Gloomy octopuses, also known as common Sydney octopuses, have always been believed to live in solitude, only meeting once a year to mate.
However, the creatures have now been found to hang out in small "cities".
The species lives in the subtropical waters off the coast of eastern Australia and northern New Zealand.
In eastern Australia's Jervis Bay, researchers observed a group of 15 gloomy octopuses communicating and living together at a site the scientists called "Octlantis".
The team of marine biologists, led by Alaska Pacific University professor David Scheel, found the octopuses "exhibiting complex social behaviours that contradict the received wisdom that these cephalopods are loners".
Dr Scheel believes it's unlikely that octopus behaviour has changed, but rather, human ability to observe their behaviour has developed.
One similar site, named "Octopolis", was found nearby in 2009 and considered a total anomaly. That site has been monitored ever since and has between two and 16 octopuses gathering there at any given time.
University of Illinois-Chicago biological sciences doctoral student Stephanie Chancellor said both locations shared similar seafloor terrain.
"In addition to rock outcroppings, octopuses who had [inhabited] 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."
It is not year clear how common these congregations, or octopus cities, are.