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

The research was presented at the in Antwerp, Belgium on July 9, 2025.

"Blue is one of the rarest colors 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 color lies in the pulp cavities of the tooth-like scales—known as dermal denticles— that armor the shark's skin.

The key features of this color-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 color saturation.

"When you combine these materials together, you also create a powerful ability to produce and change color," 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 color 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 characterize the form, function, and architectural arrangements of the color-producing nanostructures.

"We started looking at color at the organismal level, on the scale of meters and centimeters, but structural color 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 color 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 color palette is available," says Dr. Kamska.

The discovery also reveals that the shark's trademark color 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 color into greens and golds.

Dr. Kamska and her team have demonstrated that this structural mechanism of color 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 color, which 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 color 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 color 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 coloration over chemical coloration is that it reduces the toxicity of materials and reduces environmental pollution," says Dr. Kamska.

"Structural color 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 colors, but sharks and rays diverged from bony fishes hundreds of millions of years ago—so this represents a completely different evolutionary path for making color."