An octopus patrols a shallow reef, searching for food–and it’s not alone. A flurry of fish move alongside it, in a group hunting phenomenon observed in multiple marine locations around the world. Scientists have known that octopuses are smart; but social? Not so much. The eight-limbed cephalopods are generally considered solitary, going through life without other octopus interaction except for their brief (usually one-time) mating encounters. But new research is upending the idea that an octopus can’t also be a social butterfly. It just so happens their most interesting relationships may occur across species lines.
Octopuses have been observed hunting alongside fish for decades. Generally, scientists have assumed these interactions were simple cases of fish mooching off an octopus’ flexible appendages and predatory success, stealing a nibble where they could. Or, at the very least, that octopuses led the charge, dictating the actions of fishy followers. Yet the truth appears to be much more nuanced and complex, according to a study published September 23 in the journal Nature Ecology & Evolution.
Fish and octopuses hunt in packs wherein members of different species take on particular social roles. Sometimes the octopus leads and sometimes the intelligent invertebrates follow cues from members of their surrounding scaly squad, per the new analysis. Specific types of fish seem to play recurring parts, and in certain instances, fish even initiate bouts of hunting, recruiting octopuses from their dens.
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 of Animal Behavior in Germany. “Nobody really knew that octopus have this sophisticated social life–not with other octopus, but with other species,” he tells Popular Science. “What’s really novel here is that you have very different animals forming these heterogeneous groups with different types of individuals playing different roles. That’s something that we really haven’t studied broadly in animal behavior before,” he adds.
Video: Typical movement during multispecific group hunting, featuring the three major featured species in the paper (octopus, blacktip grouper, and ‘blue goatfish’). Credit: Simon Gingins
Other animals have been documented interacting and coordinating across species. Take, for instance, the badger and coyote duos observed hunting ground squirrels together. Or groupers and eels cooperating to flush prey out of coral crevices. The octopus-fish dynamic takes things up a notch, with aspects of collaboration, exploitation, and shared leadership all adding up to animal intrigue.
Over the course of 120 hours spent diving and recording video on a specially designed wide-angle camera rig in the Red Sea, researchers captured 13 separate instances of mixed-species hunting. It took “a lot of effort” to collect those observations, says Eduardo Sampaio, lead study author and an animal behaviorist at the Max Planck Institute. Sampaio managed the camera rig with his colleagues in multiple daily dives, searching for camouflaged octopuses on the ocean floor, hoping in each instance to time things just right and stumble upon the sea creatures at mealtime.
In total, all that work yielded just a few hours of relevant footage, he says. Hunting bouts can last as long as 90 minutes but their footage only encompasses snippets. The diving scientists would submerge to find a posse already on the prowl, accidentally disrupt and disperse an ongoing hunt, or run out of air and have to stop recording in the middle.
Video: Examples of octopuses punching fish, thus displacing them. Credit: Simon Gingins
Some observations were apparent during the dives, in real time. For instance Sampaio remembers the first time he witnessed an octopus “punching” one of the fish as it fell out of line–issuing a quick, forceful jab. “The first time I saw this, I was filming and we couldn’t actually use that footage because I started laughing and didn’t [keep the camera straight]. It was so unexpected,” he says. That behavior was notable enough that Sampaio and a few of his colleagues reported it in another paper published in 2020.
However, many of the more subtle social patterns hidden beneath the surface required extra time and attention to unravel. The scientists conducted in-depth video analysis, tracking the animals’ movement, angles, distance from one another, trajectory, and other variables. They found that octopuses tend to be in the center of the hunting packs and often decide whether or not to move to a new location. When the octopus moves, most of the fish follow. And octopuses use their arms to force prey out of hiding.
Blue goatfish, a species of active reef predator, also takes on a leadership position by frequently choosing where the group will head. Blue goatfish scout locations and conduct exploration out in front. “They work as a radar for the octopus,” Sampaio tells Popular Science. “The octopus doesn’t need to move around and do this speculative hunting. It can just look at the goatfish, and once it stops somewhere, the octopus knows there’s something interesting.”
Another species with a notable role are blacktip groupers. These fish are often the most common recipient of octopus punches, and seemingly act only out of self-interest, seeking to steal prey from others, Sampaio explains. Yet even the blacktips weren’t always an annoyance. Upon review of the video, the researchers noted that these fish tend to be anchors, holding the gang in one place and offering a signal that a zone might be worth a second look. Blacktip groupers are ambush predators. Their patience and drive to wait for prey to emerge offers valuable information to the whole crew. And if a blacktip does opt to move, then it prompts the rest of the fish and the octopus to do so as well. “It is a very strong signal,” he says.
“In nature, we tend to think about leadership in very simple terms,” Sampaio says. But the reality might be more akin to a human task force than a dictatorship. There’s hierarchy, but also flexibility and specialization.
Based on their findings so far, Sampaio believes the other recurring group members also have designated roles. But he and his collaborators need more data to prove it. “The problem here was sample size,” he says. Thirteen instances was only enough to determine the strongest influencers and biggest outliers. “It’s easier to look at these extreme examples and see what a species’ presence or absence does to the group,” he adds–smaller players will take an even bigger data collection effort to suss out. Already, Couzin notes they’re trying to find ways to scale up and automate aspects of their filming process.
The scientists also couldn’t readily tell individuals apart from one another. Understanding if roles are dictated at the species or individual level could reveal a whole additional layer of social complexity. Next, they’re aiming to figure out how well the animals recognize each other. As Sampaio puts it, “Does this octopus like to hunt with blue goatfish generally, or does it really want to hunt with Martha, who is a good hunter, and not Steve, who is lazy?”
Finally, the researchers are hoping to better understand how and if all the group participants benefit from the teamwork. So far, it’s been difficult to assess who is eating what and how prey gets divided. When hunting alongside fish, octopuses do appear to put less effort and time into prospecting, and instead spend more time actually eating, based on the timing and duration of “web-overs”–where an octopus expands itself out over a part of the reef, Sampaio explains. But it’s not yet known if every fish also wins out. “We still need to nail that down,” says Couzin. “But I’d be astonished if we didn’t find good evidence when we look in more detail.”
Ultimately, looking more closely at this one system could open our eyes to all sorts of undiscovered animal dynamics. Complex social collaboration and communication between species “could be very common,” Couzin, tells Popular Science. Bird flocks are often multi-species aggregations, and some research already indicates this diversity carries benefits. Ungulates on the savannah might be passing hints about resources or predators between herds, Couzin speculates. “I just find it fascinating and beautiful, and I hope that the public does too.”
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