Tiny crystals give a plain fish twinkling, colorful dots under light

As light shines steadily on a silver slip of a fish, minuscule dots on the fish start flashing: blue, yellow, blue, yellow.

The bodies “do not glow like luminous fish,” Masakazu Iwasaka, an interdisciplinary engineer at Hiroshima University in Japan has discovered. Instead of making their own light, it turns out that remarkable little photonic crystals in fish spots reflect certain wavelengths of light, alternating between blues and more greenish-yellows, he reports April 7 in Royal Society Open Science.

Lots of biological materials have evolved tricks manipulating light. The iconic morpho blue butterfly doesn’t have a flake of blue pigment. It creates its dream-perfect sky blue with stacks of microscopic light-manipulating plates. So do blue-leaved begonias (SN: 11/28/16).

Those fish reflectors are doing something similar in wide-banded hardyhead silversides (Atherinomorus lacunosus). “I found the flashing of a small spot by chance” while screening the dots no bigger than 7 to 10 micrometers across on fish backs, he says. Inside the reflective flash spots lie little platelets of the compound guanine that have grown in such a way that they can reflect colorful light depending on the angle.

Guanine may sound familiar. It’s one of the four major coding units that pair up in storing DNA’s genetic information. What gives the fish guanine platelets particular abilities though remains a puzzle. Iwasaka suspects that inside a spot, platelets move in ways that change their apparent color and dazzle power. The blue-yellow light pulses only in living silversides. Dead fish just reflect white-white.

Iwasaka hopes to create human-made counterparts to the fish reflectors. He proposes mimicking fishy structures for sensors far, far smaller than the period on a magazine page. Versions of little sparkling fish lights could fit into the world of micro-electromechanical systems (MEMS) to monitor conditions inside living tissues, responding to light or flashing themselves. In earlier work, he’s shown how guanine platelets can be manipulated in magnetic fields, suggesting that such sensors could be targeted and herded.

What the fish uses its lightshow for remains a mystery. Flashing spots aren’t unique to wide-banded hardyhead silversides, Iwasaka points out. At least two other papers reported flashing (blue to red) in other tropical fishes, probably for communication. Maybe silverside flashing communicates something too, Iwasaka says.

Or there could be safety benefits. Fish ecologist David Conover of the University of Oregon in Eugene has worked with a silvery Menidia species in the same fish family as the species Iwasaka studies. “For fish that live in bright light and near the surface, as do silversides, the reflectivity probably serves as a type of camouflage or distraction from predators lurking or striking from below,” Conover says.

Whatever drives the evolution of iridophores, those reflective spots where Iwasaka found inspiration, they’re common in the fish world. There could be plenty more places to look for flashes.