Secret of Lake Sapphires solved

Crystal plaques make Sapphirina crabs impressive color wonders

Iridescent Blue: a Sapphirinia Cancer © Kaj Maney /
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Fascinating play of colors: The small Sapphirina crabs impress with their intense blue or colorful glow - and they can even make themselves invisible. What's behind this trick, researchers have now found out. Cleverly layered crystal nanoplates create the iridescent play of sea sapphires and can even shift incident light into the UV range.

Common copepods are usually rather inconspicuous contemporaries because they are tiny and almost transparent. But there is one exception: the genus Sapphirina. Because this oceanic crust radiates in unique beauty and intensity. When light falls on the delicate tank of the males, it lights up bright blue. This gave them the surname See-sapphires.

"Gemstone Water" and invisible UV

In Japan, the sea sapphires at certain times made the whole surface of the sea light up, fishermen called this phenomenon "Tama Mizu" - gemstone water. But the only a few millimeters small Krebschen not only radiate blue, depending on the light, the color changes to iridescent red, yellow or green. Some sea sapphires can even disappear completely: they shift the light they reflect into the ultraviolet range and thus become invisible to our eyes.

The cause of the colorful glow of the sea sapphires are not pigments, but structural colors, as researchers found out a few years ago: The carapace of this copepods is covered by thin, hexagonal platelets, each consisting of several layers of crystals. If light strikes these crystal layers, it will be broken slightly differently at each layer. As a result, the wavelengths of light shift against each other and certain color components are amplified, others swallowed. The result is an extremely intense radiant color.

Play of Colors and Disappearance of a Lake Sapphire (Sapphirina) © Kaj Maney,

The separation layer is crucial

Dvir Gut and his colleagues from the Weizman Institute of Science in Israel have found out why not all sea sapphires shine in the same hue and how exactly the color change comes about. For their study, they further analyzed the fine structure of the iridescent spinal platelets and determined how each of them breaks and reflects back light of specific wavelengths. display

It showed: The crystal layers of the plates are made of guanine, the base, which also forms one of the "letters" in the genetic code. Between the layers of guanine crystals stacked one above the other lies a thin cell plasma membrane, which is crucial for the color. "The thickness of the separation layer is mainly responsible for what different colors of the copepod can produce, " the researchers report.

New ideas for artificial photonic crystals

The thicker the Trennh utchen, the further the individual crystal layers are apart - and this affects the refraction pattern of the incident light. This is also evident in the sapphirina species, which can be made invisible by UV light: its small plate structure is optimized so that it shifts the light incident at a 45-degree angle into the ultraviolet range ckwirft.

The new findings on this iridescent nanostructure could also help to develop new optical materials, the researchers said. Such photonic crystals could, for example, help to improve reflective coatings, mirrors or displays. (Journal of the American Chemical Society, 2015; doi: 10.1021 / jacs.5b05289)

(American Chemical Society, 20.07.2015 - NPO)