Captive cuttlefish require entertainment when they eat. Dinner and a show — if they can’t get live prey, then they need some dancing from a dead shrimp on a stick in their tank.
When the food looks alive, the little cephalopods, which look like iridescent footballs with eight short arms and two tentacles, are more likely to eat it. Because a person standing before them has to jiggle it, the animals start to recognize that mealtime and a looming human-shaped outline go together.
As soon as a person walks into the room, “they all swim to the front of the tank saying, give me food!” said Trevor Wardill, a biologist at the University of Minnesota who studies cuttlefish vision.
You may get a squirt of water from a cuttlefish’s siphon if you don’t feed them, though. Alexandra Schnell, a comparative psychologist at the University of Cambridge, recalled some who sprayed her if she was even a little slow with the treats. It’s the kind of behavior that researchers who’ve worked with cuttlefish sometimes remark on: The critters have character.
But they do not have the name recognition of their cousins — the octopus and the squid. Even Tessa Montague, a neuroscientist who today studies cuttlefish at Columbia University, hadn’t really heard of them until an aquarium visit during graduate school.
“Octopus are obviously part of lots of children’s story books,” she notes. Cuttlefish were not present.
During the last week of a course at the Marine Biological Laboratory in Woods Hole, Mass., though, she heard a talk by Bret Grasse, whom she called a “cephalopod guru.”
“He said they have three hearts, green blood and one of the largest brains among invertebrates,” she said. “And they can regenerate their limbs, they can camouflage. Within about 30 seconds, I had basically planned out my entire life. That lunchtime I went to the facility where he was culturing all these animals. My entire scientific career flashed in front of me. I was like, this is it, this is what I’ve been looking for.”
Dr. Montague joined the many scientists who have long studied the remarkable abilities of cuttlefish, from their camouflage to their speed when hunting. In recent years, a string of high-profile papers has reported that they are capable of surprising cognitive feats, including rejecting easy meals while holding out for better food in the future, a version of the famous marshmallow test.
Given the vast evolutionary gulf between cuttlefish and creatures like apes and crows that perform similar calculations, some scientists believe the shimmering little decapods may help us understand why these mental abilities evolve.
Invisible hunters
Cuttlefish are more closely related to insects than to humans.
They have no true bones in their bodies, just an internal shell filled with air that helps them float. Their blood gets that blue-green tint from hemocyanin, which they use instead of hemoglobin to carry oxygen. The smallest species are scarcely more than an inch long and the largest may reach more than two feet. Most species tend to live alone, and they can be found in the waters of nearly every continent except the Americas. Cold water kept them from spreading there, scientists have speculated, when they evolved millions of years ago.
Sepia officinalis is the cuttlefish species most commonly kept in labs. Roger Hanlon, a senior scientist at the Marine Biological Lab, and his colleagues have hatched wild S. officinalis eggs each year for nearly four decades, raising thousands of cuttlefish that have starred in dozens of papers. Cuttlefish hatch as miniature versions of adults rather than having a larval phase like many octopus and squid, which in his experience makes feeding and caring for cuttlefish easier.
And the creatures, which have some of the largest brains of any invertebrate, start learning while still in the egg, Ludovic Dickel and other researchers in France have found. Cuttlefish can see through the walls of their translucent homes into neighboring tanks of prey animals, and after they hatch, they much prefer the prey that they saw while unborn. Soon after they emerge, they begin to hunt, and to hide in plain sight.
A cuttlefish’s most marvelous treasure may be its skin — Dr. Hanlon has called it electric.
It is full of muscles and nerves wrapped around millions of tiny sacs of red, yellow and brown pigment. When the muscles contract, the sacs are pulled out into flat discs of color like pixels, with each pattern of contraction yielding a different effect. Below the pigments in the skin glisten blue and green reflectors and structures that scatter white light. As a cuttlefish glides over a landscape of rocks and seaweed, neurons fire and muscles twitch and it fades into the colors of its background. It can even mimic the texture of its surroundings, thanks to raised structures called papillae that open like a thousand cocktail umbrellas under its skin, creating the effect of nubbins or spires.
