Brittlestars, also known as serpent stars, are marine invertebrates that usually have five thin long arms emanating from a small, disk-shaped body. They belong to the phylum of echinoderms, which also includes sea urchins, sea cucumbers, starfish, and several other classes of marine animals.
Over the past few years, Prof. Lia Addadi, dean of the Weizmann Institute's Chemistry Faculty, and Prof. Steve Weiner, of the Institute's Structural Biology Department, have conducted a series of studies examining ways in which animals build their skeletons. The scientists have revealed that animals produce different types of proteins, some of which control crystal formation.
The idea for the current study was born when the Institute scientists met with Dr. Gordon Hendler of the Natural History Museum of Los Angeles County. Dr. Hendler brought their attention to one particular species of brittlestars, Ophiocoma wendtii; he had found that this species, which appears to be particularly sensitive to light, can change its color. Even though these animals have no specialized eyes, they are capable of detecting shadows and quickly escaping from predators into dark crevices. Hendler suspected that the arrays of spherical crystal structures on the surface of its outer skeleton serve as lenses that transmit light to the brittlestar's nervous system. This hypothesis was reinforced by the fact that within their skeletons, brittlestars indeed have relatively extensive nerve networks. Moreover, the movement of pigmented cells between the crystal structures and the nerves appeared to alter the brittlestar's response to light.
Addadi and Weiner, together with their then graduate student, Joanna Aizenberg, who now works at Bell Laboratories, began to study the phenomenon. They discovered that each skeletal element with its hundreds of lenses is a single calcite crystal; the crystal's optic axis is roughly perpendicular to the plane of the lens array. This means that the calcite lens array is capable of transmitting light without splitting it in different directions. But does the lens's geometrical shape place its optical focus precisely over the area where the brittlestar's nerves are located under the skeleton? In other words, do the lenses guide and focus light and transmit the concentrated rays inside the tissues, to the nervous system?
These questions remained unanswered for almost 10 years until recently, when the scientists found a way to examine them experimentally in a controlled manner. The experiment was conducted at Bell Laboratories with the help of lithography, a semiconductor technology. Dr. Aizenberg removed a calcite crystal array from the skeletal element of the brittlestar species Ophiocoma wendtii, placed it above a layer of photosensitive material, and exposed the system to light. She found that light had reached the photosensitive material in spots directly underneath the calcite crystals. By altering the distance between the lenses and the material, she found that the estimated focal distance of each lens - at which the lens concentrates the light by about 50 times- coincided with the depth at which nerve bundles, which presumably serve as photoreceptors, are located in the bodies of brittlestars.
Thus the crystalline lenses and the pigmented cells in the skeletons of Ophiocoma wendtii brittlestars act as "corrective glasses," filtering and focusing light on the photoreceptors. This type of "visual" system has never before been described in animals living on our planet today, but Prof. Weiner notes that calcite crystals were also used in the compound eyes of trilobites, the now extinct marine animals that inhabited the earth some 350 million years ago.
In their Nature report, the scientists conclude: "The demonstrated use of calcite by brittlestars, both as an optical element and as a mechanical support, illustrates the remarkable ability of organisms, through the process of evolution, to optimize one material for several functions, and provides new ideas for the fabrication of 'smart' materials."
The Weizmann Institute of Science, in Rehovot, Israel, is one of the world's foremost centers of scientific research and graduate study. Its 2,500 scientists, students, technicians and engineers pursue basic research in the quest for knowledge and to enhance the quality of human life. New ways of fighting disease and hunger, protecting the environment and harnessing alternative sources of energy are high priorities at Weizmann.