A jawless fish lays eggs in fresh water and it has ammocoetes larvae after metamorphosis. This fish return to the ocean as _

Explanation

Petromyzon (Lamprey) belongs to the Class Cyclostomata of Phylum Chordata. It is a jawless fish which lays eggs in fresh water. The eggs hatch in about 3 weeks into minute transparent larvae called ammocoetes. After metamorphosis, the young lam preys swim down to the sea where they remain for 3 or 4 years before reaching maturity, when they once again migrate to streams or rivers to spawn and die. Gonads become mature when adults return to rivers for spawning.