Sea urchins may use the entire surfaces of their bodies—from the ends of their "feet" to the tips of their spines—as huge eyes.
Scientists had already known the marine invertebrates react to light without any obvious eye-like structures—raising the question of how the animals see.
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Previous genetic analysis of the California purple sea urchin had revealed that the animals possess a large number of genes linked with the development of the retina—the light-sensitive tissue lining the inner eyeball in people and other vertebrates.
This and other research suggested that sea urchin vision might rely on light-receptor cells randomly scattered across their skin, which collectively function like retinas.
Scientists had theorized the animals' spines simulate the light-blocking pigmented cells found in most animals' eyes. Because light-receptor cells in the retina can soak up light from every direction, pigmented cells work to block light from the back and the sides so animals can "see" what's in front of them.
Now, however, the scientists have found two distinct groups of bristly, light-receptor cells concentrated at the bases and tips of the purple sea urchin's 1,400-plus tube feet. These long, suction-tipped tubes, located on the undersides of sea urchin bodies, help the organisms move.
The team suspects that sea urchins use their tube feet as retinas and the rest of their bodies to shield against the extra incoming light, said researcher Maria Ina Arnone, a developmental biologist at Anton Dohrn Zoological Station in Naples, Italy.
Prior studies did find the number and placement of spines on a sea urchin could affect how sharp its vision might be, and this new find "might well be part of the picture," Arnone added.
The sea urchin-eye study appeared May 2 in the journal Proceedings of the National Academy of Sciences.