Dear Elena,

The animal kingdom is full of amazing eyes. And yes, there are actually creatures on our planet that have just one.

That’s what I found out from my friend Kevin Kaiser, a veterinarian and graduate of the Washington State University College of Veterinary Medicine. He knows a lot about how different creatures see the world.

As a veterinarian working in Spokane, Wash., he also specializes in caring for animals’ eyes.

“Elena asks a great question,” Kaiser said. “There is one species that has only one eye naturally and they are from a genus called copepods.”

Unlike the mythical one-eyed giant Cyclops, these real-world creatures are pretty small. In fact, some copepods are even smaller than a grain of rice.

We find them living underwater in oceans and lakes all around the world. These water-dwellers have tiny transparent bodies, long antennae, and just one eye.

“Their ‘eye’ is called a median eye and is located near the top of their head,” Kaiser adds. “While their eye is tiny, it allows them to function in their environment.”

For example, the eye helps them see the differences between light and dark as they navigate the waters. Of course, our mammal eyes are a bit different from the eyes of these critters in the crustacean world.

Mammals usually have two eyes, but the functions can change depending on their environment or what a creature needs to survive.

A lot of mammals have round pupils, that black part of our eye that lets light in. But goats have rectangular pupils. These pupils give goats a wide panoramic view, perhaps to keep an eye out for predators.

Then there are tarsiers, small primates found in rainforests that have huge eyes. If your eyes were as big as a tarsier’s, they’d be the size of grapefruits. Their big eyes help them watch for prey.

Before an animal is born, all sorts of amazing things need to take place to help their unique eyes grow. We usually expect mammals to be born with two eyes, but sometimes the eyes don’t develop properly.

In mammals, two eyes can sometimes grow together, Kaiser said. This gives the appearance of a creature having only one eye.

There are also animals with eyes that do grow properly, but don’t really help them see too well.

While all mammals are believed to have two eyes, some species can have eyes that are vestigial. This means that their eyes are so small that they do not function or work.

We had fun looking into the answer to your question, Elena. Now you can see that in the animal world, there can be creatures with no eyes, lots of eyes, two eyes, and even one.

Sincerely,

Dr. Universe

Ask Dr. Universe is a science-education project from Washington State University. Send your own question to Dr.Universe@wsu.edu or vote in this week’s reader poll at askDrUniverse.wsu.edu.