Some cephalopods are masters of display: Not only can they adapt their skin colour to their immediate environment, thereby merging with the background, they can also produce propagating colour waves along their body. These so-called 'passing clouds' can be observed in behavioural contexts such as hunting, swimming, mating, or even in animals resting under rocks. Their function, however, remains highly speculative. Researchers at the Max Planck Institute for Brain Research in Frankfurt, Germany, have identified a cuttlefish species in which these waves are so prevalent that their properties can be studied in the laboratory. Using simple behavioural methods, they have circumscribed their possible underlying causes to a relatively small set of neuronal circuits.
Cuttlefish, squid and octopuses have remarkable control over their skin pigmentation. Cephalopod skin contains elastic pigmented cells called chromatophores. An adult cuttlefish may contain several millions of them. The size of each chromatophore can be rapidly and individually altered by neural activation of radial muscles. If those muscles relax, their chromatophore shrinks. If they contract, the chromatophore grows larger. One form of cephalopod pigmentation pattern is the passing cloud – a dark band that travels across the body of the animal. It can be superimposed on various static body patterns and textures. The passing cloud results from the coordinated activation of chromatophore arrays to generate one, or as in the present case, several simultaneous traveling waves of pigmentation.
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