Leptocephalus ~ The Transparent Fish.

A leptocephalus (meaning "slim head") is the flat and transparent larva of the eel, marine eels, and other members of the Superorder Elopomorpha. This is the most diverse group of teleosts, containing 801 species over the span of 24 orders, 24 families, and 156 genera. It is supposed that this group arose in the Cretaceous period over 140 million years ago.