Squid Skin: Why Pigment (But Not Glitter) Will Dance to the Beat
Squid and their relatives–a group of animals known as cephalopods–have the world's best skin. And it's not because they moisturize, lack pimples, or tan without ever burning. It's because their skin is a canvas of endless possibilities.
Other animals can change their looks, of course, with chameleons being the most famous. What sets cephalopods apart is a combination of speed (squid can flash between colors several times per second) and diversity. Virtually no hue is off-limits to these creatures, thanks to two separate layers of color-changing skin.
On top is a layer of chromatophores–tiny elastic sacs of pigment surrounded by muscle fibers, each connected to neurons that run straight back to the squid's brain. When the squid tells the muscles to contract, they stretch the sac wide, turning the chromatophore "on" to show the pigment. When the muscles relax, the sac turns "off," shrinking to a nearly invisible dot.
Chromatophore behavior is spectacularly showcased in this viral video by Backyard Brains, in which an iPod was wired up via an electrode to stimulate a squid’s nerves:
As you can see, chromatophores are awesome. But they are also limited to yellow, red, brown or black. To achieve fantastic glittering hues of blue and green, cephalopods rely on the second layer of skin, which is full of a different kind of structure called iridophores.
Iridophores make structural color, which means they reflect certain wavelengths of light because of their shape. Most familiar instances of structural color in nature (peacock feathers, mother of pearl) are constant–they may shimmer when you change your viewing angle, but they don't shift from pink to blue.
Cephalopods, of course, aren't satisfied with that. Researchers have known for a long time that they can change the colors of their iridophores–but the mechanism of control has remained obscure.
A new paper in the Proceedings of the Royal Society B finally sheds some light on the phenomenon. (Sorry, I couldn't resist!) Trevor Wardill, Paloma Gonzalez-Bellido, and their colleagues found that squid use neurons to control their iridophores just as they do their chromatophores. Muscles may be involved as well–though their exact contribution remains unclear.
Changing iridophore color is rather more complicated than changing chromatophore color. Iridophores are composed of nanoscale layers of material that have to be moved closer together or further apart in order to reflect the desired color, a process that can take many seconds to minutes. This video is shown at 4X speed, but that doesn't really make it less impressive:
One of the authors of the study, Gonzalez-Bellido, helped Backyard Brains prepare their squid skin, so I had to wonder: why didn't the iridophores respond to the music? You can see a couple of them in the background, underneath the chromatophores, and they're just sitting there.
Iridophores, it seems, are not only slower than chromatophores to activate, but they're extremely slow to deactivate. Greg Gage of Backyard Brains points out that the process can take tens of minutes. So the first electrical signal from the music probably activated the iridophores, then they stayed that way for the rest of the recording.
And that's why chromatophores dance to Cypress Hill, while iridophores appear to be sitting it out at the bar.
Update: Trevor Wardill also notes that iridophores seem to require a much longer continuous stimulus than was provided by the music, so they may never have become activated at all. Picky little things.