Octopuses, once thought to be loners who lived and hunted with little interaction from others of their kind, are actually quite social.
Scientists have discovered a village of sorts used by more than 50 octopuses on the sea floor about 17 metres deep in Jervis Bay south of Sydney.
The site is a flat area formed around an unidentified, partially buried artifact which provides places to hide, or dens, for octopuses.
Remains of scallops eaten by octopuses have accumulated as an extended midden, forming a shell bed of rough oval shape around three meters along its longest diameter.
They like living near each other and regularly interact with their neighbours, using their colour changing abilities to communicate, and sometimes get into brawls.
This footage from the University of Sydney documents octopuses fighting:
The researchers watched more than 52 hours of underwater film footage of one small area, witnessing 186 octopus interactions and more than 500 actions.
“There’s a lot of pushing other animals around, kicking them out of the site, and sometimes vigorous fights,” says Peter Godfrey-Smith, a professor from the Faculty of Science at the University of Sydney.
“We showed when octopuses change colour they are signalling their degree of aggression. Darker colours go with aggressive behaviours, and these are combined with other displays.”
Here are some examples:
The researchers were tipped off about the octopus site by a diver who alerted an online community of people interested in cephalopods that he had seen something interesting.
Researcher David Scheel, a professor at Alaska Pacific University, says octopuses use body patterns and postures to signal to each other during disputes.
“The postures and patterns can be quite flashy, such as standing very tall, raising the body mantle high above the eyes, and turning very dark,” he says.
And when an octopus with a dark body colour approached another dark octopus, the interaction was more likely to escalate to grappling.
“Dark colour appears to be associated with aggression, while paler colours accompany retreat,” says Scheel.
Octopuses also displayed on high ground, standing with their web spread and their mantle elevated. The researchers suspect the octopuses’ believe this makes them appear larger and more conspicuous.
The findings, the first to document the systematic use of signals during agonistic interactions among octopuses, are published in the journal Current Biology.