octopus at the Jervis Bay
Peter Godfrey-Smith

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Scientists discover an underwater city full of gloomy octopuses

Octopuses are reclusive animals, and the gloomy octopus (Octopus tetricus) is no exception. During the day, it retreats into its den in the rocky reefs of Australia, which it often blocks with rocks. It comes out most often at night, to catch lobsters, crabs, and other creatures with its meter-long arms. But in 2012, researchers reported that the species is surprisingly social. Diving in Jervis Bay, Australia, the scientists documented as many as 16 gloomy octopuses all living in a large pile of discarded shells—dubbed Octopolis—mating and fighting, even during the daytime. Now, they have studied a second congregation a few hundred meters away—the slightly less mellifluous Octlantis. The structure, which hosts 23 dens, consists of three patches of rocks, each surrounded by mounds of shells. Video surveillance over 8 days revealed up to 15 octopuses that mated and fought there, the researchers report this month in Marine and Freshwater Behaviour and Physiology. The octopuses kicked each other out of dens, went on chases, and appeared to threaten each other by standing up and darkening their bodies. At one point, they didn't even seem to notice a bottom-dwelling wobbegong shark sneaking up. The researchers suspect that limited den space might have led to the crowded conditions—and the dust-ups.