Cuttlefish match the seafloor in steps, not at once: skin-pattern space has about 60 dimensions

A team led by Sam Reiter at the Okinawa Institute of Science and Technology (Japan) and Gilles Laurent at the Max Planck Institute for Brain Research (Germany) studied how the cuttlefish Sepia officinalis matches its skin to the background. The work was published in Nature in June 2023 nature.comNaturenature.comOpen source ↗. The researchers filmed a dozen young animals in a tank of about 70 cm per side, with printed photos of pebbles and rock on a roll of fabric under the water; every 5 to 10 minutes the background was changed by hand. Seventeen high-resolution cameras recorded the skin at 25 frames per second.

High-dimensional patterns, no fixed route

Earlier studies sorted camouflage patterns into three classes built from a small repertoire of components, and behavioural experiments suggested that executing a pattern needs no feedback. The new analysis, based on hundreds of thousands of images, found that the space of skin patterns is high-dimensional. For these animals the number of independent ways a pattern can vary came out close to 60, and the authors note that their method usually underestimates it.

After a background swap, the change took about a minute and was uneven: the skin changed quickly, nearly stopped, sped up again, and settled only after several pauses. In 85 such changes across three animals, each restart after a pause moved the pattern toward its final state. The closer the pattern came to that state, the more often it paused and the longer each pause lasted. Co-first author Theodosia Woo said the animals often overshot the target pattern, paused and came back. The authors suggest that the cuttlefish receive feedback about their own skin; how they receive it is not known.

Components and blanching

Chromatophores, the pigment sacs in the skin, could be grouped into pattern components by how they covaried during camouflaging. The components differed in shape and size and overlapped one another nature.comNaturenature.comOpen source ↗. When the same animal repeated the same background change, the final patterns looked identical, but different sets of sacs changed together. In one case, about 1,100 sacs that changed as one group in the first transition split into 15 groups in the second. Components also differed in their sensitivity to spatial frequency.

The team compared camouflage with blanching, the skin-lightening reaction to a threat. The threat was a researcher's hand stopping 20 cm from the animal or a growing dark circle on a screen. Blanching was fast and direct, consistent with open-loop motion in a low-dimensional pattern space. Faint traces of the earlier camouflage remained in the pale skin, and in 16 of 17 trials in one animal the skin returned to nearly the same pattern afterwards.