An octopus and a fish join forces to hunt together.

An octopus patrols a shallow reef for food—and it’s not alone. A school of fish is moving alongside it, in a group-hunting phenomenon seen in many marine locations around the world. Scientists know that octopuses are intelligent; but social? Not really. The eight-limbed cephalopods are generally thought to be solitary, going through life without any other interactions with octopuses beyond brief (usually one-time) mating encounters. But new research challenges the notion that an octopus can’t also be a social butterfly. As it happens, their most interesting relationships may be between species.

Octopuses have been observed hunting alongside fish for decades. Scientists generally assumed that these interactions were simple cases in which the fish fed on the octopus’ flexible limbs and succeeded as predators, stealing morsels wherever they could. Or at least that the octopuses led the pack, dictating the actions of their fish mimics. But the truth appears to be much more nuanced and complex, according to a study published Sept. 23 in the journal Nature ecology and evolution.

Fish and octopuses hunt in groups, with members of different species taking on specific social roles. Sometimes the octopus leads the way, and sometimes the intelligent invertebrates follow the cues of members of the scaly troop around them, according to a new study. Certain species of fish appear to play recurring roles, and in some cases, fish even initiate hunts by recruiting octopuses from their lairs.

Previously, “the sophistication of these relationships was not understood,” says study co-author Iain Couzin, an evolutionary biologist and director of the Max Planck Institute for Animal Behavior in Germany. “Nobody really knew that octopuses had such sophisticated social lives—not with other octopuses, but with other species,” he says. Popular science. “What’s really novel is that we have very different animals forming these heterogeneous groups with different types of individuals playing different roles. That’s something we really haven’t studied extensively in animal behavior,” he adds.

Video: Typical movement during a multi-species group hunt, including the three main species covered in this article (octopus, blacktip wrasse and blue goat). Author: Simon Gingins

Other animal interactions and coordination between species have been documented. Take the badger-coyote duos that hunted ground squirrels together, for example. Or the groupers and eels that worked together to flush prey from coral crevices. The octopus-fish dynamic takes things a step further, with aspects of cooperation, exploitation, and shared leadership all adding to the animal intrigue.

During 120 hours of diving and video recording with specially designed wide-angle equipment in the Red Sea, the researchers captured 13 separate cases of mixed-species hunting. Collecting these observations took “a lot of effort,” says Eduardo Sampaio, the study’s lead author and an animal behaviorist at the Max Planck Institute for Environmental Protection. Sampaio operated the camera equipment with his colleagues during multiple daily dives, searching for camouflaged octopuses on the ocean floor, hoping to time each one just right and stumble upon the creatures at mealtime.

In all, all that work yielded just a few hours of suitable footage, he says. Hunts can last up to 90 minutes, but their footage only captures fragments. The diving scientists would dive to find a group already hunting, accidentally interrupt and disperse a hunt in progress, or run out of air and have to stop filming halfway through.

Video: Examples of octopuses hitting fish that move them. Author: Simon Gingins

Some of the observations were visible during dives, in real time. For example, Sampaio remembers the first time he witnessed an octopus “slap” one of the fish as it came off the line—with a quick, powerful thrust. “The first time I saw it, I was filming it, and we couldn’t use the footage because I started laughing and I didn’t (hold the camera straight). It was so unexpected,” he says. The behavior was remarkable enough that Sampaio and several of his colleagues reported it in another paper published in 2020.

But many of the more subtle social patterns hidden beneath the surface required extra time and attention to unravel. The researchers conducted in-depth video analysis, tracking the animals’ movements, angles, distance from each other, trajectory, and other variables. They found that octopuses tend to stay at the center of hunting groups, often deciding whether to move to a new location. When an octopus moves, most of the fish follow. And the octopuses use their arms to coerce prey out of hiding.

The blue goatfish, a species of active reef predator, also takes the lead, often choosing Where the group will move. Blue ibex scout locations and explore outside. “They serve as radar for the octopus,” Sampaio says Popular science.The octopus doesn’t have to move around and do this speculative hunting. It can just look at the chamois, and whenever it stops somewhere, it knows there’s something interesting there.”

Another species that plays a significant role is the blacktip groper. These fish are often the most common recipients of octopus blows and seem to be acting solely in their own self-interest, trying to steal prey from others, Sampaio explains. But even the blacktip groper wasn’t always a nuisance. After reviewing the footage, the researchers noticed that these fish tend to act as anchors, holding the group in one place and offering a signal that the area might be worth a second look. Blacktip gropers are ambush predators. Their patience and willingness to wait for prey to appear provide valuable information to the entire crew. And if the blacktip decides to move, it prompts the rest of the fish and the octopus to do the same. “That’s a very strong signal,” he says.

“In nature, we tend to think of leadership in very simple terms,” Sampaio says. But the reality may be more like a human task force than a dictatorship. There is hierarchy, but also flexibility and specialization.

Based on what Sampaio has found so far, he believes the other returning members of the group also have their roles marked out. But he and his colleagues need more data to prove it. “The problem here was the sample size,” he says. Thirteen cases were enough to identify only the strongest influences and the biggest outliers. “It’s easier to look at these extreme examples and see what the presence or absence of a species does to the group,” he adds—smaller players will have to put in even more data to figure that out. Couzin already notes that they’re trying to find ways to scale and automate aspects of their filming process.

Scientists also couldn’t easily tell individual individuals apart. Understanding whether roles are dictated at the species or individual level could reveal a whole additional layer of social complexity. Next, they plan to figure out how well the animals recognize each other. As Sampaio says, “Does this octopus like to hunt with the blue goats, or does it really want to hunt with Martha, who’s a good hunter, and not Steve, who’s lazy?”

Ultimately, the researchers hope to better understand how and whether all participants in the group benefit from teamwork. Until now, it has been difficult to assess who eats what and how the catch is shared. When hunting together with fish, octopuses appear to spend less effort and time searching and instead spend more time actually eating, based on the time and duration of “net crossings”—where the octopus expands to part of the reef, Sampaio explains. But it’s not yet known whether every fish wins, too. “We have yet to determine that,” Couzin says. “But I would be surprised if we didn’t find good evidence when we looked at it in more detail.”

Ultimately, taking a closer look at this one system could open our eyes to all sorts of undiscovered animal dynamics. Complex social cooperation and communication between species “may be very common,” Couzin says Popular science. Flocks of birds are often multi-species, and some studies already suggest that this diversity has benefits. Couzin speculates that ungulates on the savannah may be passing on clues to resources or predators to each other. “I just find it fascinating and beautiful, and I hope the public does, too.”