Creature with flat, transparent larvae called "leptocephali"

EEL

Think of a slippery, snake-like fish that starts life as a nearly see-through ribbon in the ocean.

This clue points to an eel because eels are the only creatures whose larvae are called "leptocephali"—flat, see-through drifters that look nothing like adult eels. The word itself is a scientific term (from Greek leptos for "thin" and kephalē for "head"), but the clue skips jargon and just describes the larvae directly. If you know eels have bizarre, ribbon-like young, the answer clicks right into place.