Image credit: georgeparrilla/Flickr

We humans make do with only two eyes. Spiders typically have eight eyes. Some insects and arthropods have compound eyes, made up of hundreds, even thousands, of lenses. According to new research, however, sea urchins might even top that. Indeed, scientists believe that an urchin's entire body might act as one giant eye.

Sea urchins don't have eyes as we think of them: a pair of orbital organs attached to a brain. In fact, as far as researchers can tell, sea urchins don't have brains at all. Instead, they use a distributed nervous system in which each part responds to local stimulus. Still, scientists have long wondered how these spiny creatures could be so adept at avoiding predators and finding dark corners without dedicated organs for sight.

Through careful genetic analysis, marine biologists have discovered light-sensitive molecules concentrated along urchin spines. Sönke Johnsen, who led the research, explained that, "it looks like the entire surface of their bodies are acting as one big eye."

In the lab, Johnsen and his team exposed urchins to very bright light. They then placed a black disk on the wall. Two thirds of the urchins quickly moved away from the disk while the remaining third "raced" towards it. Why some chose to approach the disk while others fled is unknown, but the response showed that sea urchins respond to visual stimulus. Johnsen explained that, "it's hard to examine their nervous systems, since their nerves are very, very small and the animals are more or less made of rock."

He went on to say that:

We think of animals that have a head with centralized nervous systems and all their sense organs on top as being the ones capable of sophisticated behavior, but we're finding more and more some animals can do pretty complex behaviors using a completely different style.

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