The mystery of the colourblind cephalopods

It doesn’t make sense that octopuses can camouflage themselves so well—because they can’t see any of the colours they’re matching.

Written by       Illustrated by Giselle Clarkson

Giselle Clarkson

The octopus is hiding in a curve of PVC pipe, pulling a scallop shell against the entrance like a trapdoor.

University of Auckland marine scientist Luis Nahmad-Rohen lifts the pipe out of one aquarium and into another, holding it near the surface of the water. We wait a few seconds. Then the animal’s soft, bright body unfurls from her hiding place—salmon-pink suckers, peach-coloured siphons. Her mantle and limbs are speckled white and maroon and a delicate mint. When she reaches the golden kelp at the bottom of the tank, she turns paler, greener, browner, and sidles in among its fronds.

Octopuses are famous for their mimicry and colour-rippling skin. They have a lightning-fast ability to match their environment, or to pretend to be something poisonous. Indonesia’s mimic octopus, for instance, does masterful impressions of lionfish, sea snakes and jellyfish.

And yet, octopuses are colour-blind, with just one type of visual pigment in their eyes. Humans have three. Even blind Iberian moles have colour vision—they can sense colour through the skin covering their eyes. So how do octopuses camouflage themselves with colours they can’t even see?

Every few weeks, Nahmad-Rohen sets a trap in Omaha Bay, north of Auckland, and brings an octopus to the tank at the Leigh Marine Laboratory for testing. Some of the octopuses are curious about the experiments. Others seem shy and prefer to hide. Then there are the ones who squirt jets of water at Nahmad-Rohen with their siphons and leave puddles all over the tank room. “I do try to stay on their good side,” he says.

Eventually, every single octopus loses interest. “They actually get bored,” says Misha Vorobyev, a University of Auckland vision scientist and Nahmad-Rohen’s supervisor. In a previous experiment requiring the octopuses to look at fish on a screen, one subject even covered its eyes with shells, as if to demonstrate the depths of its apathy. “At this stage, we release the octopus.”

The female octopus I’m looking at was collected the day before yesterday. (Nahmad-Rohen can tell she’s a female because all her arms are the same; on males, the third arm on the right doubles as a penis—a mating tentacle shorter than the rest.) She’s an Octopus tetricus, a common species, and Nahmad-Rohen named her Īrilya after the word for “eight” in Ho, a language from eastern India.

Like chameleons, octopuses change colour by altering three types of chromatophores (colour cells) in their skin. By contracting and releasing tiny muscles, octopuses can reproduce a wide range of colours and patterns in a fraction of a second.

Still, “some colours are easier than others”, says Nahmad-Rohen. Brown, red and yellow pigments are already present in the chromatophores, but not blue. “Matching a blue object for them would be virtually impossible.”

Īrilya is in a tank filled with natural objects: rocks, brown Ecklonia kelp and short green seaweed. But next door is another aquarium decorated in Barbie technicolour: pink pebbles with magenta-coloured fake weed on one half, fluoro-yellow pebbles and neon-green fake weed on the other.

The scientists want to see how well the octopuses can match their colours to the different backgrounds—and so far, the results are pretty clear. Īrilya is better at camouflaging to her natural environment than the psychedelic one. But why is that, if she can’t see either aquarium properly in the first place?

Vorobyev and Nahmad-Rohen are also testing whether octopuses are using polarised light as a stand-in for colour. Light rays are polarised—aligned in certain directions—when scattered, and water is great at scattering light. Human eyes generally can’t detect polarised light, but lots of animals can, and previous studies have shown that octopus eyes are highly sensitive to polarisation.

Anyway, just because colour is the first thing humans notice doesn’t mean it’s important under the sea. Five metres below the surface, the colour red is lost—water absorbs red light. Below 50 metres, orange and yellow are gone too. The deep sea is a world of blues, greens and violets. As Nahmad-Rohen points out, an octopus that humans can see may be invisible to things that want to eat it.

How an octopus first interprets what it sees then copies that on its skin is still a mystery, but a recent study on its close cousin the cuttlefish raises some intriguing possibilities.

Cuttlefish receive less attention than octopuses, but they’re no slouches on the intelligence front: they can pass the so-called marshmallow test, and can count better than a one-year-old human. They’re also better lab animals than octopuses—more cooperative, less bored.

Like octopuses, cuttlefish change colour in less than a second. When researchers slowed down videos of them changing colour, they found the cuttlefish altered their chromatophores in bursts, seemingly stopping to check how close their skin matched the background, then fine-tuning their camouflage.

What remains to be discovered is how cuttlefish and octopuses know what colour pattern they’re expressing. Are they checking their body with their eyes—or can they “feel” the arrangement of their chromatophores in some way? As the New York Times put it, “Does a certain kind of speckling feel different to the animal than, say, stripes?”

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