The marine eels and other members of the Superorder Elopomorpha have a leptocephalus larval stage, which are flat and transparent. This group is quite diverse, containing 801 species in 24 orders, 24 families and 156 genera. They arose in the Cretaceous period 140 million years ago.
Fish with a leptocephalus larva stage include eels like the conger, moray eel and garden eel. The conger eel is the one whose larva was captured by the Mie Prefecture Fisheries Research Institute in Japan.
Leptocephali have laterally compressed bodies that contain jelly-like substances on the inside, with a thin layer of muscle with visible myomeres on the outside. They have a simple tube as a gut. They have dorsal and anal fins, but they lack pelvic fins. They also don’t have any red blood cells, which they only begin produce when the change into the juvenile glass eel stage. They also possess fang-like teeth that are present until metamorphosis, when they are lost.
Leptocephali differ from fish larvae because of their size and their long larval periods, which last between three months to more than a year. Their anguilliform swimming motions allow them to move forwards and backwards. They appear to feed on marine snow, tiny free-floating particles in the ocean.
[via Twisted Sifter]