A day octopus hunting with a blue goatfish
Eduardo Sampaio and Simon Gingins
Octopuses are even more sophisticated than we thought. Despite generally being solitary animals, they can work with fish to find prey and recognise which team members aren’t helping.
That is the conclusion of a study of “hunting packs” that consist of a single octopus and several kinds of fish. The fish scout out potential prey and then call the octopus to flush them out of crevices that they can’t reach.
What’s more, the octopus will punch away fish that are just hanging around the pack hoping to catch something, rather than actively helping to find prey. “They have an understanding that these fish are exploiting them,” says Eduardo Sampaio at the Max Planck Institute of Animal Behaviour in Germany.
Day octopuses (Octopus cyanea) are common in the Indo-Pacific, found everywhere from the Red Sea to Hawaii. They hunt by wrapping themselves around objects like rocks, forming a bag with the membrane between their tentacles that traps small animals, a technique called a web-over.
Hunting groups that consist of a single day octopus and a number of fish, usually a mix of different species, were first described in the 1990s, says Sampaio, but it was assumed that the fish simply follow the octopus to try to grab any prey that escape its clutches.
During 120 hours of diving in the Red Sea, Sampaio recorded 13 of these group hunts with a double camera set-up. His team then manually recorded the three-dimensional movements of group members in the recordings so they could be statistically analysed.
“Not only are the fish following the octopus around, but the octopus is definitely following the fish around as well,” says Sampaio. “If a fish moves straight and fast towards one location, this is a strong signal to everyone in the group that there’s something interesting there.”
If this movement is ignored by the octopus, the fish then swims back and forth. “They will go between the location and the octopus, trying to get its attention,” says Sampaio.
How octopuses behave is also different in packs. When a day octopus is hunting alone, it will do lots of brief web-overs in quick succession. When with fish, it does fewer web-overs, but they tend to last longer, sometimes for more than a minute. A separate study by the team showed that web-overs last longer when an octopus has caught something.
The researchers found six fish species are most likely to be in the hunting packs. Of these, blue goatfish (Parupeneus cyclostomus) were the best hunting partners, actively seeking out prey and leading the octopus to it. Blacktip groupers (Epinephelus fasciatus) were the least likely to find prey.
Octopuses seem to recognise this. They were observed punching blue goatfish on just three occasions, compared with 27 for blacktips. “You have to have at least a species-level recognition of, ‘OK, this species is good to hunt with’,” says Sampaio.
Whether an octopus can remember individual fish that were previously helpful or exploitative is unclear. It is hard to study this aspect because it is virtually impossible to tell individual day octopuses apart unless they have some obvious mark such as a missing tentacle, says Sampaio.
“What I will say is that octopuses are territorial. Some of these fish are also territorial. So there is a high likelihood that these interactions are happening with the same individuals over time,” he says.
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