In the blink of an eye, a squid’s “smart skin” can switch color and pattern for the purpose of camouflage or sexual signaling—a virtuosic display that has long fascinated scientists.
Now, scientists from the UChicago-affiliated Marine Biological Laboratory and from Northeastern University report a paradigm-shifting discovery in how specialized organs in squid skin, called chromatophores, contribute to the feat via an elegant interplay of pigmentary action and structural coloration. Their study, which brings bio-inspired engineers closer to building smart skin, was recently published in Nature Communications.
“People have been trying to build devices that can mimic cephalopod color change for a long time by using off-the-shelf components,” said Leila Deravi, an assistant professor of chemistry and chemical biology at Northeastern, whose lab led the study. “Nobody has come anywhere near the speed and sophistication of how they actually work.”
Anterior mantle of live squid. Reflective chromatophores sparkle as angle of incident light changes. (Courtesy Stephen Senft, Hanlon Lab, MBL)
Deravi and MBL Senior Scientist Roger Hanlon, a leading expert on camouflage in cephalopods (squid, octopuses and cuttlefish), led an interdisciplinary team of researchers to investigate squid dynamic coloration on a molecular level.
Squid skin contains two types of structures that manipulate light to produce various colors. The chromatophores contain elastic sacs of pigment that stretch rapidly into discs of color when the muscles around them contract. When light strikes the pigment granules, they absorb the majority of the wavelengths and reflect back only a narrow band of color.
Deeper in the skin, cells called iridophores reflect all the light that hits them. By scattering this light, a method known as structural coloration, they bounce back a bright sheen of iridescence.
For decades, all available data had indicated that these separate structures could only produce one type of coloration or the other: pigmentary or structural. But when co-author and MBL researcher Stephen L. Senft looked closely at the squid chromatophores, he spotted iridescence shimmering in perfect alignment with the pigment.
“In that top layer, embedded into the chromatophore organ, is structural coloration,” Hanlon said. “No one had found anything like that.”
Hanlon, who has spent the better part of four decades studying cephalopod biology, went back through his old Kodachrome slides of chromatophores. Sure enough, he found a photograph of blue iridescence reflecting from a chromatophore. At the time, he had assumed the shimmering blue was from an iridophore deeper in the skin.
“I saw this in 1978, and I didn’t realize what I was looking at,” Hanlon said. “It’s incredible.”
Excised adult squid skin. Multi-hued reflectance coincides with moving shapes of chromatophores. (Courtesy Stephen Senft, Hanlon Lab, MBL)
This time, the researchers are sure the iridescence is coming from the chromatophore. The team found the proteins that create iridescence in the cells surrounding the pigment sacs.
This unexpected discovery—that the chromatophore is using both pigmentary and structural coloration to create its dynamic effects—opens up new opportunities for biologists and chemists alike.
“We kind of broke up the known paradigm of how the skin works in the cephalopod world,” Hanlon said.
Biologists like Hanlon can use this new information to better understand these fascinating species. Applied chemists like Deravi can use it to work on reverse-engineering the color-change abilities of cephalopods for human use.
“We’re piecing together a roadmap, essentially, for how these animals work,” Deravi said. “Our ultimate goal is to try to create something like a material, a wearable device, a painting or a coating, that can change color very quickly like these animals do.
“It’s not as far-fetched of a goal today as it was even three years ago.”
The Marine Biological Laboratory is dedicated to scientific discovery—exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.
Citation: “Dynamic pigmentary and structural coloration within cephalopod chromatophore organs” Nature Communications, March 1, 2019. doi: 10.1038/s41467-019-08891-x