‘Underwater city’ reveals mysterious octopus world

In 2009, researchers discovered gloomy octopuses (Octopus tetricus) living in close proximity at a site in Jervis Bay, Australia - dubbed 'Octlantis'. Mating and fighting were observed among a group that numbered between two and 11. It was thought that a scrap of metal or man-made object may have provided the first bit of shelter around which the animals could congregate.

A few years later, scientists were surprised to discover another site of wild octopuses living at a high density. Even more interesting: the site was completely natural.

This second site, on a patch of rocky outcrop in the middle of a silted area, accommodated between 10 and 15 animals of varying sizes. As well as mating and fighting, the scientists saw frequent, complex social interactions including eviction of octopuses from dens and exclusion of individuals.

Such aggressive interactions could be costly, so why do these octopuses choose to live together? It turns out that a chain of activity creates an ideal living environment. The octopuses go out to find food, which they bring back to eat in the safety of a den. Then, they toss the shells out to the silted area, which build up over time. The shell bed stabilises the soft sediment and allows the animals to build more dens and create more habitats. Corresponding author Professor David Scheel of the Alaska Pacific University thinks the habitat area wouldn’t be stable without the shells.

“We suspect that this is an area where there’s a lot of food, relatively scarce shelter and plenty of predators,” said Scheel. “If you put that story together it seems that shelter is the most important thing.”

“It looks fairly costly but probably better than going it without a den because there’s predators cruising by fairly commonly.”

These are the first occasions scientists have had to watch this species interacting repeatedly over time. Filming and observing octopus behaviour in the wild is challenging. Researchers have a limited amount of time because of camera battery power and light (artificial lights at night would disturb the animals and possibly attract predators). Also, octopuses often change their body colours and patterns so they’re tricky to tell apart.

However, the group did observe differences between the animals. Some seem to be busier than others. “There definitely do seem to be individuals that are at least considerably more active than others or considerably less active,” said Scheel.

Active behaviour involved being out of their den, reaching out to other animals and trying to chase off other animals. “Sometimes it looks like they might be trying to sort of persuade it to go another direction, like ‘don't go that way’,” said Scheel. “Sometimes one will restrain another by holding onto its body. They all start out with reaching. We’re trying to figure out what they mean.”

Analysis on the differences between the two sites and why octopuses might engage in various reaching behaviours are underway.

While most octopuses are likely to be solitary for most of the time, said Scheel, scientists may have underestimated the importance of the moments of interaction. “They do happen for most or at least sometime in their lives. And for some octopuses, very commonly. Octopuses can use behaviour that they don’t need when they’re solitary.”

The research joins findings about other unusual social behaviours such complex visual signal use in the gloomy octopus and beak-to-beak mating, as well as co-occupancy of dens in the Larger Pacific Striped Octopus whilst in a mating pair.

Matt Slater, Marine Awareness Officer of the Cornwall Wildlife Trust, worked with giant Pacific octopuses (Enteroctopus dofleini) as a keeper and curator at the Blue Reef aquarium in Cornwall. “They seem to like interaction with people,” he said. “They are intelligent and we were always looking for ways to mentally stimulate them and keep them thinking.”

However, at the aquarium where Slater worked, socialisation was not an option as one octopus would end up getting eaten, he explained. Cannibalism is a threat for octopuses in both the wild and captivity, another reason why the animals were presumed to rarely interact. A study of octopus’ cannibalism in Spain suggested it could be more energy efficient for octopuses to eat other octopuses in the wild because they are high in protein and take less energy to consume than, for example, mussels. Defense of territory is another hypothesis.

“Maybe giant Pacific octopuses do get together every now and again and have some degree of sociability we haven’t managed to witness,” says Slater. “That’s the beauty of the sea. It’s that unknown. There’s still so many questions to be answered.”

Featured image © BBC 2017