How do fish get their shimmering, iridescent scales? It’s a question that has intrigued scientists for centuries. Robert Hooke first attempted to decipher the outer structure of the silvery insects called silverfish in the mid 17th century, and by the 1860s researchers had discovered the material responsible: guanine, later identified as one of the four nucleic acids that make up DNA.
Yet only with the advent of techniques for observing materials at really high resolution have scientists begun to understand how structures that don’t have color themselves can produce such colorful effects. Dr. Avital Levy-Lior, and Profs. Stephen Weiner
and Lia Addadi
of the Structural Biology Department in the Faculty of Chemistry, together with Dr. Eyal Shimoni of the Electron Microscopy Unit, recently applied some of the most advanced microscopy methods available to take yet another look at nanoscale guanine structures in fish scales, and also in spiders.
“Guanine forms crystals, and it alone of the nucleic acids is widely used outside of the DNA in all sorts of organisms – almost always for manipulating light. In addition to shiny skin and scales, it is sometimes found in the eyes of animals,” says Weiner.
When left to grow on their own, guanine crystals are thick and chunky. Indeed, several kinds of spider the team investigated, on the basis of information from Dr. Geoff Oxford of York University, UK, had this kind of guanine crystal in their tissues. But these spiders aren’t shiny silver – their color is matte white. By contrast, in both the fish scales and the silver spiders, the guanine crystals _ which are in the form of thin, flat plates _ reflect light strongly in one direction. When these plates are stacked one atop the other, the light reflecting back from the various layers interferes with the incoming light rays, causing the shimmering effect. Weiner: “The specialized cells in which the crystals form have to control their growth to get them to develop in the right direction, as well as to arrange them into stacks. They do this in a separate vesicle for each crystal.”
The researchers found that the crystal plates in fish and spiders are about the same size and thickness. But in addition to how well they reflect light, the spacing and orientation of the plates are what determine how shiny the final product will be. Both form stacks of plates: In fish scale structures, there are about 30 in a stack; while in spiders far fewer; they compensate for this by building sandwich-like structures in which a “filling” of uncrystallized guanine sits between pairs of plates. Together with Dr. Dan Oron
and Osip Schwartz of the Physics of Complex Systems Department, the scientists calculated the expected reflectivity and concluded that fish and spiders achieve approximately the same levels of reflectivity.
Weiner: “Both structures seem to work equally well. But our findings suggest that fish and spiders may use somewhat different means of directing crystal growth. This implies that even though the two colors appear similar to the naked eye, they evolved separately. In fact, it’s likely that the use of guanine crystals evolved many times over in different species.”
Prof. Lia Addadi's research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research. She is Head of the The Aharon Katzir-Katchalsky Center. Prof.Addadi is the incumbent of the Dorothy and Patrick Gorman Professorial Chair.
Prof. Stephen Weiner's research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Helen and Martin Kimmel Center for Archaeological Science; the Maurice and Vivienne Wohl Charitable Foundation; and the estate of Hilda Jacoby-Schaerf. Prof. Weiner is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.