Author: Emily Vialls
Artist: Shangyu Chen
Editor: Niru Varma
In their early evolutionary history, the common ancestor of modern-day squids, octopuses and cuttlefish made one of the largest U-turns in evolution. 400 million years ago, in the late Devonian period, Coeloid cephalopods did away with their shells, favouring better motility, and the ability to squeeze into small spaces. But there was a catch: reducing themselves to a squishy piece of nutritious muscle would make them an easy score for predators. In response to this new selective pressure, cephalopods adapted to some of nature’s most novel and challenging niches, including complex cognition, diet generalism, and adaptive camouflage. In particular, their adaptive camouflage (a.k.a. metachrosis) system possesses complexity unlike any other animal, making it a unique research target.
The Biological Benefits of metachrosis camouflage
In tandem with their well-developed cognitive abilities, cephalopods use metachrosis in hunting, communication, and, primarily, as an antipredatory weapon. To fend off predators they can mobilise several camouflage mechanisms including countershading (a gradient colouration matching their oceanic backdrop), masquerade (resembling an inedible object), and mimicry (disguising themselves as an entirely different animal).
Another notable use of adaptive camouflage is in interspecies communication – specifically antagonistic and courtship displays. Male cuttlefish have been observed using honest and deceptive signals simultaneously to fool a rival male. The male presented typical female patterns towards the rival, whilst presenting normal male patterns towards a receptive female. The rival was confused, providing the two-faced cuttlefish with an opportunity to mate. This example elucidates how cuttlefish decision-making used alongside their metachrosis capabilities can give them a selective advantage.
The mechanisms underpinning cephalopod metachrosis
Ironically, cephalopods are colour-blind! Somehow cephalopods interpret colour in the surrounding environment without using their eyes. Individual colour-changing cells are thought to be capable of detecting light and responding by altering colour expression independently. It is thought that light-detecting proteins (known as opsins) in cephalopod skin, which only differ from proteins in the eyes by a single amino acid, can detect the presence and wavelength of peripheral light and signal the brain. After processing this visual information, the brain responds with both hormonal and neuronal cues. These signal myriads of light-refraction-altering cells, with each puzzle piece integrating seamlessly to produce a cohesive and convincing disguise. These specialised cells alter wavelength by either light reflection or refraction:
Light reflection involves chromatophores. Described as biological pixels, these consist of radial muscle cells arranged around a pigment-containing electric sac. Following a neural impulse, these muscles contract to stretch out the central sac, spreading out the pigment molecules. This amplifies the expression of the pigment colours – reds, yellows, and browns. Reflector cells called leucophores scatter the full spectrum of light, producing white by similar mechanisms that produce the polar bear’s fur colour. Leucophores match the light level of the surroundings, producing a backdrop that aids the appearance of patterns.
Light refraction works through the iridescent iridophores – cells which change colour as the angle of view changes. The layers of protein in these cells selectively reflect different wavelengths of light, due to differences in the refractive index between layers and the spaces separating them. From above these appear blue, but reflect red light if viewed from a more oblique angle. Mediated by neurohormones, or another diffusible cue, the position and orientation of these in a cell can change.
Different combinations of these substituent elements can alter wavelength collectively, individually, or not at all. Getting a pixel-by-pixel match would be impossible, nonetheless, cephalopods’ well-developed cognitive and visual systems extract an approximation of their environment, and then use this to select a camouflage out of an in-built library of patterns. The camouflage does not have to be perfect – just sufficient to fool any potential predators.
Applications in research
The nervous and camouflage systems in cephalopods are highly interconnected. Cephalopod neural networks have long been favourable research targets, providing some of the earliest insights into the inner workings of the brain, with continued use in modern neuroscience. Cephalopod studies could provide a window into another form of intelligence – one possessed by animals whose lineage split from humans 540 million years ago.
A multitude of mysteries still confound cephalopod metachrosis and its association with their complex neuronal systems. Most recently, scientists have utilised the activity of chromatophores to give direct insight into the activity of cuttlefish motor neurones and visualise extensive populations of neurones in free-living animals. Future studies could help us define a more precise link between brain activity and behaviour (a field called neuroethology), or perhaps be utilised in the growing discipline of computational cognitive neuroscience.
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