Gloomy octopuses—also known as common Sydney octopuses, or octopus tetricus—have long had a reputation for being loners. Marine biologists once thought they inhabited the subtropical waters off eastern Australia and northern New Zealand in solitude, meeting only to mate, once a year. But now there’s proof these cephalopods sometimes hang out in small cities.

Scheel et al. Evicted citizen of Octlantis.

In Jervis Bay, off Eastern Australia, researchers recently spotted 15 gloomy octopuses congregating, communicating, dwelling together, and even evicting each other from dens at a site the scientists named “Octlantis.” The international team of marine biologists, led by professor David Scheel of Alaska Pacific University, filmed these creatures exhibiting complex social behaviors that contradict the received wisdom that these cephalopods are loners. Their study was published in the journal Marine and Freshwater Behavior and Physiology (paywall).

The discovery was a surprise, Scheel told Quartz. “These behaviors are the product of natural selection, and may be remarkably similar to vertebrate complex social behavior. This suggests that when the right conditions occur, evolution may produce very similar outcomes in diverse groups of organisms.”

The mystery of Octopolis

Peter Godfrey-Smith Life in Octlantis.

It’s not yet clear what led to the creation of Octlantis or if these sorts of congregations are common.

At least one other gloomy octopus site was found recently, though; it was discovered in 2009, not far away in Jervis Bay, and named Octopolis. At that time it was considered a total anomaly. Researchers believed that the cephalopods gathered there because an unidentifiable human object happened to have formed a central point that the cephalopods surrounded with dens. The unknown artifact was a single object about 30 cm long, heavily encrusted, possibly made of metal. The site has been observed for seven years now, and at any given time, there are somewhere between two and 16 octopuses there.

Scheel et al. Gloomy posse (each letter indicates an octopus).

In Octlantis, however, there is no similarly mystifying human object to explain the gloomy octopus congregation.

The likely explanation, said Stephanie Chancellor, a study co-author and doctoral student in biological sciences at the University of Illinois-Chicago, in a statement, is that in both Octopolis and Octlantis there were several seafloor rock outcroppings dotting otherwise flat and featureless areas. “In addition to the rock outcroppings, octopuses who had been inhabiting the area had built up piles of shells left over from creatures they ate, most notably clams and scallops. These shell piles, or middens, were further sculpted to create dens, making these octopuses true environmental engineers,” she said.

Gritty city life

Peter Godfrey-Smith Civilization & its discontents.

Like any urban environment, Otocopolis and Octlantis can be tough places to live. Citizens must be scrappy. The company and food are abundant but all the activity in the cities also attracts predators, including sharks.

There’s also a lot of aggression apparently, although the researchers can’t yet explain why. Gloomy octopus males seem to spend a great deal of time chasing each other out of dens. Scheel is hesitant to speculate about what exactly this behavior means. “We are still studying this,” he said.

Lonely no more?

Gloomy octopuses aren’t the only supposedly solitary cephalopods to have been spotted interacting socially in the last decade.

Scheel believes that octopus behavior probably hasn’t changed in that time. Rather, humans’ ability to observe the behavior has. Today more divers are in the water with cameras and better technology to quickly communicate findings amongst divers and scientists.

P. Godfrey-Smith Complex social behavior evolves over generations.

However, that doesn’t mean cities like Octopolis and Octlantis are super common. “Congregations such as these probably occur wherever shelter is limited to small patches of habitat, and food is plentiful,” he posits. But the marine biologist does suspect the findings show gloomy octopuses have been socializing for a very long time. “Most commonly, the gloomy octopus seems to den by itself,” Scheel writes. “For these complex behaviors to occur, I think that they must encounter one another and interact regularly over generations, even if at any time there are more octopuses living a solitary life than interacting consistently throughout every day.”