How can octopuses, squid and cuttlefish suddenly melt into the background or send vibrant technicolour mating or aggressive displays through their multi-hued skin if they lack eye cells that perceive colour?
The answer may be that they do see colour – not from light hitting special retinal “photoreceptors” but thanks to chromatic aberration, where different colours of light focus at different distances behind a lens.
The explanation by Christopher Stubbs, a biologist from the University of California, Berkeley and his physicist father Alexander Stubbs at Harvard University was published in the Proceedings of the National Academy of Sciences.
Eyes of cephalopods – the family that includes octopuses, squid and cuttlefish – are quite unlike anything seen on land: U- and W-shaped pupils backed by a lens that moves back and forth, like a camera, rather than fattening or thinning like ours.
Humans, for the most part, are able to distinguish millions of colours thanks to red, green and blue cells (photoreceptors) in our retinas.
But cephalopods have just one photoreceptor. So how do they know how to send and detect colourful messages?
The younger Stubbs realised that by adjusting the focal point of their eyes, like a photographer adjusts a lens, cephalopods might be able to detect different wavelengths – or colours – of light.
This is known as chromatic aberration. When blue light passes through a lens it bends, or is refracted, more than red light. This means it focuses closer to the lens, while redder light focuses further away.
Unsurprisingly, this can be a real pain for photographers. It looks a little like a coloured fringe on the edge of a dark surface. If you wear glasses, you might see the effect through the edge of your lens.
To test his idea, Stubbs created a computer model of how the animals’ eyes work and see if chromatic aberration was possible.
He found not only could a shifting lens do the trick, but the cephalopods’ quirky pupils only served to maximise the effect.
As the lens moved forwards and backwards, the different wavelengths focusing on the retina at different times built up a colour picture.
While it’s not definitive evidence that the animals use chromatic aberration, it does explain how they’re able to perceive colour with just one photoreceptor.
“High-acuity ‘camera style’ lens eyes in octopus, squid and cuttlefish represent a completely independent evolution of complex eyes from vertebrates so in some sense we shouldn’t necessarily expect that this lineage would solve problems like colour vision in the same way,” Alexander Stubbs says.