Taking a crab's-eye view of the world.

Taking a crab's-eye view of the world

Some of the earth's simplest creatures have a most complicated way of seeing the world. In trying to explain how certain crabs see, a Swedish scientist has described the most complex eye structure known to exist. The newly discovered eye--which the researcher calls "a remarkable combination of ordinary lenses, cylindrical lenses, parabolic mirrors and light-guides"--joins eight other imaging systems found in animals.

In a paper published last week, Dan-Erik Nilsson at the University of Lund reports that many crabs and hermit crabs, plus a few mayflies, have eyes more complicated than previously thought. Their eyes--deluxe models of the imaging systems called compound eyes -- collect light beams through an array of lenses and focus them at a point inside. N ilsson, who calls this imaging system a "parabolic superposition" eye, found that it combines reflection, different lens shapes and slim light-guides that carry light from the lenses to the retina.

While looking at crab eyes that had been in the dark, Nilsson accidentally discovered the system, which he says may have remained hidden from other scientists because studies are ordinarily done in the light. "If you look at these eyes just anatomically [structurally] and look at them in ordinary light, they all act as ordinary compound eyes," he says. But when using dark-adapted eyes, Nilsson noted that different units did not process light as expected -- suggesting an unknown mechanism was responsible.

In a commentary accompanying Nilsson's March 3 NATURE article, Michael F. Land of the University of Sussex in Brighton, England, says the novel eye is so difficult to decipher because of its "real deviousness" in bending light differently in different planes.

Such eyes probably exist only in crustaceans and insects, says Nilsson, who suspects the system evolved because it was more efficient in collecting light in darker environs. Although previous imaging systems found in animals have been translated into fiber optics and X-ray telescopes, Nilsson says it is too early to tell whether the parabolic superposition model will likewise illuminate the field of optics.