Development of mandibular dislocation during the larval stage of Japanese eels (Anguilla japonica): a lethal skeletal abnormality at the glass eel stage identified by jaw opening angle

Abstract In the artificial seed production of Japanese eels, Anguilla japonica , the frequent occurrence of glass eels with jaw abnormalities has become a serious problem. To gain fundamental insights into this issue, we evaluated the effects of lower jaw abnormalities on survival during the glass eel stage and revealed that these abnormalities are lethal. Bone staining of glass eels and cartilage staining of leptocephali larvae showed that the abnormality was caused by mandibular dislocation in both stages. Focusing on the jaw opening angle (OA), we examined the relationship between OA and the occurrence of mandibular dislocation during the glass eel stage using logistic regression and receiver operating characteristic (ROC) analyses. The results indicated that the probability of mandibular dislocation increases when OA exceeds 69.3°. Therefore, individuals with an OA of 70° or greater were defined as having mandibular dislocation, and their appearance timing and body size were investigated. As a result, individuals with an OA of 70° or greater appeared when the total length reached approximately 35 mm, and their occurrence increased with age. These findings provide important insights into the mechanisms underlying mandibular dislocation and may contribute to its prevention in artificial eel seed production.