Dear Dottie,
When I was a kitten, I learned to swim by diving for coins in the pool. It wasn’t easy seeing the coins underwater. It was blurry down there.
I talked about how fish see with my friend Nora Hickey. She’s a fish veterinarian at Washington State University.
She told me that fish eyes don’t work the same way mammal eyes do—even though we have the same eye parts.
Your cornea is the clear, squishy tissue right at the front of your eye. It’s in front of the colored ring in your eye—called the iris. There’s a dark dot in the center of your iris—called the pupil. It gets bigger or smaller depending on how much light there is. Your lens is deeper in your eye, behind your pupil. The very back of your eye—called the retina—has lots of nerve cells that connect to your brain.
Your cornea does about 75% of the work to focus your eye. When light travels from the air into your cornea, the light bends. Then, the light travels to the lens and bends some more. That directs the light to your retina. There, a message is sent to your brain to make sense of what you’re seeing.
That works because our eyes adapted to function in air. When light travels from one thing to another, it bends or slightly changes direction. When it goes from the air to your wet, squishy cornea, that’s a big change. So, the light bends a lot.
But it’s not a big change when light travels from wet water to a fish’s wet cornea. The light doesn’t bend much.
That’s why things look unfocused when you try to see underwater. Your cornea can’t do its job there. But if you put on goggles, you trap a bunch of air next to your eye. Then you can see more like you do on land.
Since a fish’s cornea doesn’t pitch in much, all the focusing work is done by a fish’s lens. That’s why a fish’s lens has a different shape than yours. Yours is like an olive. It’s mostly round, but it’s taller than it is wide. When you look at things close or far away, muscles relax and contract to squish the lens or stretch it out.
But a fish’s lens is round like a marble. A fish focuses by moving the whole lens to zoom in or zoom out.
“To adjust their vision, fish move the lens back and forth in their eye,” Hickey said. “If you look, you can see that the lens protrudes through the pupil, extremely close to their cornea.”
Hickey told me that fish don’t use their vision as much as we do. They have lots of ways to sense the world around them.
They have sensory receptors on their skin. They’re like the cells inside our ears, so fish can “hear” subtle changes in the water. Catfish have taste buds all over their skin. So, they taste the water they swim in.
Some fish like sharks, rays and skates have a sense we don’t have. They can detect electric impulses from things around them.
There are even fish—called four-eyed fish—that have evolved duplicate parts of their eyes. One half sees underwater, and the other half sees above water.
You could say that, when it comes to seeing in all circumstances, they’re true visionaries.
Sincerely,
Dr. Universe