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The 'chameleon of the sea' reveals its secrets

29 January 2014

The natural nanoscale photonic device that enables a small marine animal to dynamically change its colours could inspire new camouflage technologies.

Image: Shutterstock
Image: Shutterstock

The cuttlefish, or 'chameleon of the sea', can rapidly alter both the colour and pattern of its skin, helping it blend in with its surroundings and avoid predators. Now, scientists at Harvard University and the Marine Biological Laboratory (MBL) at Woods Hole, Massachusetts, have described the sophisticated biomolecular nanophotonic system underlying the cuttlefish’s colour-changing ways.

"Nature solved the riddle of adaptive camouflage a long time ago," says Professor Kevin Kit Parker of the Harvard School of Engineering and Applied Sciences (SEAS), who is also a core faculty member at the Wyss Institute for Biologically Inspired Engineering at Harvard. “Now the challenge is to reverse-engineer this system in a cost-efficient, synthetic system that is amenable to mass manufacturing."

In addition to textiles for military camouflage, the findings could also have applications in materials for paints, cosmetics, and consumer electronics.

The cuttlefish (sepia officinalis) has neurally controlled, pigmented organs called chromatophores that allow it to change its appearance in response to visual clues. Hitherto, scientists have had an incomplete understanding of the biological, chemical, and optical functions that make this adaptive colouration possible.

To regulate its colour, the cuttlefish relies on a vertically arranged assembly of three optical components: the leucophore, a near-perfect light scatterer that reflects light uniformly over the entire visible spectrum; the iridophore, a reflector comprising a stack of thin films; and the chromatophore. This layering enables the skin of the animal to selectively absorb or reflect light of different colours.

See base of article for an explanation of these images
See base of article for an explanation of these images

"Chromatophores were previously considered to be pigmentary organs that acted simply as selective colour filters,” says SEAS co-researcher, Leila Deravi. “But our results suggest that they play a more complex role; they contain luminescent protein nanostructures that enable the cuttlefish to make quick and elaborate changes in its skin pigmentation."

When the cuttlefish actuates its colouration system, each chromatophore expands; the surface area can change as much as 500 percent. The Harvard-MBL team showed that within the chromatophore, tethered pigment granules regulate light through absorbance, reflection, and fluorescence, in effect functioning as nanoscale photonic elements, even as the chromatophore changes in size.

"The cuttlefish uses an ingenious approach to materials composition and structure, one that we have never employed in our engineered displays," says SEAS co-researcher, Professor Evelyn Hu. "It is extremely challenging for us to replicate the mechanisms that the cuttlefish uses. For example, we cannot yet engineer materials that have the elasticity to expand 500 times in surface area.

"And were we able to do so, the richness of colour of the expanded and unexpanded material would be dramatically different. The cuttlefish may have found a way to compensate for this change in richness of colour by being an 'active' light emitter, not simply modulating light through passive reflection."

The MBL's Roger Hanlon and colleagues has examined adaptive colouration in the cuttlefish and other invertebrates for many years. "Cuttlefish skin is unique for its dynamic patterning and speed of change," he says.

See base of article for an explanation of these images
See base of article for an explanation of these images

"Deciphering the relative roles of pigments and reflectors in soft, flexible skin is a key step to translating the principles of actuation to materials science and engineering. This collaborative project expanded our breadth of inquiry and uncovered several useful surprises, such as the tether system that connects the individual pigment granules."

As for Professor Parker, an Army reservist who completed two tours of duty in Afghanistan, using the cuttlefish to find a biologically inspired design for new types of military camouflage is more than an academic pursuit. He understands first-hand that poor camouflage patterns can cost lives on the battlefield.

"Throughout history, people have dreamed of having an invisible suit," he adds. "Nature solved that problem, and now it’s up to us to replicate this genius so, like the cuttlefish, we can avoid our predators."

Key to illustrations:

(Upper illustration) Left: Cuttlefish chromatophores change from a punctuated to expanded state in response to visual cues. The scale bar measures one millimeter. Right: This illustrated cross-section of the skin shows the layering of three types of chromatophores. Iridophores and leucophores would be positioned beneath the chromatophores (images courtesy of Lydia Mathger)

(Lower illustration) Chromatophores were previously thought to be simply sacs of pigment that acted as filters; scientists have now discovered that nanostructures (labelled here as 'granules') within the cells are capable of fluorescing (images courtesy of George Bell)

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