The larva of some bony fishes such as the eel that has a remarkably different appearance to that of the adult fish. For example the eel larva has a leaf-shaped, almost transparent body. This contrasts to the long, thin form of the adult eel.

The term is mostly used to refer to the larva of eels of the genus Anguilla which migrates from its hatching ground in the Caribbean to European freshwater habitats

The word is derived from the Greek words leptos (fine, small) and kephale (head).