Sea Urchins Can See Just Fine With Their Feet

Like their spines, a sea urchin's tube feet cover their bodies. Natalia Siiatovskaia/Shutterstock

The first thing anyone notices about a sea urchin is how uninviting it looks: this "porcupine of the sea" looks downright medieval with those endless rows of black spikes pointing in every direction, daring you to get too close.

And, as anyone who's ever brushed past one will tell you, those spikes pack a screaming sting.

But like so many strange creatures of the deep, there's so much to marvel at beneath that daunting facade. Like, for instance, the sea urchin's remarkable ability to navigate the world without actually having eyes.

It turns out sea urchins don't need them. According to a new study from the researchers at Sweden's Lund University, the marine animals use their feet instead. Those feet — tiny tube-like rows of them are interspersed with the spikes — have light-sensitive cells that grant the creature a kind of vision.

"You could say that the entire sea urchin is one single compound eye," lead researcher John Kirwan explains in a press release.

deep-sea urchins
Deep-sea urchins grow from the seabed at an underwater volcano in New Zealand's Kermadec Arc. (Photo: NOAA)

Even so, sea urchins won't be passing a driving exam any time soon.

While the idea that they use their feet to see the world isn't new, it's the first time scientists have confirmed the role of those light-sensitive foot cells and gotten a sense of the sea urchin's quality of vision.

To do that, Swedish researchers introduced bogus predators to the creature, each one bigger than the last. They then recorded how big the potential predator had to be before the sea urchin took note — and duly directed a full battery of spikes at it.

"Ordinarily, sea urchins move towards dark areas in order to seek cover," Kirwan explains. "When I notice that they react to certain sizes of images but not to others, I get a measurement of their visual acuity."

A baby sea urchin looks a lot like a lunar lander.
A baby sea urchin looks a lot like a lunar lander. KQED Science

Kirwan concluded the incoming object must take up somewhere between 30 and 70 degrees of the animal's surrounding 360-degree area for it to be noticed. That's considerably poorer than human vision, which can detect an object taking up just 0.02 degrees of visual range.

"However, this is still sufficient for the animal's needs and behaviour," Kirwan notes. "After all, it's hardly poor eyesight for an animal with no eyes."

Besides, when your entire body is covered with spikes, the point may not be so much to see, as be seen — and feared.