Ask Your Weird Animal Questions: Animal Eyes

From cats to clams, the animal kingdom literally has many different ways of seeing things—including some we may not think of as having eyes at all. This week on Ask Your Weird Animal Questions, we’re taking a visionary look at nature.

Which creature has the most eyes? —Crystal Henson via Facebook

The answer may surprise you: the chiton, a type of mollusk. This ocean dweller has thousands of eyes embedded in shells on their backs, Daniel S. Speiser, of the University of South Carolina, said via email.

A photo of a lined chiton mollusk.
A lined chiton mollusk from California. Tropical species of chiton have eyes embedded into their backs. Photograph by David Liittschwager, National Geographic Creative

Most scallop species also have dozens to hundreds of eyes, as do ark clams and giant clams, Speiser said.

What’s more, certain types of tubeworms called sabellids and serpulids have hundreds of small, compound eyes on their feeling appendages. (Related: “Eyes Made of Rock Really Can See, Study Says.”)

Why are [a] cat’s pupils shaped the way they are? —Agi Muzbek via Facebook

William R. Crumley, of Penn Vet’s Ryan Veterinary Hospital in Philadelphia, explained that domestic cats have vertical pupils, which has to do with the shape of their eye muscles.

“The constrictor muscles in a [cat’s] slit-shaped pupil are like two crescents facing each other that overlap at the top and bottom and close when stretched,” Crumley said by email.

A macro photo of a black cat's eye.
A close-up of a domestic cat’s eye. Photograph by Gabriel Burns, National Geographic Your Shot

On the other hand, animals with round pupils (like us) have a round sphincter muscle that acts like a drawstring, making the pupil get smaller or bigger.

The more difficult question to answer is why cats have these pupils, Crumley said.

Some experts have hypothesized that the slit structure allows for “multiple colors to be focused at the same point,” which enhances image quality.

A paper published in Current Biology in 2012 “suggested the vertical alignment of the slit allows for improved depth discrimination along the ground,” Crumley said.

That’s ideal for feline predators, which hunt small, ground-dwelling prey such as mice.

Which animal has the most advanced vision system? —David Gohman Luke via Facebook

The mantis shrimp is a carnival of cool, and among its amazing gifts is the most complex visual system in the animal kingdom.

The eyes of a peacock mantis shrimp allow it to strike its prey lightning fast. Photograph by Steve De Neef, National Geographic Your Shot

Ed Yong’s Phenomena post eloquently explains the mechanism, but here it is in a nutshell: Their stalked eyes have three separate regions, can see UV and circularly polarized light, and have up to 16 photoreceptors, compared to our three. The crustaceans may use their photoreceptors to recognize colors right in the eye, rather than processing them in the brain, as we do. (Related: “The Mantis Shrimp Sees Like a Satellite.”)

Those eyes also contain their own sunscreen, according to a recent study published in Current Biology.

Animals’ eyes change over the course of their lives … Has this been researched much, and if so, which type of animals’ eyes change the most? —Steve Hlavac via Facebook

“Most, if not all, animals experience changes to their eyes in some form as they age,” Jennifer Bolser, of the Humane Society of Boulder Valley in Colorado, confirmed via email.

Eyes of numerous species have indeed been studied, often as part of an effort to find models for changes in human eyes. (Watch video: “Eyes: The Window to Your Health.”)

For instance, domestic dogs and cats commonly experience iris atrophy, a condition that decreases their eyes’ ability to narrow its pupils. This makes the animals more sensitive to light; many experience lenticular nuclear sclerosis, a “subtle, gray cloudiness of the lens” that’s often confused with cataracts, Bolser said.

Because iris atrophy also occurs in humans, studying how it happens—and how to prevent it—in domestic animals may help us, too.

Got a question about the weird and wild animal world? Tweet me or leave me a note on Facebook.

Liz Langley is the award-winning author of Crazy Little Thing: Why Love and Sex Drive Us Mad and has written for many publications including Salon, Details and the Huffington Post. Follow her on Twitter @LizLangley and at www.lizlangley.com
  • ali

    How the fish check the safety of their environment ? All the creatures first try to find out if their surrounding is safe to live or not. This checking is performed momentarily. The fish,I have observed, check the depth of the water they live in . Each time , they come on the surface , touching the water line of the place with their nostrils and go down to the bottom . They do this to find out if the water level is enough for their safety. If the water level decreases, they get tension and speeds up and down to find the deepest place of the watering.

  • Teresa Coppens

    Boy am I so behind in my reading. I thought the most eyes honour would have gone to an organism in the insect world. Mollusk, I wouldn’t have guessed. Interesting article. I love reading about weird animals!

  • Liz Langley

    @Teresa Coppens I’d never have guessed that either, Teresa! I get a lot of great surprises from the awesome questions people send in!

    @ali I’m working on that one, thank you for the question!

  • Liz Langley

    @ali Thanks for the question about how fish check for the safety of their environment! It’s answered here in Weird Animal Questions – they have sensitive lines on their bodies and are able to smell an alarm substance secreted by other fish: http://newswatch.nationalgeographic.com/2014/08/08/animals-science-endangered-species-weird-world-zebras-fish/

  • Deva Raj

    which animals eyes is the best in the world?

  • Jack Regan

    I have to admit – you got me on the most eyes creature. We have thousands of them along the coast here; time to get out the macro lens.
    Northwest Wildlife Online

  • John

    So some of the oldest creatures on earth
    mollusk 500 million = most eyes
    shrimp 360 million = most advanced eyes

    Can someone explain to me how that happens? shouldn’t the earliest eyes be the most primitive, and shouldn’t the earliest creatures have fewer eyes, starting with one eye? That’s a pretty impressive mad-dash to the most advanced eye from no eye, by random mistakes.

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