Are There Animals with One Eye? Why Do We Have Two Eyes? 

You’re probably vaguely aware that you have some body parts that you don’t necessarily need. Sometimes it’s a vestigiality thing, and sometimes it’s a redundancy thing. You’re probably kind of aware of why we specifically have two eyes—even though we can function pretty alright with just one. But if you’re thinking about the cyclops, you might be wondering; are there animals with one eye? 

Further Reading: Why Do We Have Two Nostrils?

Predator and Prey

When someone tells you to think of “animals,” you probably think of vertebrates. Vertebrates are things with spinal cords and vertebrae. Basically, if it has a spine, it’s a vertebrate. Have fun using that to insult your local doormat friend, we guess. 

Anyway, “animal” probably meant things like fish, birds, reptiles, and of course mammals as a first reaction. The vast, vast majority of these animals have two eyes—despite only producing one image. We’re not like insects with compound eyes—even though insects are technically invertebrate animals. But back to vertebrates with two eyes. 

You probably know that we have two eyes because it gives us depth perception. There’s a reason our eyes aren’t in the same position, and it’s because they focus on different objects at any one time. Our eyes send our brains two different images, and our brains merge those two images together to form one. You can test this pretty easily by just… covering one eye and then swapping. Merging these images together allows us to perceive depth, and know how far objects are relative to each other and us. The ability to perceive depth by synthesizing two images at slightly different angles is called binocular stereopsis, and it’s pretty good for animals that hunt so they can better track their food. 

Also sidebar, it helps us tune out our own noses and our natural blind spot where the optic nerve hooks up to our eyeballs.

Animals with eyes on the sides of their heads

On prey animals, you’ll notice that their eyes are on the sides of their heads. Go take a second look at a rabbit, cow, deer, or gazelle. This gives those animals a much wider field of view, allowing them to detect predators much easier. 

Not having two eyes

So, what about animals that don’t have two eyes? Let’s start with third eyes, because that is a thing and it probably gives you the impression of being enlightened in some way. So are there any vertebrates that have a third eye naturally? Well, yes, otherwise we wouldn’t be having this conversation.  

Many lizards, frogs, salamanders—and even some aquatic animals have a parietal eye. This sits on top of their heads, and is used to regulate both a sense of time and temperature. They’re far less sophisticated than the two more “normal” eyes these animals have, and can’t really discern images. They really can only perceive changes in light. 

Can we push it to four? Actually, yes. There’s only one vertebrate (sort of) animal with four eyes, and that’s the lamprey. Lampreys have two parietal eyes instead of just one, so there you go. 

What about a cyclops?

Alright, you probably came here because you were wondering if there were any animals with only one eye. If you’re still only thinking about vertebrates, the answer is no. There aren’t any naturally cyclopean vertebrates. So we’re not going to be recreating any Greek myths or anything. Sorry. 

But we do have one genus of animals that do have a single eye. Fittingly, the genus is named Cyclops, after the mythological one (shocker). 

Cyclops are copepods, a group of tiny crustaceans (think crabs) whose name literally means “oar-feet.” They’re absolutely everywhere there is water, given that many are basically plankton. As far as our mono-eyed copepods go, they don’t get any bigger than 5 millimeters long, and can be as small as half a millimeter. Outside the novelty of having just one eye there’s nothing too special about them. 

Except for how they can be host to Guinea Worm, and drinking bad water with them can give it to you. 

See if you know your animals based just on their eyes here.