Nerve endings, iridescence, and camouflage amongst the squid

Iridescence is a magical thing as far as I’m concerned. I know the scientists at the Marine Biology Laboratory (MBL) in Woods Hole, Massachusetts have mundane reasons for studying the iridescence in squid but I detect a hint of the fascination in the description of their work in the Aug. 27, 2012 news item on ScienceDaily,

Squid skin is extraordinary because it has two ways to produce color and pattern. Pigmented organs called chromatophores create patterns with yellow, red, and brown colors. Underneath the pigments, iridophores, aggregations of iridescent cells in the skin, reflect light and add blue, green, and pink colors to the overall appearance of the skin. Collectively these two groups of skin elements can create spectacular optical illusions with patterns of color, brightness, and contrast change.

“For 20 years we have been wondering how the dynamically changeable iridescence is controlled by the squid,” says study co-author Roger Hanlon. “At long last we have clean evidence that there are dedicated nerve fibers that turn on and tune the color and brightness of iridophores. It is not an exaggeration to call this “electric skin.” The complex nerve network distributed throughout the squid’s skin instantly coordinates tens of thousands of chromatophores with iridescent reflectors for rapidly changing behaviors ranging from camouflage to signaling.”

The Aug. 24, 2012 MBL news release, which originated the news item, provides details about the study,

Working with longfin inshore squid (Doryteuthis pealeii), the researchers took a new approach to investigating the mystery behind the iridophore control mechanism.  By tracing a highly branched network of nerves and stimulating them electrically, they found that they could activate progressive color shifts from red and orange to yellow, green, and blue in just 15 seconds. The findings suggest that the specific color of each iridophore, as well as speed of change, is controlled by the nervous system, as is spatial chromatophore patterning that occurs in the skin layer just above.

The scientists have provided some images to illustrate the process,

Nerves in red can be easily traced among the distinctive chromatophores and iridophores that they innervate. (Credit: Wardill, Gonzalez-Bellido, Crook & Hanlon, Proceedings of the Royal Society B: Biological Sciences)

Neurally stimulated squid iridophore. (Credit: Wardill, Gonzalez-Bellido, Crook & Hanlon, Proceedings of the Royal Society B: Biological Sciences)

They’ve also created a brief, silent video showing the process of becoming iridescent in action,

What I found particularly interesting about iridescence and colour  in squid was this (from the Aug. 24, news release),

How squid choose and hold particular skin colors to help camouflage themselves remains unknown and is particularly interesting because the animals are completely colorblind.

For anyone interested in reading the study, here’s the citation from the ScienceDaily news item,

T. J. Wardill, P. T. Gonzalez-Bellido, R. J. Crook, R. T. Hanlon. Neural control of tuneable skin iridescence in squid. Proceedings of the Royal Society B: Biological Sciences, 2012; DOI: 10.1098/rspb.2012.1374

The article is behind a paywall.

ETA Aug. 28, 2012 1:15 pm PDT: I forgot to mention the ‘camouflage’ part of the headline in the context of this story. The ability to change colour in response to stimulae of one sort of another is often for the purpose of camouflage/concealment, a matter of some interest to the military. In this case (from the Aug. 24, 2012 news release),

The work was funded by grants from the Office of Naval Research (ONR), Defense Advanced Research Projects Agency (DARPA), and Air Force Office of Scientific Research.

I last wrote about squid and camouflage in my Aug. 17, 2012 posting on soft robots.

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