Metamorphosis in Fish
Little known is that some fish, both bony fish (Osteichyes) and non-bony fish (Agnatha), undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone.
Examples among the non-bony fish include the lamprey. Among the bony fish, mechanisms are varied.
The salmon is diadromous, meaning that it changes from a freshwater to a saltwater lifestyle.
Many species of flatfish begin their life bilaterally symmetrical, with an eye on either side of the body; but one eye moves to join the other side of the fish - which becomes the upper side - in the adult form.
The European eel has a number of metamorphoses, from the larval stage to the leptocephalus stage, then a quick metamorphosis from leptocephalus to glass eel at the edge of the continental shelf (8 days for the Japanese eel), two months at the border of fresh and salt water where the glass eel undergoes a quick metamorphosis into elver, then a long stage of growth followed by a more gradual metamorphosis to the migrating phase. In the pre-adult fresh water stage, the eel also has phenotypic plasticity because fish-eating eels develop very wide mandibles, making the head look blunt. Leptocephali are common, occurring in all Elopomorpha (Tarpon- and eel-like fishes).
Most bony fishes undergo metamorphosis after absorption of the yolk sac because after that phase they need to be able to feed for themselves.
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