Eyes of the Urchins

During the daytime in the subtidal zone, when light filters through the water just below the ocean's surface, sea urchins scramble for a place to hide. They're looking for a crevice where they can lie low until evening, when predators are few and food is plenty. As it turns out, they find their hiding places without the benefit of a specialized sense organ like the eye.

Sönke Johnsen, associate professor of biology, designed an experiment to determine how and how well sea urchins "see." He found that they could sense when an object was nearby (the test made sure this recognition couldn't have been caused by factors like vibration or sound) but couldn't tell exactly what it was. Some sea urchins moved toward the object; others moved away.

Johnsen explains that vision is a continuum: Organisms receive details about their environment in varying degrees. The human eye has a lens that directs light to a specific point on the retina. Sea urchins' spines direct light to their hard shells, which are photosensitive. This mechanism is unique in the animal kingdom.

In effect, he says, the sea urchin's entire body "acts as one large eyeball."

Eyes of the Urchins


Ralph A. Clevenger/CORBIS


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