Scientists have discovered Octlantis - an underwater site built entirely by octopuses, where they communicate with each other by posturing, chasing or colour changes.
Researchers from University of Illinois in the US found the site in the waters off the east coast of Australia, that is the home of up to 15 gloomy octopuses.
It is the second gloomy octopus settlement found in the area, and the discovery lends credence to the idea that octopuses are not necessarily loners.
The first gloomy octopus site was found in Jervis Bay off the east coast of Australia in 2009.
The new site is located just a few hundred meters away from the first site, and has been dubbed Octlantis.
The site is about 10 to 15 metres under the water's surface and is about 18 metres in length and four metres wide, researchers said.
It is composed of a few patches of exposed rock and beds of discarded shells from prey animals. A total of 13 occupied and 10 unoccupied octopus dens - holes excavated into sand or shell piles - were found at the site, they said.
"At both sites there were features that we think may have made the congregation possible - namely several seafloor rock outcroppings dotting an otherwise flat and featureless area," said Stephanie Chancellor, a PhD student at the University of Illinois.
"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," Chancellor said.
Researchers placed four cameras at the new site to film for a day, recording 10 hours of footage that showed numerous social interactions among the inhabitants. The number of octopuses observed at the site ranged from 10 to a high of 15.
"Animals were often pretty close to each other, often within arm's reach. Mating, signs of aggression, chasing, and other signalling behaviours were observed," Chancellor added.
"Some of the octopuses were seen evicting other animals from their dens. Often another animal observing this behaviour would quickly swim away," she added.
The study was published in the journal Marine and Freshwater Behaviour and Physiology.
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