Opalescent coastal squid (Doryteuthis opalescens) are some of the most sophisticated shape changers on Earth. These curious cephalopods are lined with a special skin that can be precisely adjusted to a kaleidoscope of colors.
Scientists have long been fascinated by the remarkable camouflage and communication of this squid. New research has brought us even closer to figuring out how to pull out such an eclectic closet that allows them to hunt near the shine of the coast, slip through unseen predators, or even evade aggressive suitors making pair of false testicles.
Previous studies have shown that opalescent squid has a complex molecular machine in the skin: a thin film of stacked cells capable of expanding and contracting like an accordion to reflect the entire visible spectrum of light, from from red and orange to yellow and green, to blue. and violet.
These tiny grooves are like what you see on a compact disc, the researchers say, reflecting a rainbow of colors as you tilt it under the light. But, like a CD, this skin also needs something to amplify its colorful noise.
When the researchers tried to genetically design the skin of this squid, they noticed that something was slightly off.
The “motor” that tunes the skin grooves of squid is driven by reflectin proteins, which respond to different neural signals and control the pigment’s reflective cells.
Synthetic materials containing reflectin proteins have shown an iridescent appearance similar to what we see in squid, but these materials could not blink or shine in the same way.
Something was clearly missing and recent studies on live squid and genetic engineering have shed light on the mystery. It turns out that reflectin proteins can only shine if they are enclosed in a reflective membrane envelope.
This wrapper is what closes the structure of the accordion and, looking down, you can begin to see how it works.
Reflectin proteins usually repel each other, but a neural signal from the squid brain can disable this positive charge, allowing the proteins to bind very close together.
When this happens, it triggers the membrane it coats to push water out of the cell, reducing the thickness and space of the grooves, which divide the light into several colors.
This collapse between the slots also increases the concentration of reflectin, which allows light to be reflected even brighter.
Thus, the authors explain, this complex process “dynamically [tunes] the color while increasing the intensity of the reflected light, ”and this is what allows the opalescent squid to glow and flicker, sometimes with color and sometimes not.
Squid skin cells, which only reflect white light, also appear to be driven by this same molecular mechanism. In fact, the authors think that this is what allows squid to mimic the bright light or footprint of the sun on the waves.
“Evolution has so exquisitely optimized not only color tuning, but brightness tuning using the same material, the same protein, and the same mechanism,” says biochemist Daniel Morse of the University of California. Santa Barbara.
Engineers have been trying for years to mimic the remarkable skin of the opalescent squid, but have never succeeded. The new research, which received support from the U.S. Army Research Office, has helped us figure out where we were going wrong.
By themselves, thin reflectin films cannot provide all the light control power we see in squid, the authors conclude, because we seem to lack this coupled amplifier.
“Without this membrane surrounding the reflectors, there are no changes in the brightness of these artificial thin films,” says Morse.
“If we want to capture the power of the biological, we have to include some kind of membrane-type enclosure to allow a reversible tuning of the brightness.”
The study was published in Letters of Applied Physics.