchial levators 2 and 3 disappear during the course of development, whereas the others remain as levator internus 1 and levator posterior, which have also been described in adult fish. In place of these regressed muscles, the levatores externi and levator internus 2 develop and regulate the branchial arches. In addition, we found that the levator posterior, which is thought to represent the fifth levator externus, and the levatores externi exhibit different origins. We also found that at least a part of the caudal fin musculature develops from the trunk myotome.