H.G. Wells knew it. H.P. Lovecraft knew it. And SpongeBob SquarePants certainly knows it. Squid are pretty fancy. Especially when it comes to their skin. Thanks to specialised pigment-filled cells called chromatophores found in their tissue, they are able to change their colour and appearance in the blink of an eye – switching from being dazzlingly iridescent one minute to dark and murky the next.
It's an effect that is thought to have evolved to mimic the shimmering quality of light as it makes its path through the water.
It comes as little surprise then, that these marine masters of disguise have long been a subject of fascination to the military, as understanding exactly how squid manage to achieve their remarkable acts of camouflage may help engineers to produce materials with similar qualities.
Now, after more than a decade of research, the University of California’s Prof Daniel Morse and team have figured out one of the mechanisms behind squids' ability to shimmer and flicker in such a mesmerising way.
“For centuries, people have been amazed at the ability of squids to change the colour and patterns of their skin - which they do beautifully - for camouflage and underwater communication, signalling to one another and to other species to keep away, or as attraction for mating and other kinds of signalling,” said Prof Morse.
“What we've discovered is that not only is the squid able to tune the colour of the light that's reflected, but also its brightness.”
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Previous studies have shown that proteins named reflectins found in squids' tissues are responsible for their iridescence. Now, the new study on the opalescent inshore squid, Doryteuthis opalescens, has pinpointed the structures and mechanisms that enable the light-reflecting cells known as iridocytes in its skin to take on virtually every colour of the rainbow.
It all comes down to tiny accordion-like structures called lamellae found on the iridocytes’ cell membranes. These lamellae change shape due to the osmotic action of water within the cell and so alter the wavelength, and therefore colour, of the light they reflect, thanks to the reflectin proteins condensing or spreading apart inside them.
At the same time, as the membrane condenses, the reflectins group closer together causing an increase in their refractive index and amplifying brightness
“Water gets squished out of the accordion-like structure, and that collapses the accordion so the thickness in spacing between the folds gets reduced, and that's like bringing the grooves of a compact disc closer together,” Morse said. “So the light that's reflected can shift progressively from red to green to blue.”
The team are now working with material engineers on replicating the effect in a synthetic material.
Reader Q&A: How do squid survive the extreme water pressure in the deep sea?
Asked by: Adam Stephens, Salisbury
A big challenge facing squid in the deep sea is keeping their cells working. Under high pressure, important molecules like proteins in cell membranes and enzymes become squashed and bent out of shape and either work more slowly or not at all. One way squid counteract this is by loading their bodies with trimethylamine oxide or TMAO, which helps large molecules keep their shape. For many deep sea animals, the deeper they live the more TMAO they have in their bodies. TMAO also gives rise to the distinctive fishy smell of many sea creatures – the deeper the species lives, the more it smells.
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