Last week we looked at how several marine animals used camouflage to protect themselves from predators. In that essay I mentioned that a flounder is not flattened top to bottom, as stingrays are, but are in fact flattened side to side, with both eyes on one side of the body. Let’s examine this a bit more closely.
The fish above, believe it or not, is flattened just as a flounder is. This body plan is known as laterally compressed. A fish that is flattened top to bottom would be described as being dorsoventrally flattened, like this stingray is.
Notice a few things about this butterflyfish: The tail is vertical, and moves side to side as the fish swims. There is an eye on each side of the head, as well as one gill opening. The dorsal fin is on top of the fish and the ventral and pelvic fins are on the belly, pointing downward. Now let’s look at the flounder:
Like a stingray, both eyes are on “top” of the fish’s head. But look at the other features (remember, we are looking at this fish from above). There is only one gill opening on this visible side, the dorsal fin is on the left side of the body and the ventral and pelvic fins are on the right side. When it swims the horizontal tail moves up and down. Clearly this fish is lying on its side. But how does it end up with two eyes on one side of its body (and obviously no eyes on the other side)?
To answer this we need to look at the developmental stages of the fish. A juvenile flounder just days old actually looks like any other normal fish. It swims rightside up, has a vertical tail and one eye on each side of the head.
As the larval flounder develops, the body gradually flattens. One of the eyes, either the left or the right depending on the species, moves across the forehead until it is sitting right next to the other eye. Once this metamorphosis is complete the young fish leaves the open water and settles down on the bottom. This process is often described as an “eye migration”, but as with many concepts in science, the term simplifies this down to where it isn’t technically correct. True, an eye that at an early stage in the fish’s life was on one side of its body is now positioned on the opposite side. But to get there the eye itself doesn’t crawl across the head, the bones of the animal’s skull actually twist as the fish grows and this skeletal twisting places the two eyes together.
The side of the body that ends up facing upward, the one whose skin cells are so adept at color changing, is called the “eyed side” of the fish. The “blind side”, called that because it has no eyes, lies beneath the fish and against the bottom substrate. This blind side is never exposed and lacks any pigmentation. It’s pure white in most species.
You may have noticed I mentioned that the eye that moves is either the left or the right depending on species. Pleuronectids, the large family of fish that is comprised of all flounders and flatfishes, is divided into two groups: Right-eyed flounders and left-eyed flounders. In right-eyed species the left eye migrates over to the right side and in left-eyed flounders the opposite happens. I’ll rotate the images and place them side by side so you can see the difference more clearly.
First is a right-eyed species called a winter flounder. It’s called a “winter” flounder because it spends summers in deep water and moves into coastal waters, where it’s easier to catch, in the winter. Notice that the dorsal fin is on the left and the ventral fin is on the right. Now turn it belly-down in your mind and you can see the eyes are on the right side of the body. The second image is a summer flounder, also called a fluke. Everything is in the reverse (including the seasonal migratory behavior that gives it its name). You can see on this one the eyes are on the left side of the body.
One last interesting note about the eyes, and you’ll have to scroll all the way to the top image to see this (this one, by the way, is a left-eyed flounder). The eyes can move independently and, like those on the one pictured, often look in opposite directions. This gives the fish a full 360 degree view of its surroundings.
Flounders are clearly a triumph of evolutionary diversity. That an adaptation such as skull twisting and eye migration occurs at all is, frankly, wondrous. One of the lamer lines of argument against evolutionary theory is the idea that only a fully realized adaptation would be useful to an animal. Only a fully functional wing, for example, is any good. An “almost wing” could not arise on its own. And so it is with the flounder’s twisted skull. How could there exist an animal with a partially migrated eye? Normally we would have to venture into the fossil record to find one of these transitional species, but in the flounder’s case we need only look as far as the Indian Ocean. Here exists several species of fish known as Psettodes. These are semi-flounders whose migrating eye only reaches about the halfway point, stopping at the fish’s forehead.
Added Quiz: Is the Psettode pictured above on its way to right-eyeddom or left-eyeddom?
Other diaries in this series can be found here.