Octopuses work together with fish to hunt – and the way they share decisions is surprisingly complex

A new study published in Nature Ecology & Evolution lifts the veil on what happens when octopuses and fish hunt together. As it turns out, this cross-species relationship is more complex than anyone expected.

Animals of the same species often cooperate – work together to reach some kind of goal. But it’s relatively rare to find cooperation between individuals from different species.

A classic example you’ll be familiar with is the close relationship between dogs and humans, whether in the context of herding sheep or hunting. In these situations, the dog and the human work together to achieve a goal.

That’s mammals. But underwater species also sometimes cooperate. A nice example is the joint hunting behaviour of moray eels and grouper. The grouper approaches the moray and signals that it wishes to hunt. The eel responds in kind, and off they go.

During these hunting forays, the grouper uses signals to indicate where prey may be hidden in the coral matrix. It’s a synergy made in heaven: the eel can scare the prey fish from hiding places among the coral, while the grouper patrols over the top. There is literally no place for prey to hide.

For the eels and grouper, the chances of catching their dinner are greatly improved when hunting together compared to hunting on their own.

Who’s in charge here?

While researchers have described these behaviours before, one question remains unanswered. Who, exactly, is in charge of these cross-species interactions?

Who decides what they are going to do, where and when? Are the different players “democratic”, in that they come to some form of compromise, or does one species take the lead and the other simply follows (that is, they are “despotic”)?

In an international collaboration, biologist Eduardo Sampaio and colleagues have investigated cross-species interactions between the usually solitary day octopus (Octopus cyanea) and several fish species, such as goatfish and groupers.

The fish and the octopus share a common goal – to increase their hunting efficiency. The traditional view of octopus-fish hunting groups assumed that the octopus is the producer, and the fish simply follow along and opportunistically pick up the scraps.

With its long, flexible arms, the octopus explores all the nooks and crannies of the hunting ground, flushing out prey the fish can then take advantage of. In this scenario, the octopus would be solely in charge of decisions and the fish just follow (that is, it’s an exploitative, despotic relationship).

However, when researchers took a closer look, it appeared perhaps this relationship is not as simplistic as previously believed. But without fine-scale analysis providing hard evidence, it is difficult to work out the precise details of how this cooperation works.

What did the new study find?

Using sophisticated behavioural analyses of 3D videos captured from 120 hours of diving, Sampaio and team found that each partner in the interaction plays a specific role. There was, in fact, no true leader – they are democratic.

The fish were responsible for exploring the environment and deciding where to move, while the octopus would decide if and when to move. Interestingly, controlled experiments showed the octopuses were guided by social information provided by the fishes.

When partnered with blue goatfish, the octopus foraging tactics where more focused and efficient. When partnered with blacktip groupers, they were less so. So, the nature of the hunting relationship varied depending on who’s involved.

The researchers concluded that, overall, success rates for capturing prey were higher for the octopus when foraging with fishy partners.

The details revealed by this study suggest this relationship is far more sophisticated than other cross-species hunting associations examined to date.

Despite the huge evolutionary gap between these animals (the equivalent of about 550 million years), both fish and octopus show clear signs of social competence and advanced cognition.