How does a flounder’s eye move from one side of its body to the other?

Some of the most amazing things happen in nature! How cool (and weird) is the process of metamorphosis—when an animal’s body changes shape as the animal grows? One of the neatest examples of metamorphosis is eye placement in flatfish such as flounder. When a flounder is young, its body looks a lot like other fish: an upright body and an eye on each side. However, as a flounder grows older, its body changes….a lot! It changes to swimming flat on the ocean floor (it can hide itself this way) and both eyes are on the top of its body.

So how does the eye move from one side of the body to the other? Scientists have evidence that the eye is PUSHED to the other side! How, you ask? By growing bones and muscles on the side that is OPPOSITE to where the eye is moving. To imagine this, you can do a little exercise. Place both arms on a table with your elbows bent and with two fists pushing into each other. Now, keeping your left elbow in the same place, slowly open your left hand, pushing against your right fist. What happens to your right fist? It moves to the right! This is just like an eyeball moving across a flounder’s body, with growing bone and muscle as the left hand. Pretty cool, huh?

Here’s a great time lapse video of a flounder’s moving eye: