leptocephalus

noun

lep·​to·​ceph·​a·​lus ˌlep-tə-ˈse-fə-ləs 

plural leptocephali ˌlep-tə-ˈse-fə-ˌlī 

-ˌlē

: a long thin small-headed transparent pelagic first larva of various eels

Word History

Etymology

New Latin, from Greek leptos + kephalē head — more at cephalic

First Known Use

1769, in the meaning defined above