Novel nanostructures in blue sharks reveal their remarkable potential for dynamic colour-change

(Press-News.org) New research into the anatomy of blue sharks (Prionace glauca) reveals a unique nanostructure in their skin that produces their iconic blue colouration, but intriguingly, also suggests a potential capacity for colour change.

“Blue is one of the rarest colours in the animal kingdom, and animals have developed a variety of unique strategies through evolution to produce it, making these processes especially fascinating,” says Dr Viktoriia Kamska, a post-doctoral researcher in the lab of Professor Mason Dean at City University of Hong Kong.

The team revealed that the secret to the shark’s colour lies in the pulp cavities of the tooth-like scales — known as dermal denticles— that armour the shark’s skin. The key features of this colour-producing mechanism inside the pulp cavity are guanine crystals, which act as blue reflectors, alongside melanin-containing vesicles called melanosomes, which act as absorbers of other wavelengths. “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 Dr. Kamska. As a result, a pigment (melanin) collaborates with a structured material (guanine platelets of specific thickness and spacing) to enhance colour saturation.

“When you combine these materials together, you also create a powerful ability to produce and change colour,” says Professor Dean. “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.”

This anatomical breakthrough was made possible using a mixture of fine-scale dissection, optical microscopy, electron microscopy, spectroscopy, and a suite of other imaging techniques to characterise the form, function, and architectural arrangements of the colour-producing nanostructures. “We started looking at colour at the organismal level, on the scale of metres and centimetres, but structural colour is achieved at the nanometer scale, so we have to use a range of different approaches,” says Professor Dean.

Identifying the likely nanoscale culprits behind the shark’s blue colour was only part of the equation. Dr Kamska and her collaborators also used computational simulations to confirm which architectural parameters of these nanostructures are responsible for producing the specific wavelengths of the observed spectral appearance. “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 Dr Kamska.

The discovery also reveals that the shark’s trademark colour is potentially mutable through tiny changes in the relative distances between layers of guanine crystals within the denticle pulp cavities. Whereas narrower spaces between layers create the iconic blues, increasing this space shifts the colour into greens and golds.

Dr Kamska and her team have demonstrated that this structural mechanism of colour change could be driven by environmental factors that affect guanine platelet spacing. “In this way, very fine scale alterations resulting from something as simple as humidity or water pressure changes could alter body colour, that then shape how the animal camouflages or counter-shades in its natural environment,” says Professor Dean.

For example, the deeper a shark swims, the more pressure that their skin is subjected to, and the tighter the guanine crystals would likely be pushed together - which should darken the shark’s colour to better suit its surroundings. “The next step is to see how this mechanism really functions in sharks living in their natural environment,” says Dr Kamska.

While this research provides important new insights into shark anatomy and evolution, it also has a strong potential for bio-inspired engineering applications. “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 Professor 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.”

Therefore, this discovery could have implications for improving environmental sustainability within the manufacturing industry. “A major benefit of structural colouration over chemical colouration is that it reduces the toxicity of materials and reduces environmental pollution,” says Dr Kamska. “Structural colour is a tool that could help a lot, especially in marine environments, where dynamic blue camouflage would be useful.”

“As nanofabrication tools get better, this creates a playground to study how structures lead to new functions,” says Professor Dean. “We know a lot about how other fishes make colours, but sharks and rays diverged from bony fishes hundreds of millions of years ago – so this represents a completely different evolutionary path for making colour.”

This research, funded by Hong Kong’s University Grants Committee, General Research Fund, is being presented at the Society for Experimental Biology Annual Conference in Antwerp, Belgium on the 9th July 2025.

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