The gloomy octopus is, as the name might suggest, something of a recluse. Usually, these despondent cephalopods live the life of a loner, shunning other octopuses to go it alone. But new observations are revealing that the gloomy octopus can sometimes form communities, upending what we thought we knew about their eight-legged brethren.

The first communal gloomy octopus (Octopus tetricus) denning site was found in 2009 in Australia and nicknamed "Octopolis". This was seen as an anomaly, with researchers suspecting that the man-made object around which it had developed played a role in the unnatural behavior of the 16 animals observed. Yet the same team have now discovered a brand new – and totally natural – octopus settlement just a few hundred meters from the original, and suspect that the cephalopod cities may be much more common than thought.

The researchers suspect that there are certain geological features at this new site that meant the 13 cephalopods were more likely to congregate in a large group, rather than striking out alone. The region has a number of seafloor rocky outcrops clustered close together in an otherwise featureless plain, and it is thought that this might be forcing the often solitary animals together, forming unusual social interactions.

“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, co-author of the paper published in Marine and Freshwater Behaviour and Physiology. “These shell piles, or middens, were further sculpted to create dens, making these octopuses true environmental engineers.”

It’s believed that it is this combination of restricted den sites coupled with an abundant food source that allows the multi-armed masters of the sea to live in such close proximity, and not due to man-made structures as previously thought. It's also giving researchers a fascinating insight into the social lives of the animals.

The gloomy octopuses were frequently found within arm’s reach of each other, and observed interacting in a whole host of ways, from mating, chasing, and signaling to each other. “Some of the octopuses were seen evicting other animals from their dens,” said Chancellor. “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.” 

The researchers still don’t fully understand the behavior of the octopuses, particularly as the animals face higher levels of aggression and potential injury from their scuffles, but something is clearly keeping them all together, and the team plans to find out what.