Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to check out their course on Algorithm Fundamentals. [♩INTRO]. Brittle stars may seem a bit boring at first glance.

They're basically skinnier, wigglier versions of their relatives, the sea stars. But those are not the only ways they differ from their meatier cousins. Sea stars have eyes—one at the end of each arm.

But brittle stars are eyeless, so clearly they must not be as cool, right? Except, it turns out they can see. Just with their skin instead of eyes!

Vision seems like it would be pretty simple: if you have eyes, you can see. If you don't have eyes, you cannot. But eyes are just a really advanced form of photoreception: the ability to perceive light.

And there's lots of animals on our planet that do not have eyes that can still sense light. And it has a name! It's known as extraocular photoreception.

Basically, they can sense light with other parts of their bodies, either in addition to or instead of having eyes. The brittle star Ophiocoma wendtii, is one such creature. These brainless, shy animals spend their time hanging out in dark crevices of coral reefs.

And they know they're in those crevices because they can react to light and shadow using their skin. Until recently, scientists thought that crystal-like structures in their skin worked together like a big compound eye. Essentially, they believed the crystals could focus light onto nerves running all over the brittle star's body, and that allowed the animals to form images of their environment.

And they believed that because they thought there was this big network of nerves running below the crystals. But once they took a closer look with high-tech microscopes, they found that these nerves actually run in between the crystals. Which sinks the whole crystals-focusing-light on them idea.

However, they also found that the skin on their arms contains lots of cells filled with light-sensitive proteins called opsins. And it turns out these are in close contact with those nerves. So now, biologists think these cells allow the brittle star to detect light.

Basically, when light hits those light-sensitive proteins, the cells send a signal to the nerves, and that causes the brittle star to react. Which, in their case, generally means moving out of the spotlight and back into the dark. It's still not fully known how exactly the nerves produce this response, what with the animals lacking a brain.

And they aren't sure whether brittle stars can see shapes or just experience light and shadow. But they're eager to learn more, because there are lots of cases where we would like machines to see without relying on a central control system. It could help cut down the circuitry needs of mobile robots, for example, since they wouldn't need to send all information to their centralized “brains” to process.

And it might make the machines better at tolerating and adapting to damage. Basically, much like brittle stars, localized injuries wouldn't have as big an impact on their overall ability to function—an idea known as graceful degradation. So not only do brittle stars have the superpower of seeing without eyes, they might be able to show us how to design robots with that same ability, so that they can travel into dangerous environments and perform all sorts of helpful tasks.

And that makes these eyeless, boneless, brainless animals anything but boring. Thanks for watching this episode of SciShow, which is sponsored by Brilliant. Brilliant offers hands-on courses that can help you cultivate your math and scientific thinking skills.

Like, if you want to get a peak at what robotics engineers do every day, you could take their Algorithm Fundamentals course. Algorithms are the codes that underlie everything our machines do. So with this course, you'll gain a deeper understanding of how we get robots, or any computer, to do what we want.

With a premium subscription, you'll get access to it and all of Brilliant's other courses, as well as their other educational content. And if you're one of the first 200 people to sign up at Brilliant.org/SciShow, you'll get 20% off an annual premium subscription. So if you want to deepen your STEM skills, be sure to head on over to that link and check out what Brilliant has to offer! [♩OUTRO].