The leptocephali are about 300 - 400 mm long and appear to be metamorphosing larvae of some type of moray eel of the family Muraenidae. The morphological similarities between these leptocephali and Ribbon Eels, Rhinomuraena quaesita, suggest they may be Ribbon Eel larvae.
Leptocephali are transparent because they have only a thin layer of muscle over a mucinous pouch inside the body. The pouch contains glucosaminoglycan (GAG) compounds that form a transparent jelly-like substance. The GAGs are converted into tissue during metamorphosis, and thus in addition to being transparent, they are also an energy storage material. (Video)