Animal Eyes

It may not look like it, but a lined chiton mollusk from California has eyes on its back. Photograph by David Liittschwager, National Geographic Creative. 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.

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 cat’s pupils shaped the way they are?—Agi Muzbek via Facebook

William R. Crumley, of Penn Vet’s Ryan Veterinary Hospital, 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 closeup of a domestic cat 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 lighting 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, 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 which 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.