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| A flatfish with a sea star. John Butler, NOAA. |
As with jellies, fish go through some major transformations. Often their juvenile form looks little like the adults, with some being wrongly classified as separate species. Some sport long appendages, such as the juvenile barbeled dragonfish whose guts exit its belly and trail behind, being as long or longer than its body.
In one of the most amazing examples of adaptation, flatfish have rearranged their bodies to adapt to life on the seafloor. While most animals are symmetric, flatfish are an exception. This includes flounders, sole, plaice, dabs, tonguefish, turbot, and halibut.
In the evolutionary scheme of things, they evolved rapidly over a three million year period, starting about sixty-five million years ago. They are born looking like regular fish, but as they mature they deform with one eye moving to the other side of their head and one side of their mouth rising up on their face so that its crooked. They end up looking like something out of a Picasso painting. Then they start swimming sideways.
These changes didn’t happen one at a time, but were coordinated through related genes. The eye didn’t move and then the mouth—the entire skull changed. This prompted further coordinated changes, such as their body flattening and their fins extending to run down the sides of their body. Their new bottom side—which can be either left or right depending on the species—remains white, while the upper side becomes pigmented for camouflage.
The fish are able to expand and contract the different-colored pigment cells in their skin, so are able to rapidly change their color and shading from light to dark to match their surroundings, confuse predators, and probably to communicate. Tropical flatfish can do this in a couple of seconds, while it can take a couple of days for a cold-water flatfish to change its camouflage. They are very good at this. When placed on a checkerboard, they do a pretty good job of matching the pattern. Well enough so that at a distance in a natural environment, they would blend in.
But fish that evolved to be flat bottom dwellers don’t all do it this way. Some, like rays and skates, flatten vertically without losing their symmetry. Still, these transformations are not nearly as drastic as caterpillars changing into butterflies, but they are striking for a non-insect.
This reformation happens during a stage of rapid growth—somewhat like our pre-teen years—when they suddenly begin leaning to one side when swimming. Then one of its eyes moves up the side of its head and over the top. Or it moves through its head to the other side. In some species this takes five days, in others just one day. And their behavior changes too, from swimming freely to a sedentary life on the bottom.
Some of these fish are large. The European plaice gets up to three feet in length (1 m). They spend their days hiding, buried in the sand, coming out at night to hunt for crustaceans and shellfish that they take into their mouths whole, crushing them with teeth in their throats. Some flatfish even eat lobsters and sand dollars, which don’t have much flesh in them.
Having both eyes on the same side of their head probably gives them 360-degree vision and might give them good depth perception where the visual fields overlap, but since their eyes swivel independently their brains probably process the input from each eye separately.
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