The eyes of the mantis shrimp have more types of photoreceptors, or colour-detecting cells, than any animal on the planet. But the bottom-dwelling sea creatures are surprisingly bad at discriminating one colour from another, a discovery that has not made sense to scientists until now.
In a new study, published Jan. 23 in the journal Science, Hanne Thoen of the University of Queensland and colleagues, say that the mantis shrimp perceives colours in a way that has not been documented in the animal kingdom before — the creatures immediately recognise basic colours just by scanning an object with their eyes, rather than using the brain to distinguish different colours of light.
The time-saving trick helps it to survive in one of the most colourful environments on Earth — tropical coral reefs.
What do animals see?
Most animals have two to four types of photoceptors — cells that detect different wavelengths in electromagnetic spectrum — in their eyes. Humans, for instance, have three different types of photoreceptors, each one tuned to pick up green, red, or blue wavelengths of light. When light hits our eyes, the receptors turn these colours into electrical signals that are sent to the brain. Our brains determine the colour that we see by blending the signals that each receptor senses — like how a screen made of millions of different-coloured pixels makes an image.
Certain species, like birds, reptiles, and butterflies, have even more receptors than humans, although scientists believe that three to four are just fine for discriminating all the colours of the visible spectrum.
But, for some reason, the mantis shrimp has 12 distinct photoreceptor types. Each photoreceptor is sensitive to a different wavelength of light, ranging from deep ultraviolet colours to red. In addition, the crustaceans are also able to perceive linear and circular polarised light, which refers to the angle and direction of travelling light waves. Previous studies have suggested that mantis shrimps use the ability to detect circular polarised light, which reflects off their bodies, as a secret channel of communication, especially during mating.
The mantis shrimp super-eye
There are more than 500 known species of mantis shrimp, which range in size from less than an inch to over a foot long.
Like most insects and other crustaceans, the mantis shrimp has compound eyes that are perched on top of moving stalks.
The eyes are made up of tens of thousands of ommatidia units — long, thin clusters of photoreceptor cells. The 12 photoreceptors that are sensitive to different wavelengths in our visual spectrum are located in four rows of ommatidia across the center of each eye, called the mid-band.
To determine the colour of an object, the mantis shrimp’s eye muscles must scan the band across a particular area.
In the study, mantis shrimps were trained to associate different colour wavelengths with a food reward. The animals had to first tap an optical fibre transmitting one colour. The food was then removed and the mantis shrimps were presented with two fibres of varying wavelengths. The mantis shrimps aced the eye test if they chose the fibre representing the colour wavelength they were originally trained to grab.
Despite their high number of colour photoreceptors, the mantis shrimps didn’t perform so well when presented with similar colours. According to the researchers, the animals had had trouble distinguishing colours that would be comparable to what humans see as light orange and dark yellow.
Instead of comparing wavelengths of the visible spectrum in their brains, the mantis shrimp uses its 12 photoreceptors to “make quick and reliable determinations of colour,” the study authors wrote in their paper.
Skipping over this neural processing step is a key to surviving in the “colour-rich world of coral reefs that [the mantis shrimp] inhabits,” the researchers said.
The mantis shrimp is a ferocious creature, known for spearing or clubbing their prey with their powerful claws (like those of a praying mantis, hence the name).
“The extraordinary sensory machinery of the eye midband may be useful for species, and even individual, recognition, as well as more generally in detecting prey and other other objectes,” Michael F. Land and Daniel Osorio, from the University of Sussex wrote in a commentary on the study, published in the same issue of Science.
Of course, without being able to get into the head of the mantis shrimp, we may never know how they actually experience those colours in the brain.
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