The first time you see a blue shark in the water, it’s almost impossible not to be taken aback by just how… well, blue they are. Now, scientists have uncovered the secret to their vivid azure hue and discovered that these animals might also be able to change colour.
Blue sharks (Prionace glauca) have special crystals inside their scales that create their distinctive shade and may also give them chameleon-like abilities, according to research presented at the Society for Experimental Biology Annual Conference.
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How blue sharks change colour
"Blue is one of the rarest colours in the animal kingdom,” says Dr. Viktoriia Kamska, City University of Hong Kong, in a statement. “Animals have developed a variety of unique strategies through evolution to produce it, making these processes especially fascinating.”
The blue shark’s strategy? Special crystals inside the tooth-like scales (known as dermal denticles) that these animals have covering their bodies.
Derman denticles have pulp-like cavities and, inside these, are guanine crystals, which reflect blue light, and tiny sacs that store melanin (a natural pigment that give skin, hair and eyes their colour), which absorb other wavelengths of light.
“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,” says Kamska.
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The scientists used a combination of advanced technologies to figure out how these tiny structures worked to create the shark’s vibrant colour.
"When you combine these materials together, you also create a powerful ability to produce and change colour,” says Professor Mason 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.”
It’s incredibly difficult to tweak the tiny structures themselves to test different scenarios so they also turned to computer simulations to see what happened when different wavelengths of light were introduced.
As well as uncovering how the shark creates its bold blue colouring, the experts also realised that it might have the ability to change hues, like a chameleon. When the crystals move closer together, the shark appears its classic blue colour, and when they drift further apart, its skin is tinted with greens and golds.
This could help the animal stay camouflaged in changing environmental conditions. "In this way, very fine scale alterations resulting from something as simple as humidity or water pressure changes could alter body colour,” says Professor Dean. These "then shape how the animal camouflages or counter-shades in its natural environment.”
In deeper, darker water, the pressure might push the nanocrystals together more closely and so the shark would turn a darker blue – helping it blend in more seamlessly with its surroundings.
The researchers now want to see how their hypothesis plays out in the wild – can they observe this happening among blue sharks in their natural habitat?
They’re also curious how the understanding of this mechanism might benefit humans by inspiring ways of creating a blue colour without using potentially harmful chemicals. "A major benefit of structural colouration over chemical colouration is that it reduces the toxicity of materials and reduces environmental pollution,” says Kamska.
Sharks aren’t the only marine animals that have remarkable colour changing abilities. Cephalopods such as octopus, cuttlefish and squid can alter their colour and pattern in the blink of an eye – sometimes even sending messages through the displays that flash up on their skin.
Even seahorses can blend into their surroundings to hide from potential danger. They are able to do this thanks to tiny balloon-like pouches within their skin called chromatophores. These are filled with pigment and can rapidly grow or shrink to change the animal’s appearance.
Studying how blue sharks (and perhaps other species, including great whites which may be able to change their shade to creep up on prey without detection) create their colour could give researchers exciting new insights.
“We know a lot about how other fishes make colours, but sharks and rays diverged from bony fishes hundreds of millions of years ago,” says Professor Dean, “so this represents a completely different evolutionary path for making colour.”
Top image: blue shark. Credit: Getty
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