Octopuses are known for leading mostly solitary lives. So, biologists were surprised to find the second of two sites off the eastern coast of Australia where as many as 15 gloomy octopuses, or Octopus tetricus, have congregated.
The discovery is reported in the journal Marine and Freshwater Behaviour and Physiology.
The first gloomy octopus site was discovered and described in 2009 by co-author Matthew Lawrence, an independent scholar. The settlement, dubbed Octopolis, contained several dens and a human-made flat object about a foot (30 centimeters) long.
The second site, called Octlantis, is located close to the first site and has a total of 13 occupied and 10 unoccupied dens.
Taken together, both sites support the notion that octopuses will form communities if conditions are right. When they do, they become true architects of their environment.
“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 co-author Stephanie Chancellor, a Ph.D. student in biological sciences at the University of Illinois in Chicago, in a statement. “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.”
The researchers recorded 10 continuous hours of GoPro camera footage, which showed the octopuses interacting with one another. The animals were frequently close to each other and were observed mating, being aggressive, and signaling through color changes.
“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.”
More research is needed to understand the significance of octopus settlements and interactions, says Chancellor, because so little is known about octopus behavior.