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How squids and octopuses are able to change their colour

Washington, July 26 : A new study has tried to look into the mechanism behind squids' and octopuses' ability to change colour.


Two years ago, an interdisciplinary team from UC Santa Barbara discovered the mechanism by which a neurotransmitter dramatically changes colour in the common market squid, Doryteuthis opalescens.

That neurotransmitter, acetylcholine, sets in motion a cascade of events that culminate in the addition of phosphate groups to a family of unique proteins called reflectins.

This process allows the proteins to condense, driving the animal's colour-changing process.

Now the researchers have delved deeper to uncover the mechanism responsible for the dramatic changes in colour used by such creatures as squids and octopuses.

Structural colours rely exclusively on the density and shape of the material rather than its chemical properties.

The latest research from the UCSB team shows that specialized cells in the squid skin called iridocytes contain deep pleats or invaginations of the cell membrane extending deep into the body of the cell.

This creates layers or lamellae that operate as a tunable Bragg reflector. Bragg reflectors are named after the British father and son team who more than a century ago discovered how periodic structures reflect light in a very regular and predicable manner.

"We know cephalopods use their tunable iridescence for camouflage so that they can control their transparency or in some cases match the background," co-author Daniel E. Morse, Wilcox Professor of Biotechnology in the Department of Molecular, Cellular and Developmental Biology and director of the Marine Biotechnology Center/Marine Science Institute at UCSB, said.

"They also use it to create confusing patterns that disrupt visual recognition by a predator and to coordinate interactions, especially mating, where they change from one appearance to another.

"Some of the cuttlefish, for example, can go from bright red, which means stay away, to zebra-striped, which is an invitation for mating," he added.

The researchers created antibodies to bind specifically to the reflectin proteins, which revealed that the reflectins are located exclusively inside the lamellae formed by the folds in the cell membrane.

They showed that the cascade of events culminating in the condensation of the reflectins causes the osmotic pressure inside the lamellae to change drastically due to the expulsion of water, which shrinks and dehydrates the lamellae and reduces their thickness and spacing.

The movement of water was demonstrated directly using deuterium-labeled heavy water.

When the acetylcholine neurotransmitter is washed away and the cell can recover, the lamellae imbibe water, rehydrating and allowing them to swell to their original thickness.

This reversible dehydration and rehydration, shrinking and swelling, changes the thickness and spacing, which, in turn, changes the wavelength of the light that's reflected, thus "tuning" the colour change over the entire visible spectrum.

The findings are published in the Proceedings of the National Academy of Science.

--ANI (Posted on 26-07-2013)

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