Anguilliform leptocephali of the genus Thalassenchelys Castle and Raju 1975 are remarkably large and peculiarly shaped eel larvae, whose adult form has been unknown since the discovery of the larvae in the 1950s. We found bigmouth conger Congriscusmegastomus (Günther 1877) collected off the Pacific coasts of Japan to have mitochondrial DNA sequences (16S rDNA and COI) nearly identical to those of Thalassenchelys coheni Castle and Raju 1975 published to date and collected recently in the north Pacific. Vertebrae counts of C.megastomus were consistent with the myomere counts of T. coheni. We conclude that T.coheni, so-called larval species described by Castle and Raju (1975), is a junior synonym of C.megastomus. Therefore, the family to which the leptocephali belong must be Congridae.
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