Sea urchins do not possess eyes, but instead can see using the tips of their tentacle-like feet. The quality of their vision through these feet has now been tested by Lund University in Sweden, in a new study that has shown that while sea urchins have fairly low resolution vision, it is good enough to fulfil their basic needs.
Sea urchins belong to the phylum Echinodermata along with sea stars and sand dollars, and typically have a spiny, globular appearance. About 950 species live throughout Earth’s oceans, where they inhabit the seabed in depth zones from the intertidal to 5,000 metres (16,000 ft). Sea urchins are sensitive to and negatively attracted to light, so seek to hide themselves in crevices or under objects.
“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. You could say that the entire sea urchin is one single compound eye,” says John Kirwan, who conducted the study as a part of his doctoral thesis, together with colleagues at Lund University.
John Kirwan tested the sea urchin species Diadema africanum using two different visual responses: a taxis towards dark objects and an alarm response of spine-pointing towards looming stimuli.
Initial experiments placed the animals in water inside strongly illuminated cylinders that had various dark images on the walls.
“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,” explains John Kirwan.
To obtain further data, he carried out another experiment in which he showed rapidly growing figures above the sea urchins, as a way of conjuring up an image of an approaching predator. He then registered how large the figures had to be before the sea urchins would defend themselves by directing their spines towards the shadow above.
The acuity of vision was calculated using X-ray tomography and electron microscopy.
John Kirwan’s calculations show that of the 360 degrees surrounding the sea urchin an object must take up between 30 and 70 degrees for the sea urchin to see it. Humans only need an object to take up 0.02 degrees in order to detect it, making it clear that their eyesight is poor in comparison with human eyesight.
“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.
John D. Kirwan, Michael J. Bok, Jochen Smolka, James J. Foster, José Carlos Hernández, Dan-Eric Nilsson. The sea urchinDiadema africanumuses low resolution vision to find shelter and deter enemies. The Journal of Experimental Biology, 2018; jeb.176271 DOI: 10.1242/jeb.176271
Long-spined urchin in Panama. Credit: Scripps Oceanography/UC San Diego