Tiny crystals give a plain fish twinkling, colorful dots under light
Puzzling fish twinkles from wide-banded hardyhead silversides might lead to ultra-tiny sensors
![scores of small silver fish swimming](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2021/04/040621_sm_flashyfish_feat.jpg?fit=1030%2C580&ssl=1)
You don't see it from a distance, but the white backs of hardyhead silverside fish (seen here swimming in the Red Sea) can twinkle with tiny flashing blue and yellow dots.
Georgette Douwma/Science Source
By Susan Milius
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.