Metamorphosis

Guest post: Emily Argo

“I think you have a dead fish,” said a concerned aquarium visitor who summoned me to a nearby tank. Pointing to the tank the visitor recounted that they had been watching for a while and had not seen the fish move from it’s position on the bottom. The fish, I am happy to report, was not dead, but alive and well. The visitor had spotted a halibut!

Halibut are flatfishes (this group also includes flounder, sole, turbot, and plaice), they begin their lives as bilaterally symmetrical larvae swimming in the water column, but eventually metamorphose over the course of a few days into a laterally compressed, pancake-like bottom dweller (and a successful sit-and-wait predator). When you think of species that metamorphose (a change in body form between life stages), fish may not be the first type of organism that comes to mind. You probably think of caterpillars and butterflies or tadpoles and frogs, but there are actually over 500 species of fish that metamorphose!

A flounder camouflaging with the bottom (photo credit: Moondigger via Wikimedia Commons [CC BY-SA 2.5]).

One of the most distinct changes that takes place during flatfish metamorphosis is the movement of one eye to the other side of the body, so both eyes are on the same side. This requires reorganization of the bones and muscles in the head of the fish and also impacts the brain and the olfactory system. Additionally, the fish will begin swimming on its side and the coloration on the top side of the body (where both eyes are) will also begin to change helping the fish camouflage with its benthic environment. All these changes are controlled in some way by the thyroid hormone, but there is still a considerable amount to learn about these mechanisms and the evolutionary benefits of these changes.