Leptocephali (singular leptocephalus) all have laterally compressed bodies that contain transparent jelly-like substances on the inside of the body and a thin layer of muscle with visible myomeres on the outside.
Their body organs are small and they possess only a simple tube for a gut. This combination of features results in them being very transparent when they are alive.
Leptocephali have dorsal and anal fins confluent with caudal fins, but lack pelvic fins.They also lack red blood cells until they begin to metamorphose into the juvenile glass eel stage when they start to look like eels.
Leptocephali are also characterized by their fang-like teeth that are present until metamorphosis, when they are lost.
Leptocephali differ from most fish larvae because they grow to much larger sizesand have long larval periods of about three months to more than a year.Another distinguishing feature of these organisms is their mucinous pouch. They move with typical anguilliform swimming motions and can swim forwards and backwards.