Yes, you read that right. Today’s animal is the family of fishes known as the four-eyed fish. I know this conjures images of fish wearing glasses (which is a hilarious thought), but these guys don’t wear glasses. Instead, they just have some very strange eye biology. Very strange.

There are only three species of four-eyed fish, the large-scale foureyes, the Pacific four-eyed fish, and the simply named foureyes. All species occur in a range spanning southern Mexico through Central America to the northern part of South America. The fish are found in brackish waters, usually in mangrove forests.

See how strange its eyes are?
Image by H. Zell, CC BY-SA 3.0, via Wikimedia Commons

Four-eyed fish can grow to a maximum length of 32 cm, though they are usually smaller than that. They are pretty boring looking fish, being brown with a pale stripe running down each side of its body. But you didn’t click on this blog to read about what colour four-eyed fish are. You came here to find out about their eyes.

So let’s get to it. Four-eyed fish don’t actually have four eyes, but what they do have is a pair of eyes that is split in two. Each eye has an above-water portion, and a below-water portion, with each having its own pupil. The top half of the eye is designed for seeing above water, while the lower half is best at seeing under water, with a thicker lens on the bottom to adjust for the different refractive index under water. Four-eyed fish sit just at the surface of the water, the top half of their eyes sticking into the air.

Another strange attribute of four-eyed fish is their reproductive behaviour. Both males and females have a ‘handedness’, a side on which they will mate. A right-handed male will only mate with a left-handed female, and vice versa. Four-eyed fish give birth to live young, keeping the eggs inside of them until they hatch.

An even weirder view of a four-eyed fish.
Image by Ruben Undheim from Trondheim, Norway, CC BY-SA 2.0, via Wikimedia Commons

So although they don’t actually have four eyes, I think the four-eyed fishes definitely are right up there as some of the coolest fish around. After all, how many animals actually have split eyes?

Cover image by Quartl, CC BY-SA 3.0, via Wikimedia Commons