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New technology moves scientists closer than ever to understanding how colour-blind cuttlefish pull off their kaleidoscopic patterns. 

By Max Kozlov, 17 November 2023

Cuttlefish are masters of disguise: in milliseconds, they can drastically alter their skin pattern to blend in with their surroundings, a feat made all the more puzzling by their apparent colour-blindness.

Now, new findings and tools are allowing researchers to come closer than ever before to understanding how cuttlefish pull off one of the most impressive camouflage displays in the animal kingdom. Scientists are closely examining the animals’ skin cells, developing tools to track their brain activity and studying their skin to see whether the cuttlefish dreams. Many presented their findings at the 2023 meeting of the Society for Neuroscience (SfN) in Washington DC this week.

Cuttlefish have one of the largest brains of any invertebrate, so scientists hope that this research will yield insights into how such complex behaviour evolves. “Chameleons don’t even come close in speed and accuracy [to] how cuttlefish control their skin,” says Horst Obenhaus, a neurobiologist at the Norwegian University of Science and Technology in Trondheim.

Full display

Neuroscientists have long been interested in these strange cousins of octopuses and squid, because their brain activity is reflected in their skin patterns1. To capitalize on this capacity to wear their thoughts on their skin, Gilles Laurent, a neuroscientist at the Max Planck Institute for Brain Research in Frankfurt, Germany, and his colleagues, took high-resolution videos of a cuttlefish’s individual skin cells in action. “This allows us to have access to an output of the brain without going into the brain,” he says.

There is plenty to catch on camera: the cuttlefish’s skin includes millions of cells called chromatophores, which contain pigments of various colours. When muscles in the skin contract, the cells change shape and modulate the amount of pigment shown. Taken together, these contractions create different colours, patterns and textures, allowing the animal to change its appearance entirely at a moment’s notice.

In July, Laurent and his colleagues reported2 that cuttlefish change their skin colour several times before settling on one that matches their surroundings, even if they have been exposed to the same place before. This suggests that they don’t have a set strategy for blending in; instead, they use trial-and-error to approximate their environment.

But studying the cuttlefish’s skin can only hint at what’s happening in the creature’s brain. The true “holy grail” for researchers, Obenhaus says, will be manipulating cuttlefish genes. But that has proved difficult, says Tessa Montague, a molecular biologist at Columbia University’s Zuckerman Institute in New York City. For example, viruses commonly enlisted to deliver gene-editing technology into cells cannot be used on cuttlefish because there are only a few viruses known to infect the animals and their close relatives.

Flashing neurons

Montague and her colleagues are now closing in on success: they have successfully edited the genome of embryos of the miniature species Sepia bandensis, which reaches only seven centimetres long when fully grown. The embryos don’t survive for long, Montague reported at the SfN meeting. But once her team can raise them to adulthood, Montague plans to insert a gene that produces a fluorescent protein into the animals’ genomes, which would make neurons light up as they fire. That would enable her team to visualize the specific neurons and activation patterns that enable the animals to change their skin with each change of scenery.

In the meantime, her laboratory has been developing other tools that will allow researchers to study the animals once the gene-edited cuttlefish are ready. After engineering animals with fluorescent neurons, researchers need to develop a method to image those neurons, which is no easy feat in animals that lack a rigid skull — on which to mount an imaging device — and are surrounded by corrosive salt water.

Another tool Montague and her colleagues are developing is a tank surrounded by screens that use e-ink, the same technology used in e-readers. The screens can be programmed with patterns, allowing the team to systematically study the cuttlefish’s reactions to their environment without disorienting them by using screens that emit bright light.

Doing the cuttlefish wave

Understanding how cuttlefish camouflage themselves is just the tip of the iceberg, Montague says. Changing their skin colour seems to be one way in which the animals communicate with one another. And there are other colour displays, such as the ‘wave’, in which colours slowly ripple across the cuttlefish’s body, that scientists haven’t been able to explain at all.

Researchers are also using cuttlefish skin to understand the evolution of sleep. Similar to octopuses3, cuttlefish go through periods of ‘active sleep’, in which their skin rapidly flashes different colours. Obenhaus is testing whether the creatures might be replaying previous social encounters while they sleep. Some scientists say this is why animals dream, which could be what’s happening to the cuttlefish. “It’s a tantalizing idea,” Montague says. The dramatic colour flashes defeat the purpose of camouflage, so it’s likely that they serve some important evolutionary function, she adds.

References

(Sources: Nature)