The number of muscles and nerves in each papilla is roughly the equivalent to what is in each human finger, Dr. Wardill estimates.
“If you can imagine a thousand fingers sticking out of something — it’s crazy. I’m not joking, it’s that complicated,” Dr. Wardill said. “And it’s all soft. There are no bones.”
A cuttlefish can drift unseen past a fish, then engulf it in an explosive flash of limbs. If a predator, like a dolphin or a shark, is nearby, it can become one with the weeds.
This visual subterfuge is all the more remarkable when you realize that cuttlefish are colorblind. In 1996, Justin Marshall and John Messenger showed that cuttlefish on a blue-and-yellow speckled background don’t notice a difference between the two colors, adopting a fine-grained tan pattern. This is because they are not actually picking up on color when they change: What they detect instead appears to be the intensity of the light bouncing off their surroundings. That is sufficient for them to create impressions of their backgrounds that are convincing enough to survive. Evolution, of course, has honed their mimicry skills ruthlessly.
“If they got it wrong, they died,” said Dr. Marshall, a marine biologist at the University of Queensland in Australia.
Their intimate control of their body’s appearance is also involved in mating. Most cuttlefish live solitary lives, gathering in threes and fours to court and mate. But inhabitants of the small Australian steel town of Whyalla tipped scientists off in the late 1990s that cuttlefish were swarming on the coast. Swarms of giant Australian cuttlefish gather to mate, with the number once nearing 200,000. In this aggregation, in which there are more males than females, big males defend their chosen mates and fight would-be challengers. Smaller males disguise themselves, however, retracting one arm and holding three others in a posture that usually only females take. They nip past the gatekeepers to deposit their own sperm, with a fair rate of success.
And then, just like that, they die. Cuttlefish live only a single year, maybe two. For all their sophistication, they have a very short existence.
“But most of what cephalopods do is so different it confuses people,” Dr. Hanlon said. And their unusual characteristics command the attention of anyone curious about the most extreme forms life can take.
Marshmallow tests, but make it shrimp
Enter, then, the comparative psychologists.
Nicola Clayton didn’t start out studying cuttlefish. A professor at Cambridge, she has built her career around the remarkable behavior of the scrub jay, a small blue-black bird that stores food for later. In 1998, she and colleagues showed that the birds can remember how long it’s been since they hid food items. They can plan for the future, hiding food in places where they have reason to believe they’ll be hungry later.
Their behavior is more sophisticated than simply learning that food will appear if you push a button or recognize a pattern, something that many animals can do. Apart from apes and other corvids, like crows, few animals studied so far possess the full portfolio of mental abilities demonstrated by these birds.
But cuttlefish and other cephalopods may be an intriguing test case. When octopus and cuttlefish hunt, they do not take the same route two days in a row, marine biologists have observed. Christelle Jozet-Alves, a comparative psychologist, wondered if that meant they had a memory like that of corvids, capable of re-experiencing what had happened to them in the past. In 2013, with Dr. Clayton and a collaborator, she published a tantalizing cuttlefish study suggesting they did. Dr. Clayton, Dr. Schnell and their colleagues have started to ask: Do cuttlefish have a sense of the future and the recent past? Can they make decisions about what they think is likely to happen in the future?
Octopus have long amazed observers with their apparent canniness — in one YouTube clip with more than 2 million views, an octopus scoops up coconut shells and carts them off, perhaps to use them later as tools. Squids also have large brains and sophisticated behavior. But cuttlefish are easier to grow in the lab than squid and easier to work with than octopus, which are often standoffish and may refuse to engage with an experimenter, Dr. Schnell said.
There is, of course, the Houdini factor, too.
“You get little escape artists with octopuses. You come in in the morning and it doesn’t matter how tight you have closed an aquarium tank, you’ll find them crawling out,” she said.
