The spiky body of a sea urchin acts as one big, spine-covered eye, confirms a new study that tested how well urchins can see.

Sea urchins, like their close relatives the sea stars (starfish), don't technically have eyes. Instead, the ball-like invertebrates detect light striking their spines and compare the beams intensities to get a sense of their surroundings.

To explore urchins' visual capabilities, Sonke Johnsen and colleagues at Duke University collected 20 Strongylocentrotus purpuratus sea urchins from the wild and tested their reactions to sets of black disks.

Each urchin was placed in an otherwise empty, well-lighted tank and presented with two disk sizes, first a disk 2.3 inches (6 centimeters) wide and then one 3.9 inches (10 centimeters) wide. Each disk was placed 20 inches (half a meter) away from the urchin.

"The urchins were really fussy to deal with. Some just wouldn't move, like deer in headlights … if you can imagine a deer as a spiked ball," Johnsen said.

"But I guess for them it was a bit like being in a Twilight Zone episode, just being stuck in a featureless, well-lit room."

Urchins Watch Out for You?

All the sea urchins seemed oblivious to the smaller disks, Johnsen's team found. But when the creatures were presented with the larger disks, some urchins fled while others moved closer.

Johnsen and his team don't know why the urchins reacted differently to the larger disks, although the researchers speculate that the herbivorous urchins were unsure whether the black spots signaled predator or food.

But the behavior suggests that even densely spined species such as S. purpuratus have limits to their visual ranges, the study authors say. Later experiments with other species could reveal the role spine density plays in how well urchins can see.