It sounds like the set-up to a joke: how does an animal with no eyes see? Researchers from Sweden trying to answer that question have found that the answer when it comes to sea urchins is that they use their whole body.
Eyes in the animal kingdom are incredibly varied. Some creatures are blind, others have incredibly complex eyes giving them vision they can rely on to stay safe - as anyone who's failed to swat a fly will know.
The eye is often so complex that since the days of Charles Darwin evolutionary biologists have marvelled at how such a structure could have evolved and it's even been used as an argument in favour of so-called Intelligent Design, the biblical theory of creation and evolution.
Despite having no eyes, however, a sea urchin is able to react to light. How?
Researchers already knew that these sea creatures had large numbers of genes which were familiar from our own retina - the light sensitive part of the human eye - and produce a protein that is found in many animal eyes, called opsin.
"It was this discovery that underpinned our research," says Sam Dupont from the University of Gothenburg's Department of Marine Ecology, one of the researchers behind the study and co-authors of the article. "We wanted to see where the opsin was located in sea urchins so that we could find the sensory light structures, or photoreceptors. We quite simply wanted to know where the sea urchin sees from."
So, while they have no eye as such, the urchins have many many light receptors, found on the end of the feet that cover their entire body.
"We argue that the entire adult sea urchin can act as a huge compound eye, and that the shadow that is cast by the animal's opaque skeleton over the light-sensitive cells can give it directional vision," says Dupont.
The study has been published in the Proceedings of the National Academy of Sciences (PNAS).
Top Image: Red Sea Urchin (Strongylocentrotus franciscanus) Credit: © naturediver