Thereâs 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. Itâs the creation of a group called The Backyard Brains,Â 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. Iâll 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 squidâs 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 doesnât 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 oneâs point of view. In essence, they act like naturally occurring diffraction gratings.
Physicists call these structures photonic crystals, an example of âphotonic band gap materials,â meaning they block out certain frequencies of light and let through others. This makes them âtunableâ, particularly the manmade varieties, because of those highly ordered arrays of periodic âholes.â 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 â" and to controlling the propagation of light at the desired frequencies.
But as the Backyard Brains point out, the Longfin Inshore squid and its relatives donât get their iridescent colors from such structures, but from pigment cells â" i.e., chromatophores â" that line the outer layer of its translucent skin. This UC-Davis site describes the pigment cell as âa flexible bag of color.â
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 âdynamically tuneable structural colorationâ 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.
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 Pachelbelâs Canon in D Minor:
But the Backyard Brains didnât 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 creatureâs 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 â" 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 MÃ¤thger and Roger Hanlon, of the Woods Hole Marine Biology Laboratory. Source.
Wardill, T.J. et al. âNeural control of tuneable skin iridescence in squid,â Proceedings of the Royal Society B, August 15, 2012.