homehome Home chatchat Notifications


Sea urchins lack eyes -- so they see with their feet

They don't see too well, but it's not bad given the circumstances.

Tibi Puiu
June 19, 2018 @ 6:51 pm

share Share

Sea urchins are the only animals that we know of that can see without having eyes. Researchers from Lund University, Sweden, recently confirmed this fact in a new study, showing that the spiny, globular animals have low-resolution-vision.

Credit: Wikimedia Commons.

Credit: Wikimedia Commons.

The sea urchin is found across warm ocean floors worldwide, but rarely in the colder, polar regions. Sea urchins are commonly found along the rocky ocean floor in both shallow and deeper water and sea urchins are also commonly found inhabiting coral reefs. People usually aren’t too fond of them, as coming into contact with sea urchins can result in an inadvertent sting.

John Kirwan conducted the research as part of his Ph.D. thesis. He and colleagues focused on a sea urchin species called Diadema africanum, which they placed in strongly illuminated cylinders that had various dark patches on the walls.

Although they lack eyes, previous research had suggested that they can see through their tentacle-like tube feet. The experiment was designed to put this assumption to the test.

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

According to Kirwan, sea urchins have light-sensitive cells in their tube feet, which — like their famous spines — cover their whole body. This makes them unique in the animal kingdom.

“You could say that the entire sea urchin is one single compound eye,” Kirwan said.

In another experiment, Kirwan and colleagues introduced rapidly growing figures above the sea urchin, as a way of mimicking an approaching predator. He then recorded how large the figures had to be before the sea urchin reacted by pointing its spines towards the towering shadow.

According to his calculations, an object must take between 30 and 70 degrees of the sea urchin’s surrounding 360-degree area in order for the animal to see it. Humans, who have much better visual acuity, need an object to take up only 0.02 degrees to detect it. This made it clear that the sea urchin has a very low-resolution vision — but it does indeed see.

“However, this is still sufficient for the animal’s needs and behaviour. After all, it’s hardly poor eyesight for an animal with no eyes,” John Kirwan concludes.

Besides sensing light, the tube feet are used for feeding and locomotion. In some species, the tube feet also assist in attaching the sea urchin to surfaces or as levers to correct their position when they fall upside down.

Scientific reference: The sea urchin Diadema africanum uses low resolution vision to find shelter and deter enemies John D. Kirwan, Michael J. Bok, Jochen Smolka, James J. Foster, José Carlos Hernández, Dan-Eric Nilsson Journal of Experimental Biology 2018 : jeb.176271 doi: 10.1242/jeb.176271 Published 8 May 2018. 

share Share

A Brain Implant Just Turned a Woman’s Thoughts Into Speech in Near Real Time

This tech restores speech in real time for people who can’t talk, using only brain signals.

Using screens in bed increases insomnia risk by 59% — but social media isn’t the worst offender

Forget blue light, the real reason screens disrupt sleep may be simpler than experts thought.

We Should Start Worrying About Space Piracy. Here's Why This Could be A Big Deal

“We are arguing that it’s already started," say experts.

An Experimental Drug Just Slashed Genetic Heart Risk by 94%

One in 10 people carry this genetic heart risk. There's never been a treatment — until now.

We’re Getting Very Close to a Birth Control Pill for Men

Scientists may have just cracked the code for male birth control.

A New Antibiotic Was Hiding in Backyard Dirt and It Might Save Millions

A new antibiotic works when others fail.

Researchers Wake Up Algae That Went Dormant Before the First Pyramids

Scientists have revived 7,000-year-old algae from Baltic Sea sediments, pushing the limits of resurrection ecology.

A Fossil So Strange Scientists Think It’s From a Completely New Form of Life

This towering mystery fossil baffled scientists for 180 Years and it just got weirder.

ChatGPT Seems To Be Shifting to the Right. What Does That Even Mean?

ChatGPT doesn't have any political agenda but some unknown factor is causing a subtle shift in its responses.

This Freshwater Fish Can Live Over 120 Years and Shows No Signs of Aging. But It Has a Problem

An ancient freshwater species may be quietly facing a silent collapse.