Sea urchins don't seem to have any problems avoiding predators or finding comfortable dark corners to hide in, but they appear to do all this without eyes. So how do they see? It appears that sea urchins may use the whole surface of their bodies as a compound eye, and the animals' spines may shield their bodies from light coming from wide angles to enable them to pick out relatively fine visual detail. Divya Yerramilli and Sönke Johnsen from Duke University explain that if this is the case, sea urchins with densely packed spines will have better vision than sea urchins with sparsely packed spines, so they decided to test the vision of Strongylocentrotus purpuratus sea urchins, with tightly packed spines, to find out how well they see (p. 249).

Placing individual urchins in a brightly lit arena with a 6 cm or 9 cm diameter dark disk on the arena's wall, the team viewed the shadows of the moving animals from beneath the arena's white floor. Would the sea urchins see the disk and respond to it, or would they be oblivious to the disk's presence? Recording 39 urchins' responses to the disk at different positions around the arena's perimeter, the duo saw that the urchins wandered randomly around the arena when the 6 cm diameter disk was in place; they didn't respond to it. But it was a different matter with the 9 cm diameter disk; the urchins either raced toward it or fled in the opposite direction.

Calculating the visual angle of the 9 cm diameter disk from a sea urchin's perspective, Yerramilli and Johnsen suggest that the sea urchin's visual resolution is at least 10 deg. And when the pair calculated the sea urchin's visual resolution based on the animal's spine density, they found that it could be as good as 8 deg., but not good enough to see the smaller 6 cm diameter disk.

But why did some of the sea urchins career toward the disk while others turned away? Yerramilli and Johnsen suspect that it depends on the sea urchin's interpretation of the dark object. Some of the animals may interpret the object as a predator and flee, while others identify it as shelter and head towards it. What is more surprising is that the urchins' vision is as good as Nautilus and horseshoe crab vision, which is quite impressive for an echinoid that has turned its whole body into an eye.

Spatial vision in the purple sea urchin Strongylocentrotus purpuratus (Echinoidea)
J. Exp. Biol.