The larvae of eels and other related species are called small heads or in the fancier Greek, Leptocephalus. The video above should give you some insight into this moniker. Unlike fish larvae, Leptocephali can grow quite large from a few inches to well over a foot in length. Also unlike fish larvae, Leptocephali do not feed on plankton but rather feed on microscopic particles in the water called marine snow. As can be seen in the video above they are transparent, undoubtedly an adaptation to avoid predation. Indeed, Leptocephali do not possess red blood cells which could be easily seen in a transparent body. Red blood cells are not acquired until the next phase of their life.