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Study Suggests Brainless Brittlestars Have Sophisticated Visual System

Certain species of sea-dwelling brittlestars¿relatives of starfish and sea urchins¿have long impressed scientists with their ability to respond to changes in light intensity¿a talent that enables them to detect and evade predators. Yet exactly how the creatures, which lack specialized eyes, detect such changes has eluded explanation. Now a report appearing in the journal Nature finally offers insight. Microscopic crystals embedded in the brittlestar's calcite skeleton appear to serve as lenses that together form a visual system unique among all extant animals.

Joanna Aizenberg of Bell Laboratories/Lucent Technologies in Murray Hill, N.J., and her colleagues began to suspect that the crystals held the key when they noticed that light-indifferent species do not have such structures. The crystals found in their light-sensitive cousins, the researchers reasoned, thus might well function to focus light onto underlying photosensitive tissues. To assess the lensing effect, the team placed a calcite crystal array from the species Ophiocoma wendtii above a layer of photosensitive material and exposed it to light. Subsequent examination of the photosensitive material showed that the light had left its mark in spots that had been directly underneath calcite crystals. Furthermore, the distance at which each lens focused the light on the photosensitive material corresponded to the distance between the lenses and the presumably photosensitive nerve bundles in the brittlestar itself.

The authors observe that the brittlestar's arrays of dome-shaped crystals (which focus light better than the spherical variety) are positioned such that the animal could, in theory, gather a considerable amount of visual information about its environment. "Although we have only limited evidence that the lens apparatus operates at a distance, which would conform to the definition of an eye, our results suggest that the presence of such structures may be sufficient to elicit the rapid, coordinated behaviors, such as detection of predators and retreat toward crevices, that imply the occurrence of vision," they write. "These are the very abilities that are central to the survival of individuals of O. wendtii in their natural habitat."

In a commentary accompanying the Nature report, Roy Sambles of the University of Exeter writes, "Human ingenuity came up with microlens arrays only a few years ago, and they are used in directional displays and in micro-optics, for example as signal-routing connectors for signal processing. Once again we find that nature foreshadowed our technical developments."

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