Cephalopods: Colour on demand

Octopuses come in many colours and they keep changing. Kate Chmiel explores their exceptional use of colour and what it might mean to humans. As Denis Baylor points out in his 1995 book 'Colour: Art and Science': "The rich colours that we see are inventions of the nervous system rather than properties of light itself."

Above: Pfeffer’s Flamboyant Cuttlefish (Metasepia pfefferi), Stradbroke Island, 2008. Photo: Mark Norman.

What colour is an octopus? How do you describe the look of an animal when its colour and shape are in constant flux? John James Wild 'figured' the Australian Giant Cuttlefish, Sepia apama, as a uniformly dark creature1, an utterly inadequate representation of its flashing skin that shimmers and shifts in the blink of an eye. A drawing of a cuttlefish, even when exquisitely rendered, preserves just a moment with no sense of what came before or after.

This is an interesting question for the non-scientist, but a compelling problem for scientists that study cephalopods - the group of molluscs that includes cuttlefish, squid, octopuses and nautiluses. Trying to understand and describe the colour of a cephalopod has produced a type of artistry of its own. The language of description varies from clunky terms that try to pin down that which wriggles away from classification – stripes, streaks, bands, bars, lines – to scientist Martin Moynihan’s depictions seventy years later that read like lavish dishes on a menu. "Individual in Dark with Ring and Glittering (Golden) Brows2" is one particularly delicious description by Moynihan whose detailed black and white line drawings capture the drama, if not the hue, of colour change. But nothing does the job like colour photography and videography, prompting many marine biologists to include a camera in their arsenal of specialist tools. The camera submerges to take measurements, but in some cases, it emerges, almost by accident, with visual art.

Dr Mark Norman is the Head of Sciences at Museum Victoria. As one of the world’s leading experts in cephalopods, he has spent thousands of hours underwater, coaxing and cajoling squids, octopuses and cuttlefish to show him what they can do. He turned to photography as a way to document the performance art of his subjects in a way that museum specimens cannot. “By the time we got back to the museum they were shrivelled pieces of rubber, yet while alive they were doing the most spectacular shows.”3 He is reluctant to claim his photographs are anything but diagnostic pictures, a filter he calls the “field guide mentality” where he is driven to preserve the animal’s best profile with all its identifying characters neatly recorded. “That doesn’t mean that the inherent beauty of the creatures doesn’t come to life.”

Cephalopods have mastered colour change in a manner unparalleled in the animal world. The fabled chameleon’s ability to match its surroundings is paltry compared to the cephalopods of shallow coral and rocky reefs. Here the waters are filled with sunlight and predators that long to devour the soft fleshy bodies of octopuses, cuttlefish and squids. These evolutionary pressures have shaped mimicry, camouflage and bluffing displays often incorporating dramatic colour shifts. They also talk to one another in the language of colour to woo or warn members of their own species.

“The rich colours that we see are inventions of the nervous system rather than properties of light itself.”4 Baylor refers to colour perception in the eye and brain of the beholder, whatever its species. The cephalopods add another aspect to Baylor’s summary since they actually construct colour with their nervous systems. Where the colour change of chameleons is driven by a relatively slow hormonal signal, cephalopods can go from one colour to another as if flicking a switch, because each of their millions of colour-creating skin cells have a direct connection to their brains.

Three types of cell combine forces to paint the skin of a cephalopod. The upper layer is populated by chromatophores, tiny sacs of pigment surrounded by an asterisk of muscle. The animal can expand each chromatophore at will, spreading the pigment over a larger area, or contracting it to a dense pinprick to reveal the layers beneath. Across all groups of cephalopods there are five pigments – yellow, orange, red, brown and black – but most species have just two to four of these.

Two other cell types, deeper in the skin, tamper with ambient light to fill out the spectrum. Iridophores and reflector cells comprise stacked plate-like bodies that refract and polarise light, beaming out brilliant metallic pinks, yellows, silvers, greens and blues, such as the vivid azure warning circles of a blue-ringed octopus. In the deepest layer, leucophores scatter light to produce snowy white patches.5

The visual acuity of most cephalopods is highly evolved, the structure and function of their eyes being remarkably similar to our own. But the most extraordinary thing is that cephalopods themselves are colourblind; their world is a tonal one. Perhaps Moynihan’s stippled illustrations are a better approximation of the visual experience of an octopus than the precise pixels of Norman’s digital camera, and help us understand the ‘artist’s’ intention as an exercise in chiaroscuro rather than technicolour. However, many cephalopods can see different polarisations, a property of light to which the human eye is completely oblivious. Where we see colour, some cephalopods see another secret signal of polarised light. Like their predators, we may miss the main message, bamboozled by the camouflage and gaudy displays when the real communication, from one cuttlefish to another, is beyond our ken.

