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