Using video footage from divers and camera traps, the researchers watched how the octopuses behaved and interacted. At least three pairs of neighbors mated along with a couple near-miss mating attempts. (Since octopuses are normally solitary creatures, their reproductive systems are tailored to long-distance fertilization— typically, males shoot a package of sperm at a female, which then burrows into the female’s skin and releases the sperm.)

Off the coast of Australia, marine biologists discovered a city of sorts grouped around some rocks and built from scavenged junk and discarded prey shells. Ten to 15 gloomy octopuses—the common name for Octopus tetricus —live there, which David Scheel, a professor at Alaska Pacific University, and his colleges call “Octlantis.” Octopuses—typically a solitary animal—built a small city using scavenged junk and discarded prey shells.

Octopuses are typically solitary animals—they goes about their days hiding in a den and emerging at night to hunt. But new research suggests they may not be as solitary as we once thought.

Not all the interactions were harmonious, though. Researchers also witnessed octopus evictions, where one octopus would yank another from a den and take its place. Sometimes this would lead to a fight, exposing the octopuses to predators like sharks. In fact, Scheel and his colleagues said that sharks would routinely circle above the city, waiting for their prey to emerge from their dens.

This isn’t the first the time researchers find a colony of octopuses living in close quarters. In 2009, divers off the coast of Australia found a small group living together in burrows around a piece of discarded metal and called it “Octopolis.”

But what might make these animals want to live together? Perhaps the same reason other species cluster into groups: limited shelter and an abundance of food. Here’s Annalee Newitz for ArsTechnica:

The rocky outcropping and metal debris at the cores of Octlantis and Octopolis are rare examples of shelter-ready regions on a generally flat ocean floor. Plus, the shell mounds of the cities create perfect habitats for scallops and other octopus food. It’s likely that octopuses gathered in these spots for the shelter and food, and their social behaviors evolved over time.

The existence of octopus cities such as “Octopolis” and “Octlantis” calls into question how much we actually know about how these underwater creatures live, especially given that gloomy octopuses also aren’t the only species of octopus to be observed living in clusters of dens. Perhaps octopus cities are more common than we knew.