The discovery of an underwater city built by octopuses, dubbed Octlantis, has led researchers to conclude the mysterious mollusks aren’t the loners we thought they were.

Octopuses have long been thought solitary creatures, with mating typically the only meeting between the two sexes. The creatures always go their separate ways afterward.

However in Jervis Bay, off the coast of New South Wales, Australia, researchers recently discovered a ‘city’ built by octopuses on rocky outcrops using piles of old shells.

“Environmental engineers”

More than a dozen ‘gloomy octopuses,’ also known as common Sydney octopuses, were found congregating, socializing and even fighting at the site.

“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,”explained Stephanie Chancellor of the University of Illinois at Chicago, who is an author on the paper.

“These shell piles, or middens, were further sculpted to create dens, making these octopuses true environmental engineers,” she said.

The ‘city’ lies about 15 meters under the water’s surface, and is about 18 meters in length and four meters wide. Researchers found 23 octopus dens, 13 of which were occupied while the other 10 were vacant.

Aggressive behavior

The research team used GoPro cameras to film how the cephalopods interact in Octlantis. What they discovered was mating, chasing and a huge amount of aggressive behavior. Some of the octopuses were seen evicting other animals from their dens.

“There were some apparent threat displays where an animal would stretch itself out lengthwise in an ‘upright’ posture and its mantle would darken. Often another animal observing this behavior would quickly swim away,” Chancellor explained.

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The researchers said these observations are unique to octopuses living in close quarters.

Octlantis is the second gloomy octopus city found off the coast of New South Wales. The earlier site, dubbed Octopolis, was found in Jervis Bay in 2009.

A significant difference between the two cities is that Octopolis was built around a man-made object. It was initially thought that the cephalopods needed an artificial structure on which to base their settlement, however the discovery of Octlantis has shown otherwise.