Eyeless urchins can "see " with spines

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Sea urchins, like their close relatives the 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.

According to a report in National Geographic News, to explore urchins' visual capabilities, Sonke Johnsen and colleagues at Duke University, US, 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," he added.

Johnsen's team found that all the sea urchins seemed oblivious to the smaller disks.

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 urchins were unsure whether the black spots signaled predator or prey.

But the behavior suggests that even densely spined species such as S. purpuratus have limits to their visual ranges, according to the study authors.

Later experiments with other species could reveal the role spine density plays in how well urchins can see.

The finding also suggests that, even though urchins don't have eyes, their visual abilities are similar to those of marine invertebrates that do have eyes, such as the nautilus and the horseshoe crab.

--ANI