You can see right through the whitespotted conger larva. This flat-shaped fish found in the waters around Japan is about six inches long now. But when fully grown, it will lose its transparency and be up to one metre or three feet long.
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. They also lack red blood cells until they begin to metamorphose into the juvenile glass eel stage when they start to look like eels. Learn more here.