How a star avoids the limelight: some echinoderms have thousands of eyes on their backs. When the lights come on, they switch to wearing shades

Natural History, May, 2004 by Adam Summers

By day, gaudy reef fishes dominate the scene at a coral reef, but by night, invertebrates steal the show. Coral polyps--at least the ones that can't depend on nutrients provided by photosynthesizing symbiotic partners--extend their tentacles in the darkness to feed. Feather stars spread their arms, each grooved to direct the flow of food to the mouth. Spiny lobsters march on parade. And night is the time to get a good look at one of the shiest of all the reef dwellers: the brittle star.

Only at night does the brittle star venture out of its hiding place to snake across the surface of the reef in search of food. But divers looking for one should be careful with their lights, because shining a light on an outstretched limb might just cause the animal to hide.

The reaction has always intrigued me, because brittle stars have no obvious eyes. Nevertheless, they can detect light, and a team of investigators has now figured out how they do it. It turns out the dorsal side of the brittle star is covered with microscopic lenses embedded in its skeleton, making the entire back of the creature into a compound eye.

Brittle stars are echinoderms--relatives of sea stars and sea urchins--and can be distinguished from the other groups by their long, thin, highly flexible arms and their small central disk. But, like all echinoderms, they have a hard internal skeleton made up of small calcite plates, held together by catch connective tissue [see "Catch and Release," by Adam Summers, November 2003]. It has been known for decades that some of these animals can sense light. In the 1980s Gordon Hendler, a zoologist at the Natural History Museum of Los Angeles County, and Maria Byrne, a developmental biologist at the University of Sydney in Australia, pointed out that members of one brittle star species change from their nighttime color scheme of banded gray and black to deep brown in the daytime.

Responding to light does not require an eye. A sea snake in the subfamily Hydrophiinae, for instance, can perceive its tail being exposed as the snake forages on the reef, and move it out of the light. But that response would merely require the rail to have sensory neurons that respond to light--or to heat, the by-product of light on a dark surface. Some brittle stars, though, have responses to light that do seem to require an eye. When placed in bright sunlight, members of those species will make for a shadowed area as fast as their five little arms can carry them. Such behavior suggests they can detect shelter at a distance and even form an image of their surroundings.

Joanna Aizenberg, a materials scientist at Bell Laboratories in Murray Hill, New Jersey; Alexei Tkachenko, a physicist now at the University of Michigan in Ann Arbor; and Stephen Weiner and Lia Addadi, both structural biologists at the Weizmann Institute of Science in Rehovot, Israel, teamed up with Hendler to examine the hidden eye of the brittle star. The investigators used a scanning electron microscope to look at skeletal calcite plates from the upper surface of a member of the light-sensitive brittle star species Ophiocoma wendtii. What they saw was an unusual pattern of densely packed, crystal-clear bumps, each thinner than a human hair. Similar plates from a non-light-sensitive species (O. pumila) had no such bumps, and neither did plates from the underside of the arms of O. wendtii.

Might the bumps serve as lenses to focus light? Aizenberg and her colleagues established good theoretical reasons for thinking they could. The calcite crystal that makes up a bump is aligned so as to conduct light from the outside surface of the plate to the inside surface. Furthermore, because the bumps are not perfect spheres, they don't have the blurriness characteristic of spherical lenses. By measuring the shape and size of the bumps, the investigators calculated that they would focus light between four and seven micrometers (just a few ten-thousandths of an inch) beyond their inner surface.

Maria Byrne had previously examined a plate and surrounding tissue with a transmission electron microscope and found a bundle of nerves at the appropriate distance--at the calculated focal point--from each bump. Cells containing pigment reside alongside and below each bump. During the day, however, the pigmented cells migrate to the upper surface of the calcite ossicles, where they shield the top of the lens from the bright sun and, incidentally, darken the animal. The color change is not a camouflage response; rather, it is the echinoderm version of sunglasses.

Following up this work, Aizenberg exposed a sheet of film to light passing through the presumed array of lenses. Her results were striking. A small spot of light hit the film under each bump. The size of each spot varied with the size of the bump, and the brightness of the spot depended on the incident angle of the light with respect to the surface of the brittle star arm. In other words, each tiny, embedded bump acts as a directional light sensor, responding most strongly to light coming from a particular direction. The bump is thus a rudimentary lens.