Colour-changing cuttlefish can inspire new military camouflage

Scientists have found how a marine animal known as the "chameleon of the sea" dynamically changes its colours, an advance that could help develop improved protective camouflage for soldiers on the battlefield.
The cuttlefish can rapidly alter both the colour and pattern of its skin, helping it blend in with its surroundings and avoid predators.
Scientists at Harvard University and the Marine Biological Laboratory (MBL) in the US uncovered new details about the sophisticated biomolecular nanophotonic system underlying the cuttlefish's colour-changing ways.
Neurally controlled, pigmented organs called chromatophores allow the cuttlefish (Sepia officinalis) to change its appearance in response to visual clues.

However, 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, said co-author of the study Leila F Deravi at the Harvard School of Engineering and Applied Sciences.
"Chromatophores were previously considered to be pigmentary organs that acted simply as selective colour filters," Deravi said.

"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," Deravi said.
When the cuttlefish actuates its colouration system, each chromatophore expands; the surface area can change as much as 500 per cent.
The team showed that within the chromatophore, tethered pigment granules regulate light through absorbance, reflection, and fluorescence, functioning as nanoscale photonic elements, even as the chromatophore changes in size.
"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 (per cent) in surface area," said co-author Evelyn Hu, also at SEAS.

"And were we able to do so, the richness of colour of the expanded and unexpanded material would be dramatically different - think of stretching and shrinking a balloon.
"The cuttlefish may have found a way to compensate for this change in richness of colour by being an 'active' light emitter (fluorescent), not simply modulating light through passive reflection," Hu said.
The paper was published in the Journal of the Royal Society Interface.