Findings hint at shark’s unexpected ability to change colour

The blue shark (Prionace glauca) is a striking member of one of nature’s most exclusive clubs – organisms which appear blue.

New research suggests that the nanostructures responsible for producing this lovely hue may also allow the shark to change colour depending on its environment.

Dr Viktoriia Kamska, a post-doctoral researcher at the City University of Hong Kong (CityUHK), says blue is one of the rarest colours in the animal kingdom.

“Animals have developed through evolution a variety of unique strategies to produce it, making these processes especially fascinating.”

Pigments, which absorb certain wavelengths of light but not others, are one way animals produce colour. Structural colour, nanometre-scale structures small enough to interfere with the pathways of light, are another.

Kamska and her colleagues found that “pulp cavities” are responsible for their metallic blue colour. These sit within the blue shark’s tooth-like scales (dermal denticles).

Guanine crystal nanostructures of specific thickness and spacing reflect the blue light. Melanin pigment-containing vesicles, known as melanosomes, absorb the other wavelengths to enhance the colour saturation.

“These components are packed into separate cells, reminiscent of bags filled with mirrors and bags with black absorbers, but kept in close association so they work together,” explains Kamska.

“When you combine these materials together, you also create a powerful ability to produce and change colour,” adds Professor Mason Dean, a laboratory lead at CityUHK.

“What’s fascinating is that we can observe tiny changes in the cells containing the crystals and see and model how they influence the colour of the whole organism.”

Computer simulations revealed that while narrower spaces between the guanine layers creates iconic blues, increasing this space shifts the colour into greens and golds.

“It’s challenging to manually manipulate structures at such a small scale, so these simulations are incredibly useful for understanding what colour palette is available,” says Kamska.

The researchers think this colour change mechanism could be driven by environmental factors that affect guanine crystal spacing.

“In this way, very fine scale alterations resulting from something as simple as humidity or water pressure changes could alter body colour, which then shapes how the animal camouflages or counter-shades in its natural environment,” says Dean.

For example, a shark’s skin is subjected to increasing pressure the deeper it swims. This would likely push the guanine crystals closer together, darkening the shark’s colour to better suit its surroundings.

“Not only do these denticles provide sharks with hydrodynamic and antifouling benefits, but we’ve now found that they also have a role in producing and maybe changing colour too,” says Dean. “Such a multi-functional structural design – a marine surface combining features for high-speed hydrodynamics and camouflaging optics – as far as we know, hasn’t been seen before.”

“The next step is to see this mechanism really functions in sharks living in their natural environment,” says Kamska.

The findings may have applications in producing environmentally friendly structural colours for manufacturing.

“A major benefit of structural colouration over chemical colouration is that it reduces the toxicity of materials and reduces environmental pollution,” says Kamska. “Structural colour is a tool that could help a lot, especially in marine environments, where dynamic blue camouflage would be useful.”

The research has been presented at the Society for Experimental Biology Annual Conference in Antwerp, Belgium on 9 July.