Fish aren鈥檛 often inspired by Italian design. But the colourful neon tetra 鈥� with its striking horizontal stripe that changes from blue to yellow 鈥� looks like it might have been. The stripe changes colour because tiny plates inside the tetra鈥檚 scales move like venetian blinds, changing the way they reflect light.
The neon tetra, or Paracheirodon innesi, is one of the world鈥檚 most popular aquarium fish, in part thanks to its bright colour-shifting abdominal stripe. It was already known that the tetra鈥檚 ability to change colours was linked to tiny platelets inside the light-reflecting cells on the stripe鈥檚 surface, but until now no one has known how the colour shift happens.
To find out, Shinya Yoshioka and his team at , Japan, built a microscope that could measure the angle and the colour of the tiny plates simultaneously. From the way octopuses use the orientation of similar platelets to camouflage themselves, Yoshioka predicted the tetra鈥檚 platelets would move like venetian blinds. 鈥淲hen plates of the blind are tilted, the light is reflected into a different angle,鈥� he explains.
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Yoshioka鈥檚 microscope shines a strong light onto the platelets and records where it is reflected, thereby determining their angle. Colour is measured using a spectrometer, which measures the strength and type of wavelength bouncing off the plates.
Chemical switch
Potassium is known to make the tetra鈥檚 scales change from blue to yellow, so Yoshioka put cells from a tetra鈥檚 stripe under his microscope and added potassium. As the colours changed, so did the angle of the platelets 鈥� confirming Yoshioka鈥檚 venetian blind theory.
When blue, the plates were 12 degrees in relation to the floor of the light-reflecting cells. But when they turned yellow, they shifted to around 16 degrees. 鈥淭he small change in angle can cause a large shift in colour,鈥� Yoshioka says.
鈥淭he venetian blind model is a simple mechanical movement,鈥� he says, which would be very attractive as a design for colour-changeable products.
Just how the fish move the platelets is not yet known, however. , an ecologist at the University of Queensland in Brisbane, Australia, who was not involved in the study, says it鈥檚 likely that the fish control the angle of their platelets through neural networks which kick in when they respond to threats from their environment.
Potassium is an important messenger in neural networks, so it鈥檚 conceivable, he says, that it triggers some sort of neural response which tugs on tiny filaments attached to the platelets.
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