Sea urchins are notorious for their spiky appearance. These round-shaped echinoderms are nothing but a ball of thorns, a globe of prickly spines without other discernable features. You can’t even make out where their eyes are, but that’s because they simply don’t have any.
But the lack of eyes is no catastrophe, as sea urchins manage to see well enough without them, thank you very much. In fact, they are the only creatures known to pull that off and can actually see with their feet, Science Daily reports.
Concealed among their threatening thorns are tentacle-like tube feet that the sea urchins use for, well, pretty much everything — from moving around from one place to another, to grabbing food and directing it into their mouths, to, perhaps the most surprising usage of all, vision.
In a new study published in The Journal of Experimental Biology, a team of scientists from Lund University in Sweden argue that, while these marine creatures have relatively poor eyesight — understandable in an animal with no eyes — they still see well enough to cover their basic needs.
The idea that sea urchins can see with their feet is not a new one, notes Discover Magazine. This oddity of nature was first described in a 2011 study on purple sea urchins (Strongylocentrotus purpuratus), published in the Proceedings of the National Academy of Science.
Building on that research, the Lund team devised a series of experiments to test just how good the sea urchins’ vision really is. What they uncovered is that sea urchins have a fairly low resolutions vision and mostly see dark and light, but that’s enough to get them through the day.
“Sea urchins are currently the only animals that have been shown to see without having eyes. They see using light-sensitive cells in their tube feet, which resemble tentacles and, like the spines, are all over the body,” explains study lead-author John Kirwan. “You could say that the entire sea urchin is one single compound eye.” In other words, an eyeball with spikes.
Kirwan’s study focused on specimens belonging to the long-spined sea urchin species called Diadema africanum, which the team placed in strongly illuminated cylindrical tanks and then proceeded to give them a sort of an eye chart test featuring dark images of various sizes.
Another test involved displaying black circles against the walls of the tanks. Made to look like predators, these circles tested how large an object needs to be so that it can be spotted by a sea urchin and trigger a defensive reaction.
“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,” Kirwan talks us through the process.
As it turns out, sea urchins have a considerably poorer eyesight than humans do. These thorny globes can’t spot an object unless it takes up between 30 and 70 degrees of the 360-degree field surrounding the animal. Meanwhile, we can detect things at 0.02 degrees.
“This means that the sea urchin’s picture of the world is very crude in human standards,” notes biologist Dan-Eric Nilsson, co-author of the study. “But it is good enough to guide movements towards suitable structures in their environment.”