Octopuses Join Fish in Multi-Species Hunting Parties

Octopuses and fish form hunting groups

Although solitary octopuses seem to enjoy joining mixed-species hunting parties, they are not typically known for seeking out members of their own species. Their exceptional intelligence - a behavioural characteristic more commonly linked with social animals - has therefore long presented a mystery.

A new study now offers another compelling illustration of their impressive mental abilities.

Behavioural ecologist Eduardo Sampaio of the Max Planck Institute and his colleagues followed 13 hunting groups comprising several species. They uncovered an unexpectedly sophisticated system of cooperation between octopuses and fish.

Footage collected across 120 hours of diving showed that each species brings distinct abilities to the interspecies hunt. Fish including goatfish (Mullidae) search for prey and guide the group towards promising areas. The big blue octopus (Octopus cyanea), meanwhile, decides when the hunt begins.

"The octopus basically works as the decider of the group," Sampaio told Helena Kudiabor at Nature News. "There's a sign that some cognition is occurring here, for sure."

How big blue octopuses and fish share the hunt

Despite some participating species being divided by more than 550 million years of evolution, they are able to coordinate their efforts for mutual gain. Field experiments showed that both the octopuses and fish catch more crustaceans, fish and molluscs when hunting together.

"One item of prey is not divided, it is taken by whoever catches the prey first!" explains Sampaio. "However, because the interaction between the fish and octopus repeat several times during a hunt, prey is 'shared' in the sense that sometimes the octopus catches the prey, and other times fish catch the prey."

Fish that patrol the area signal the presence of prey to the octopus, reducing the time the cephalopod must spend searching. In turn, when an octopus probes crevices that its companions cannot enter, prey may be driven out towards the waiting fish.

"This beneficial interaction enables fish to acquire otherwise unreachable prey," says Sampaio. It allows "octopuses to conserve energy by focusing on high-quality food sources, while exerting control and providing feedback within the group, highlighting the sophisticated dynamics of marine life collaboration."

As octopuses and their principal hunting partners are both territorial, the same individuals probably cooperate repeatedly, although distinguishing one animal from another is difficult.

Octopuses discipline opportunistic hunting partners

Mixed-species hunting and foraging arrangements are seen elsewhere, from groups of bird species to badgers working alongside coyotes and groupers partnering with moray eels. However, none of these other non-human systems is known to involve animals directly correcting one another's behaviour.

The new recordings, by contrast, show an octopus striking fish that step out of line with a punch. Blacktip grouper (Epinephelus fasciatus) were frequently the target of this aggression. The groupers appeared to deserve the treatment: they generally contributed little to the group, instead lingering nearby to take advantage of the work done by the rest of the team.

"The [octopuses'] exhibited range of partner-dependent behavioral flexibility, especially concerning the use of social information when deciding to switch foraging strategies and whom and when to punch, indicates that day octopuses have hallmarks of (heterospecific) social competence and cognition," the researchers write in their paper.

This behaviour is only one of many ways in which octopuses attend to the other species around them, a quality that may help explain their unusually sharp intelligence.

"These results broaden our understanding of leadership and sociality, emphasizing the complexity and adaptability of social interactions in nature," Sampaio concludes.

The research was published in Nature Ecology & Evolution.