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Shiny! On the Iridescence of Squid


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There’s a killer video making its way around the Intertubes, showcasing the color-changing skin of a Longfin Inshore squid shifting hues in sync to Cypress Hill. It’s the creation of a group called The Backyard Brains,  who manufacture do-it-yourself brain activity recording kits for educational purposes.What better way to learn about neurons than to observe them in action? And just for giggles, the Backyard Brains decided to make this latest video. I’ll let them explain:

During experiments on the giant axons of the Longfin Inshore Squid (loligo pealei) at the Marine Biological Laboratory in Woods Hole, MA; we were fascinated by the fast color-changing nature of the squid’s skin. Squids (like many other cephalopods) can quickly control pigmented cells called chromatophores to reflect light. The Longfin Inshore has 3 different chromatophore colors: Brown, Red, and Yellow. Each chromatophore has tiny muscles along the circumference of the cell that can contract to reveal the pigment underneath.

Here, check it out and groove to the funky beat:

Iridiscence is actually pretty common in Nature: we see it in the wings of butterflies and dragonflies, in the shells of beetles, in opals, and yes, in squid and other sea creatures. But not all iridescence is identical when it comes to underlying mechanisms. As I wrote back in 2007, the color we see in the wings of butterflies doesn’t come from actual pigment molecules, but from the precise lattice-like structure of the wings (or shells, or feathers), which forces light waves passing through to interfere with itself, so it can propagate only in certain directions and at certain frequencies. And the brilliant colors that result change depending on one’s point of view. In essence, they act like naturally occurring diffraction gratings.

Physicists call these structures photonic crystals, an example of “photonic band gap materials,” meaning they block out certain frequencies of light and let through others. This makes them “tunable”, particularly the manmade varieties, because of those highly ordered arrays of periodic “holes.” Anything tunable is by definition controllable, and therefore useful for practical applications. Photonic crystals are used most often as waveguides for light in telecommunications/fiber optics systems, or other places where scientists want to be able to control either the frequency or the direction of light.

Butterfly wings get their color from naturally occurring photonic crystal structures in scales made of chitin, a polysaccharide that shows up in all kinds of insects. Those scales are arranged like tiles on a roof, except they measure a mere tens of micrometers across. Each side of the wing contains different photonic structures: a metallic blue produced by single crystals, and a dull-ish green that results from a more random arrangement of crystals. Precisely ordering the lattice structure is critical to achieving the most brilliant colorful effects — and to controlling the propagation of light at the desired frequencies.

Close-up view of chromatophores in squid skin.

But as the Backyard Brains point out, the Longfin Inshore squid and its relatives don’t get their iridescent colors from such structures, but from pigment cells — i.e., chromatophores — that line the outer layer of its translucent skin. This UC-Davis site describes the pigment cell as “a flexible bag of color.”

Each cell is attached to muscle fibers along the surface of the skin, and those muscles in turn are connected to a nerve fiber. Those nerves can be stimulated by electrical pulses, for example, and those impulses then travel along the nerve fiber to the muscles, causing them to contract. (The activation mechanism for this “dynamically tuneable structural coloration” appears to be linked to a neurotransmitter called acetylcholine, according to the new paper that inspired the video.)

This actually expands the cell, since the muscles are pulling in different directions.

Why does this cause a change in color? As the cell expands, so does the pigmented area; as it shrinks back down, the pigmented area also shrinks. So, if all the black cells suddenly retract, the squid will lighten. And it the animal expands all its red chromatophores, it will flush red.

UPDATE: Pulled from a comment by marine biologist Danna Staaf, who knows a little something about squid: “Chromatophores are the bags of pigment that we see expanding and contracting to the beat of Cypress Hill. Beneath that layer of chromatophores, the squid’s skin has a totally separate layer of iridophores, which are more similar to (but more awesome than) the crystals in butterfly wings. Squid iridophores aren’t pigment-based; they’re reflective structures that can be tuned to reflect different wavelengths, and the neural control of that tuning is the subject of the recent Royal Society paper. Tuning iridophores is a lot slower than expanding and contracting chromatophores, as illustrated in this video.”

Just last month, Ed Yong highlighted this unusual ability of squid. That post focused on a similar video created by marine biologist Michael Bok, who was dissecting a Longfin Inshore squid one day when he noticed the chromatophores were still active. He recorded the color changes in action, and set the whole thing to Pachelbel’s Canon in D Minor:

But the Backyard Brains didn’t just set video imagery to cool music. They actually used the music of Cypress Hill as the stimulus to induce the color changes. Per The Verge:

[They] simulated the nerve signals that trigger the muscles by connecting an iPhone to one of the creature’s dorsal fins with a suction electrode. When audio was sent to the squid in the form of electrical signals, the voltage changes made the pigment cells dance in sync with the music.

In essence, they turned the cells into actuators, a type of transducer — i.e., a device that turn one kind of signal (usually electrical) into motion. In this case, they converted sound (mechanical energy) into electrical impulses, which in turn produced motion in the chromatophore cells in the squid skin. Pretty ingenious, huh?

Image of chromatophores in squid skin from 2006 paper by Lydia Mäthger and Roger Hanlon, of the Woods Hole Marine Biology Laboratory. Source.

Reference:

Wardill, T.J. et al. “Neural control of tuneable skin iridescence in squid,” Proceedings of the Royal Society B, August 15, 2012.

Jennifer Ouellette About the Author: Jennifer Ouellette is a science writer who loves to indulge her inner geek by finding quirky connections between physics, popular culture, and the world at large. Follow on Twitter @JenLucPiquant.

The views expressed are those of the author and are not necessarily those of Scientific American.



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  1. 1. Physicalist1 10:50 pm 08/25/2012

    That really is amazingly cool.

    Link to this
  2. 2. Raoul 11:37 pm 08/25/2012

    There are many animals who change the color of their skin.
    Mimetism. Camaleons for example.Do they have chromatophores and muscles, too?

    Link to this
  3. 3. Jennifer Ouellette 12:11 am 08/26/2012

    Yes, chameleons also have chromatophores: http://magma.nationalgeographic.com/ngexplorer/0210/articles/mainarticle.html

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  4. 4. Danna Staaf 1:02 am 08/27/2012

    Hurrah for all the squid love! But this post confuses two separate structures: chromatophores and iridophores. Chromatophores are the bags of pigment that we see expanding and contracting to the beat of Cypress Hill. Beneath that layer of chromatophores, the squid’s skin has a totally separate layer of iridophores, which are more similar to (but more awesome than) the crystals in butterfly wings. Squid iridophores aren’t pigment-based; they’re reflective structures that can be tuned to reflect different wavelengths, and the neural control of that tuning is the subject of the recent Royal Society paper. Tuning iridophores is a lot slower than expanding and contracting chromatophores, as illustrated in this video: http://www.youtube.com/watch?v=26oqIW0E2Oo

    Link to this
  5. 5. mettarefuge 5:20 pm 08/29/2012

    This is amazingly interesting science, and squids are remarkable creatures. I will never forget seeing a school of them skin diving one time in Laguna Beach, CA. But am I the only one that wonders about what this was like for the squid?

    “When audio was sent to the squid in the form of electrical signals, the voltage changes made the pigment cells dance in sync with the music.”

    I hope the “electrical signals” were not painful to the animal; suffering matters, and we should always try to alleviate it when at all possible.

    Link to this

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