Their medium is enriched further – not only is their paint changeable, but the texture and shape of the canvas fluctuates at will. Exquisite muscular control smooths out the skin, or ripples it into grotesque crenulations; a handy skill to have for mimicking the rubble of the sea floor or an algal bloom on a rock. Norman astutely observes the phantasmagorical surface thus: “imagine your own body was an Etch-a-Sketch you could play with in three dimensions, in five colours and high resolution.”

It takes an artist to deconstruct the layers of colour in human skin; the greens and purples are built up through layers of underpainting and skilled glazing. Perhaps in a similar way, we can better appreciate the complexity of the sea floor when it’s reported to us through the medium of an octopus’s skin. “Together they must create a pictorial reality which is credible – so a painter has to find a way of uniting all the elements in a picture to make a whole.”6

But what is all this artistry for? For some species, it’s body language that communicates honest information. The animal uses visual signals to attract or rebuff a mate, threaten battle or warn of poison-filled flesh. Other instances are omissions of truth or, sometimes, audacious lies. Some cephalopods pretend to be benign features of their own landscape such as blades of kelp or rocks encrusted with tropical life. The colour range of each species correlates closely with that of their environment, an observation that suggests that a Darwinian art critic has biased their perfect palettes. Norman likens it to council-stipulated Federation colours in heritage-protected suburbs: anything that doesn’t match gets weeded out. Over thousands of generations, he says, selective pressure has “distilled the core essence of what it is that deceives a predator.” Sometimes it’s a simple deceit that jams the vision of hungry fish such as sharp-edged white patches that tear the eye away from the outline of the animal. More startling are the instances of mimicry and the abstraction of characteristics such as body shape and movement. The mimicry isn’t perfect but it’s a startlingly effective caricature. The Mimic Octopus of Indonesia has been caught pretending to be more dangerous and less delicious than it actually is. Norman has recorded this species emulating predatory fishes, sea snakes and toxic lionfish, combining colour, shape-shifting and impeccable acting to appear and behave as something else.7

Norman’s affection for his subjects is clear in his descriptions, his photographs and films. He envies their remarkable skills and wants to share his wonder with us surface-dwellers. “They’ve got the eye contact that that we as humans understand, bifocal eyes with alertness and high motion, then you put them in the most alien body shape you can think of. We relate but we don’t relate at the same time.” His photograph of the Striped Pyjama Squid is an excellent reference for taxonomists, but there’s something more to it, too. The viewer is drawn into its world. Eye to eye, it’s difficult to comprehend that these bright-eyed, curious, responsive animals have more in common with oysters than ourselves.

Scientists may be uneasy with the art/science tension and whether their own records of cephalopod behaviour could be considered art. Moynihan’s book was pilloried by colleagues who considered his work too whimsical and poetic to be science. The camera saves the scientist from this quandary if he or she chooses to preserve cephalopod performances in an objective way, with all artistic flourishes courtesy of the subject alone. Yet, it’s a stretch to claim that cephalopods are creating art, and certainly not one that Norman would make. Those of us who have made no commitment to objective analysis are permitted a bit of anthropomorphism. If art is a provocation, a communication of an internal state, or an intention to deceive, stimulate, bewilder or dazzle, cephalopods certainly fit the bill. And if art is an expression of beauty, could anyone deny the glory of a cephalopod colour show?

Kate Chmiel is the Online Writer/Editor at Museum Victoria. She has also worked in biological research and exhibition curation.

1 John James Wild was a colonial scientific illustrator who worked for James McCoy, Director of the Melbourne Museum in the 19th Century. Caught and Coloured http://museumvictoria.com.au/caughtandcoloured/

2 Martin Moynihan Communication and Noncommunication by Cephalopods, Indiana University Press, Bloomington 1985.

3 Conversation with Dr Mark Norman, 6 Sept 2011.

4 Denis Baylor, ‘Colour mechanisms of the eye’ in Trevor Lamb and Janine Bourriau (eds) Colour: Art and Science, Cambridge University Press, Cambridge 1995.

5 Roger T Hanlon and John B Messenger, Cephalopod Behaviour, Cambridge University Press, Cambridge 1996.

6 Bridget Riley ‘Colour for the painter’ in Trevor Lamb and Janine Bourriau (eds) Colour: Art and Science, Cambridge University Press, Cambridge 1995.

7 Mark D Norm

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