leptocephalus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of the small, flat, transparent larvae of eels and certain other fishes, characterized by a long, narrow head.

Century Dictionary and Cyclopedia

1. n. A spurious genus of eel-like fishes, having a very thin diaphanous body, formerly regarded as a valid generic type and hence giving name to the Leptocephalidæ, but now generally considered to be the larval form of conger.
2. n. The larval or aborted stage of the conger and allied fishes, when the body is much compressed and hyaline and no generative organs are developed.
3. n. The genus otherwise called Conger.

Wiktionary

1. n. the larva of an eel, flat in shape and transparent

WordNet 3.0

1. n. slender transparent larva of eels and certain fishes