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A few small crystals give a simple fish with bright, colorful spots under the light

As the light shines constantly on a silver slip of a fish, tiny dots begin to flash: blue, yellow, blue, yellow.

The bodies "do not shine like luminous fish," discovered Masakazu Iwasaka, an interdisciplinary engineer at Hiroshima University in Japan. Instead of making their own light, it turns out that the small photonic crystals noticeable in fish spots reflect certain wavelengths of light, alternating between blue and greener yellows, reports April 7 in the Royal Society Open Science.

Many biological materials have developed tricks that manipulate light. The iconic blue morph butterfly does not have a blue pigment flake. Create your perfectly blue sky with lots of microscopic plates that manipulate light. So do the blue-leafed begonias (SN: 28/11/16).

Those fish reflectors are doing something similar on the broadband hard silver (Atherinomorus lacunosus). “I discovered the flickering of a small spot by chance” while examining points no larger than 7 to 10 micrometers in diameter on the fish’s backs, he says. Inside the reflective spots are small platelets of guanine compound that have grown in such a way that they can reflect colored light depending on the angle.

When a constant light shines on the back of the resistant broadband plates (Atherinomorus lacunosus), the small spots pulsate from blue to greenish yellow. A new study finds that everything is a trick to reflect light.

Guanine may seem familiar. It is one of the four major coding units that are paired in the storage of DNA genetic information. What gives fish guanine platelets particular abilities remains an enigma. Iwasaka suspects that inside a stain the platelets move in such a way that they change their apparent color and dazzle the power. The blue-yellow light only pulses on the bright silver. Dead fish only reflect white-white.

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Iwasaka hopes to create human-made counterparts for fish reflectors. He proposes to mimic fish structures for sensors much, much smaller than the period of a magazine page. Versions of small shimmering fish lights could fit into the world of micro-electromechanical systems (MEMS) to control conditions within living tissues, responding to light or flashing by themselves. In previous work, he showed how guanine platelets can be manipulated in magnetic fields, suggesting that such sensors could be targeted and snatched.

What the fish uses for its lights remains a mystery. Intermittent spots are not unique to resistant broadband silver, Iwasaka points out. At least two other papers reported blinking (blue to red) in other tropical fish, probably for communication. Maybe the silver flicker also communicates something, Iwasaka says.

Or there may be security benefits. Fish ecologist David Conover of the University of Oregon at Eugene has worked with a silver Menidia species from the same fish family that studies the Iwasaka species. “For fish that live in bright light and close to the surface, like silver, reflectivity is likely to serve as a kind of camouflage or distraction from predators lurking or hitting from below,” says Conover.

Anything that drives the evolution of iridophores, those reflective points in which Iwasaka found inspiration, are common in the world of fish. There could be many more places to look for flashes.

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