Tag Archives: iridophores

Chameleons (male panther chameleons in particular)—colour revelation

Caption: These are male panther chameleons (Furcifer pardalis) photographed in Madagascar. Credit: © Michel Milinkovitch

Caption: These are male panther chameleons (Furcifer pardalis) photographed in Madagascar.
Credit: © Michel Milinkovitch

Researchers at Switzerland’s University of Geneva/Université de Genève (UNIGE) have revealed the mechanisms (note the plural) by which chameleons change their colour. From a March 10, 2015 news item on phys.org,

Many chameleons have the remarkable ability to exhibit complex and rapid color changes during social interactions. A collaboration of scientists within the Sections of Biology and Physics of the Faculty of Science from the University of Geneva (UNIGE), Switzerland, unveils the mechanisms that regulate this phenomenon.

In a study published in Nature Communications [March 10, 2015], the team led by professors Michel Milinkovitch and Dirk van der Marel demonstrates that the changes take place via the active tuning of a lattice of nanocrystals present in a superficial layer of dermal cells called iridophores. The researchers also reveal the existence of a deeper population of iridophores with larger and less ordered crystals that reflect the infrared light. The organisation of iridophores into two superimposed layers constitutes an evolutionary novelty and it allows the chameleons to rapidly shift between efficient camouflage and spectacular display, while providing passive thermal protection.

Male chameleons are popular for their ability to change colorful adornments depending on their behaviour. If the mechanisms responsible for a transformation towards a darker skin are known, those that regulate the transition from a lively color to another vivid hue remained mysterious. Some species, such as the panther chameleon, are able to carry out such a change within one or two minutes to court a female or face a competing male.

A March 10, 2015 University of Geneva press release on EurekAlert (French language version is here on the university website), which originated the news item, explains the chameleon’s ability as being due to its ability to display structural colour,

Besides brown, red and yellow pigments, chameleons and other reptiles display so-called structural colors. «These colors are generated without pigments, via a physical phenomenon of optical interference. They result from interactions between certain wavelengths and nanoscopic structures, such as tiny crystals present in the skin of the reptiles», says Michel Milinkovitch, professor at the Department of Genetics and Evolution at UNIGE. These nanocrystals are arranged in layers that alternate with cytoplasm, within cells called iridophores. The structure thus formed allows a selective reflection of certain wavelengths, which contributes to the vivid colors of numerous reptiles.

To determine how the transition from one flashy color to another one is carried out in the panther chameleon, the researchers of two laboratories at UNIGE worked hand in hand, combining their expertise in both quantum physics and in evolutionary biology. «We discovered that the animal changes its colors via the active tuning of a lattice of nanocrystals. When the chameleon is calm, the latter are organised into a dense network and reflect the blue wavelengths. In contrast, when excited, it loosens its lattice of nanocrystals, which allows the reflection of other colors, such as yellows or reds», explain the physicist Jérémie Teyssier and the biologist Suzanne Saenko, co-first authors of the article. This constitutes a unique example of an auto-organised intracellular optical system controlled by the chameleon.

The press release goes on to note that the iridophores have another function,

The scientists also demonstrated the existence of a second deeper layer of iridophores. «These cells, which contain larger and less ordered crystals, reflect a substantial proportion of the infrared wavelengths», states Michel Milinkovitch. This forms an excellent protection against the thermal effects of high exposure to sun radiations in low-latitude regions.

The organisation of iridophores in two superimposed layers constitutes an evolutionary novelty: it allows the chameleons to rapidly shift between efficient camouflage and spectacular display, while providing passive thermal protection.

In their future research, the scientists will explore the mechanisms that explain the development of an ordered nanocrystals lattice within iridophores, as well as the molecular and cellular mechanisms that allow chameleons to control the geometry of this lattice.

Here’s a link to and a citation for the paper,

Photonic crystals cause active colour change in chameleons by Jérémie Teyssier, Suzanne V. Saenko, Dirk van der Marel, & Michel C. Milinkovitch. Nature Communications 6, Article number: 6368 doi:10.1038/ncomms7368 Published 10 March 2015

This article is open access.

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.