Look though you might, you won’t see anything that looks like an eyeball on a sea urchin -- a critter that looks more like a spine-covered cactus than an animal. But it turns out that the urchin may have the equivalent of thousands of eyes -- all of them on their feet.
Sea urchins don't have eyes, but they do respond to light. Credit: Wikipedia.Researchers have known for a long time that sea urchins respond to abrupt changes in light. But they’ve been unsure about how they do it, because there are no structures that even remotely resemble eyes.
In 2009, though, researchers discovered that urchins have the same genes as those found in the retinas of humans and other creatures. The retina is the part of the eye that perceives light.
And two years later, they found bundles of light-sensitive structures on the bases and tips of their tube feet. Since a sea urchin has more than a thousand of these feet, it means they could have a couple of thousand “eyes.” The urchin’s vision isn’t very sharp, though -- it reacts to big objects, but not to small ones. So it’s unclear whether it forms actual images of what’s around it, or just perceives changes in light.
Of course, the sea urchin does use its feet to clamber around the bottom of the ocean. But it doesn’t use all of them at the same time. So it’s possible that it uses some to get around, and some to keep an eye out for food, shelter, and predators.
Those bristly spines may also play a role in the sea urchin’s vision. They may shade some of the “eyes” from bright light -- helping turn the entire creature into one big, prickly eyeball.
