Credit: Wikimedia Commons.

Sea urchins lack eyes — so they see with their feet

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. 

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