Newswise — Tiny ocean creatures known as sea sapphires perform a sort of magic trick as they swim: One second they appear in splendid iridescent shades of blue, purple, or green, and the next they may turn invisible (at least the blue ones turn completely transparent). How do they get their bright colors and what enables them to “disappear?” New research at the Weizmann Institute of Science has solved the mystery of these colorful, vanishing creatures, which are known scientifically as Sapphirinidae.
The findings, which recently appeared in the Journal of the American Chemical Society, could inspire the development of new optical technologies.
Sapphirinidae belong to a subclass of crustaceans called copepods, and can live in fresh or salt water. These animals are barely visible to the human eye, ranging from around one to several millimeters in length. It is the male Sapphirinidae that display striking, iridescent colors, whereas the female is transparent. Scientists think that their unique magic trick could help Sapphirinidae escape predators when necessary, but still display their flashy colors when a female of the species – or possibly another male – is nearby.
The scientists – Prof. Lia Addadi, Prof. Steve Weiner, Dvir Gur, and Maria Pierantoni of Weizmann’s Department of Structural Biology; Prof. Dan Oron and Ben Leshem of Weizmann’s Department of Physics of Complex Systems; and Dr. Viviana Farstey of the Interuniversity Institute for Marine Sciences, Eilat, Israel – investigated the makeup of a crystal layer on the backs of several species of male Sapphirinidae. They first measured the reflectance, which determines the color, and then, using a microscope technique called cryo-SEM, observed the organization of the crystals and the cellular material holding them in place.
These colors are due to iridescence – the result of light reflecting off periodic (repeating) structures. These multilayer reflectors – a type of structure known to scientists as a photonic crystal – are composed of thin, transparent crystals of guanine. Guanine is more generally known as one of the nucleic acid bases found in DNA.
The research group found that the guanine plates in Sapphirinidae are stacked in incredibly precise periodic arrays. What gives each species its unique color? Their analysis revealed that the main factor determining whether an animal will be yellow, blue, or purple is the spacing between plates, which is controlled by the thin layer of cellular material separating them.
The researchers also showed how this complex arrangement of plates enables some Sapphirinidae to disappear from sight: When certain species of male Sapphirinidae rotate their backs to the light at a 45-degree angle as they perform a spiral swimming maneuver, the wavelength of the reflected light is shifted out of the visible light range and into the invisible ultraviolet. In contrast, light hitting the sea sapphire directly returns the beautiful blue color. In the ocean’s light, which comes from above, the tiny creature can control its visibility, from neon to none, just by adjusting its rudder.
The spacing between the plates acts as a sort of “tuner” for the wavelength of the light and, thus, the organism’s color: The closer the plates are to one another, the shorter the wavelength – that is, the bluer the light reflected from them. This sophisticated strategy for manipulating light, say the scientists, could be used in the design of artificial photonic crystal structures – nanoscale structures that can manipulate the flow of photons. Such structures could have many potential uses, including adaptive or changeable reflective coatings, optical mirrors and optical displays.
Prof. Lia Addadi’s research is supported by the Jeanne and Joseph Nissim Foundation for Life Sciences Research. Prof. Lia Addadi is the incumbent of the Dorothy and Patrick Gorman Professorial Chair.
Prof. Dan Oron’s research is supported by the Crown Photonics Center; the Deloro Institute for Advanced Research in Space and Optics; the Willner Family Leadership Institute for the Weizmann Institute of Science; the Leona M. and Harry B. Helmsley Charitable Trust; and the Wolfson Family Charitable Trust.
Prof. Stephen Weiner’s research is supported by the Helen and Martin Kimmel Center for Archaeological Science, which he heads; the Exilarch’s Foundation; the European Research Council; and the estate of George and Beatrice F. Schwartzman. Prof. Weiner is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.
The Weizmann Institute of Science in Rehovot, Israel, is one of the world’s top-ranking multidisciplinary research institutions. The Institute’s 3,800-strong scientific community engages in research addressing crucial problems in medicine and health, energy, technology, agriculture, and the environment. Outstanding young scientists from around the world pursue advanced degrees at the Weizmann Institute’s Feinberg Graduate School. The discoveries and theories of Weizmann Institute scientists have had a major impact on the wider scientific community, as well as on the quality of life of millions of people worldwide.