It's not exactly news--the Navy's decision to fund a huge, interdisciplinary research project on squid skin is so last year--but the topic cropped up again and started me wondering: why do squid use different techniques to make different colors?

Here's an explanatory bit from MSNBC:
Hanlon and colleagues plan to extract the "operating principles" that make the skin of squid, octopuses, and cuttlefish observant, adaptive and responsive to the environment. The information they gather from looking at interactions of pigments and reflectors at the cellular and molecular levels will be used to inform the engineers and scientists building the materials that emulate these properties. . . . According to Hanlon, this work isn't just for secretive military applications. Industry and society may also benefit from the effort, which will reveal knowledge about combining pigments and reflectors.
That's a key thing I hadn't picked up on before: the interaction of pigments and reflectors. Squid skin (and octopus and cuttlefish skin) has two different ways of making color. Chromatophores use pigment to make red, yellow, brown, and black. Iridophores use reflection to make blue and green. And leucophores just reflect ambient light, whatever it is.

But if you've already got chromatophores, why not use pigment to make blue and green? Or, put another way, if you've got iridophores, why not use reflection to make red and yellow? Why invest in two totally different systems?

As I understand it, biological pigment has a much easier time making reds, browns, blacks, and yellows (think of human skin and hair tones). Reflection, which has a lot more to do with physics than biology even when biological creatures use it, has a much easier time with blues and greens (think of the scattering and reflection that make the sky and water blue).

Very clever of cephalopods to maximize their spectral coverage by using both . . .

Piece of skin from a juvenile Humboldt squid. The splotches are chromatophores; the reflective band in the upper right in composed of irido/leucophores.

P.S. Sorry this has been much less than Squid-A-Day lately. I'm working on another big project that's cutting into my blogging time . . . but it'll be wrapped up soon and I'll share some fun snippets! For now, let's just say my relationship with Illustrator has been taken to a whole new level.