She added, “I’ve never had that happen with cuttlefish,” perhaps because their internal shells, which keep them buoyant, make them less able to squeeze through small spaces.
To see if cuttlefish can make decisions about the future based on their experience, in a pair of experiments published in 2020, the team gave cuttlefish crabs each morning. At night, they gave shrimp, a much preferred goody, to some, but not all. Cuttlefish that reliably got shrimp stopped eating crab, waiting for the better food, while those that got them only occasionally stuck with crabs. Even if shrimp was only provided once every 48 hours, as the time drew near the creatures would still refrain from eating crabs, saving room.
Then, Dr. Schnell and colleagues reported in 2021 that cuttlefish would forgo an easy meal in favor of waiting for food they preferred, a version of the assessment of self-control known as the “marshmallow test” that is administered to human children the world over. C.C. Chao and a colleague at National Tsing Hua University in Taiwan had found something similar in 2020, that cuttlefish chose a smaller meal if they had been rewarded for this in the past.
These studies suggest that cuttlefish are capable of self-control and of remembering their own past experiences. The next step will be tests of whether, like the jays, they are aware of how they will feel in the future, and can plan for it.
“We’re adapting these experiments that have been done in chimpanzees and corvids,” Dr. Schnell said, “to see if these animals that diverged from this lineage 550 million years ago have the same capacity.”
If they do, cuttlefish will have an important role in illuminating how and when intelligence evolves. Corvids and certain primates — including humans — each developed the ability to plan for the future, but they seem to have arrived at it independently, rather than inheriting the capacity from a common ancestor. Both kinds of creatures have complex social lives and lengthy life spans to learn from, commonalities that make it hard for biologists to say what traits or environment make intelligence a good investment for an organism.
The cuttlefish promises to add another dimension to the study of intelligence because they must have developed it in a completely different context.
“They don’t live a long time, unlike the corvids. They’re not highly social, unlike the corvids,” Dr. Clayton said. “It was very unlikely that it was social intelligence that was driving the evolution.”
There are still more tests to come. It’s not clear whether cuttlefish will turn out to have all the same skills as apes and corvids, or just a handful. If what they have is similar, then it’s possible that profound vulnerability, rather than long life or social complexity, is what has forced them to become so canny.
The ancestors of cuttlefish and other cephalopods had hard external shells to protect them. Over the eons, these shells were lost or adapted into internal structures, like the cuttlefish’s. And unlike octopuses, which are reclusive den dwellers, cuttlefish are exposed in open water.
“They’re essentially a big lump of protein, which is very tasty to anything that lives in the ocean,” Dr. Schnell said.
It might be that the best protection for these little roving meatballs with arms, alongside their formidable camouflage, was their brain.
Light-up cephalopods
At the same time that comparative psychologists are bringing the cuttlefish to a wider audience, the sea creatures may become even easier for more scientists to work with.
At the Marine Biological Laboratory in Massachusetts, a project started in 2017 and led by Mr. Grasse is now growing five different cephalopod species with the aim to make cuttlefish and other cephalopods more mainstream in labs.
And at Columbia, Dr. Montague is developing genetic tools that will allow researchers to manipulate cuttlefish genes, a practice that until recently was only possible with run-of-the-mill lab animals, like mice and fruit flies. With the advent of CRISPR gene editing technology, it is becoming possible to edit the genes of stranger, more offbeat creatures. If she can develop CRISPR tools for cuttlefish, it could be the beginning of a new chapter for these cephalopods in science.
For Dr. Montague, it could mean a way into understanding how cuttlefish look at their environment and, in less than a second, make their skin blend in. She hopes to engineer cuttlefish whose neurons light up as they carry that visual message to the brain and out to the skin. Then she can map the precise path that information takes and start to explore how each change of scenery affects the cuttlefish’s neural world.
Along the way, she has accumulated a number of unusual collaborators, from artists to designers to scientists, all unified in their fascination with the cuttlefish and its charms.
“All kinds of people want to work on this project,” she said, “and that’s been one of the real joys of it.”
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