On Supernumerary of Dorsal Fin and the Complicated Abnormality in Anomalous Ayu Fish, Plecoglossus altivelis

The external configuration and the bones of the malformed Ayu fish (Plecoglossus altivelis) which have the second dorsal fin were examined through X-rays in the present study.The number of soft rays for the second dorsal fine of the malformed fish was measured as approximately 10.0 and the number of spines for the interneural spine was approximately 3.0. It is noted that the number of spines for the interhemal spine was about 4 fewer than for the normal fish. On the other hand, no difference was observed in the external configuration, the vertebral number, the number of the rays of the first dorsal fin and the anal fin, the number of the spines of the interneural spine which supports the first dorsal fin and that of the free interneural spine, between the malformed and the normal fish.It was also observed that the malformation of the caudal vertebra, the interhemal spine and the hemal spine of the malformed fish which have the second dorsal fin took place almost at the same time.