My Octopus Teacher is an Academy Award-winning Netflix documentary about the clever capers of an octopus off the South African coast, filmed by a man who became besotted with the creature. One of the most eye-watering revelations of the doco is the octopus’s ability to dramatically alter its shape, texture and colour.
No other animal can camouflage itself in such a range of environments. An octopus changes colour to make itself cryptic in a fraction of a second when it detects a threat, says Misha Vorobyev, the head of the ecology and colour vision laboratory at the University of Auckland. Yet it can’t see colour.
“We are trying to find out how octopuses know what colour is there,” says Vorobyev. “Unlike most animals, including fish, the octopus is colour-blind. It sees everything in black and white.
“It’s much worse than for colour-deficient humans. It has only one photo-sensitive type of cell in its eye, hence it doesn’t have the capacity to see colour.”
But, as in a black-and-white movie, octopuses can distinguish well between light and dark. This explains their ability to discern shapes and textures, although their colour-matching ability remains a mystery.
The plot is thickened by the fact that part of the octopus visual world is foreign to us. Unlike us, they can see light polarisation.
“Light consists of electrical field waves oscillating in different directions,” says Vorobyev. “The direction of these oscillations is called the direction of light polarisation.”
The ability to see polarisation may partly compensate for the lack of colour vision.
Vorobyev’s team borrows octopuses from the wild and houses them, short-term, in tanks at the Leigh Marine Laboratory. To learn more about their vision and colour changes, the creatures are exposed to various backgrounds and images and their responses are recorded.
The team has some potential explanations for the colour-matching enigma. One is that octopuses instinctively know the likely colour of some objects – that sea grass is green, for example.
“Not ‘knows’ in the conscious sense,” says Vorobyev, “but during natural selection, those octopus that got it wrong were eaten before they could reproduce. Those that approached green sea grass and became green survived.”
Their boneless bodies mean octopuses are sought-after meals by predators, and their chief defences are camouflage and hiding (squirting ink is a last resort).
It’s also possible that octopuses understand some correlation between brightness and colour – for example, dark tends to be greener, or brighter objects tend towards yellow.
Or, finally, different colours may have different polarisation. “We don’t know how polarisation relates to colour.”
Vorobyev thinks octopuses might combine cues like computer programs that assess both shape and brightness in order to imbue old movies with colour. “You give the computer a particular algorithm, and it does quite a good job at producing reasonable colouration. But what is the algorithm of the octopus? We don’t know. It’s possible it uses polarisation as part of it.”
Whatever the octopus’s strategy, it happens in its central brain. “It doesn’t just imitate its background but may imitate an object in its vicinity. A third of octopus nerve fibres coming from the brain are connected to the cells that change the colour of the skin.” This is sophisticated camouflage.
The work is challenging because as My Octopus Teacher showed, octopuses relax if they feel safe. “In captivity, we feed it and it’s calm, so it doesn’t need the colour and it doesn’t hide. It needs motivation in order to become invisible.”
More motivation would make it easier to investigate the connection between what octopuses see and their colour fluctuations.
Another challenge is that octopuses sometimes rebel. Vorobyev describes how after a week of being shown nothing but a screen with lines of various thicknesses and spacing, one octopus placed a shell over each eye. “It’s as if it was deliberately demonstrating it did not want to see it any more.”
