Japanese eel jaw and vertebra ossification occurring respectively during the larval stage and metamorphosis

Morphological deformities of the jaw and vertebrae arise as hindrances to seed production of Japanese eel Anguilla japonica . To obtain basic information related to development in these areas, we analyzed eel larva skeletal system formation by staining with alizarin red S. Results demonstrated that dentary and maxillary ossification at the jaw begins by 10 days after hatching, about 8 mm total length. Subsequent progression of ossification is slow until metamorphosis. For instance, ossification of the angular and quadrate, which are connected to the dentary, begins remarkably later than ossification of the dentary. Ossification of the head develops quickly during metamorphosis and produces a primitive skull in glass eels. Ossification of the vertebral centrum progresses rapidly from the posterior end to the anterior-most during metamorphosis. Our results demonstrate that the process of ossification at the head is shared among some species of Elopomorpha, but that ossification of vertebral centra differed. Furthermore, our results imply consistency of the period between ossification and the occurrence of some morphological deformities around the jaw and vertebrae.