Scheel, et. al.
The first time that divers discovered a "city" of octopuses off the coast of Australia, it seemed like a fluke. Octopuses are notoriously solitary animals. Divers found a small group of them in 2009 living together in burrows built around a piece of discarded metal, and they called it "Octopolis." At the time, scientists believed it was a rarity, perhaps caused by human meddling in the environment. But, in 2016, divers found another community of octopuses living in dens built from discarded shells. And this time there was no hunk of metal that had disturbed the natural environment. Researchers now suspect octopuses have been building group habitats for a long time.
Alaska Pacific University marine biologist David Scheel and his colleagues described the new discovery in the journal Marine and Freshwater Behavior and Physiology. They call it "Octlantis," and, over several months of observation, they determined that the settlement is made up of roughly 10 to 15 gloomy octopuses (yes, that is actually the common name for Octopus tetricus). Octlantis is hardly a metropolis, though—it's more like a tiny village of dens clustered around rocks, built up over several generations. These octopuses only live for about three years, so each generation is relatively short. But they leave behind mounds of discarded shells from their prey, as well as junk they've scavenged, like beer bottles and lead fishing lures. Over the years, octopuses pushed these mounds against the rocks, burrowed inside, and created dens next to each other.