Computational analysis elucidates cuttlefish’s extraordinary camouflage skills
Scientists using computational analysis shed light onto the extraordinary ability of cuttlefish, squids and octopuses to camouflage and hide in the surrounding environment.
Cuttlefish, squids and octopuses are marine animals of the order Sepiida. They belong to the class Cephalopoda and about 150 million years ago abandoned their shell and developed a stronger predator-like behaviour. This development was accompanied by an increase in brain size—modern cuttlefish and octopuses have the largest brain relative to body size among invertebrates. Considered to be highly intelligent creatures (relative to their ecology), cuttlefish are known mostly for their ability to alter their appearance in a fraction of a second and mimic the environment with striking accuracy. This is achieved by their brain controlling millions of specialized skin cells called chromatophores which act as biological color pixels on the skin. Each chromatophore is attached to small radial muscles controlled by motor neurons in the brain. When these motor neurons are activated, they cause the muscles to contract, expanding the chromatophore and displaying the pigment. When neural activity ceases, the muscles relax, the elastic pigment sack shrinks and the underlying skin is revealed.
This means that the expansion state of a chromatophore can provide an indirect measurement of motor neuron activity—which is what a group of scientists at the Max Planck Institute (MPI) for Brain Research and the Frankfurt Institute for Advanced Studies/Goethe University set out to do. “We set out to measure the output of the brain simply and indirectly by imaging the pixels on the animal’s skin” says senior author of the study and MPI Director Gilles Laurent. The task was not easy and required years of work as tens of thousands of individual chromatophores needed to be tracked, in parallel, at high-resolution and speed from one image to the next, from one pattern to the next, over weeks and as the animal moved and grew. One key insight was “realizing that the physical arrangement of chromatophores on the skin is irregular enough that it is locally unique, thus providing local fingerprints for image stitching” says co-author Matthias Kaschube of FIAS/GU.
Laurent’s team was able to start peering into the brain of the animal and its camouflage control system. They found, for instance, that to camouflage cuttlefish do not try to match their skin to the surroundings pixel by pixel. Instead, they seem to extract, through vision, a statistical approximation of their environment, and select accordingly an adaptive camouflage texture out of a large but finite repertoire of patterns, selected through evolution. This ability is present from birth implying that their biological solutions to this statistical-matching problem are innate. They also found that when a cuttlefish changes appearance, it changes in a very specific manner through a sequence of precise intermediate patterns, suggesting internal constraints on pattern generation. They also realised that chromatophores systematically change colours over time, and such that the time necessary for this change matches the rate of production of new chromatophores as the animal grows.
“This study opens up a large range of new questions and opportunities,” Laurent concludes. “Some of these concern texture perception and are relevant to the growing field of cognitive computational neuroscience; others help define the precise link between brain activity and behavior, a field called neuroethology; others yet help identify the cellular rules of development involved in tissue morphogenesis. Finally, this work opens a window into the brain of animals whose lineage split from ours over 540 million years ago. Cephalopod brains offer a unique opportunity to study the evolution of another form of intelligence, based on a history entirely independent of the vertebrate lineage for over half a billion years.”
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