todropscience:

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 (super diverse). 

Leptocephali have compressed bodies that contain jelly-like substances on the inside, with a thin layer of muscle with visible myomeres on the outside, a simple tube as a gut, dorsal and anal fins, but they lack pelvic fins. They also don’t have any red blood cells (most likely is respiration by passive diffusion), which they only begin produce when the change into the juvenile glass eel stage. Appears to feed on marine snow, tiny free-floating particles in the ocean.

This large size leptocephalus must be a species of Muraenidae (moray eels), and probably the larva of a long thin ribbon eel, which is metamorphosing, and is entering shallow water to finish metamorphosis into a young eel, in Bali, Indonesia.

• Video: Filmed by Barry Haythorne and Rob Rutgers, HRF U/W Production