Some octopuses intimidate their neighbors by turning black, standing tall and looming over them threateningly, like an eight-armed Dracula.
That's according to a study published Thursday that helps show that octopuses aren't loners, contrary to what scientists long thought; some of the invertebrates have an exciting social life.
"There can be over a dozen octopuses or more at this site," says David Scheel of Alaska Pacific University. "Generally, during the Australian summer there are more and we see a lot of activity then."
A local diver, Matthew Lawrence, first noticed there was a lot of octopus interaction going on there. His observations piqued the interest of Scheel, who is a marine biologist, and Peter Godfrey-Smith, a philosopher who had been thinking about octopus consciousness.
The research team eventually recorded 52 hours of underwater video, showing 186 octopus interactions.
"I took a look fairly early on at one sequence in which one octopus approaches another in a fairly menacing way," recalls Scheel. "He gets all dark, stands up very tall, and the other octopus crouches down and turns very pale. And then, when the approaching octopus persists, the other one flees. And this is immediately followed by the first octopus approaching a third octopus that's nearby. And the third octopus turns dark and doesn't crouch down. He just stays where he is, holds his ground."
It looked like they were signaling to each other, says Scheel. That was surprising, because the changing color patterns on an octopus's body are generally just associated with camouflage from predators. As the researchers watched more video, they became convinced.
"The dark color and some of the behaviors that go with it are associated with aggression, or at least approach," Scheel says. "The paler colors signify that the octopus is not going to stand its ground — that it's going to retreat or withdraw."
An aggressive octopus would stretch out the web of its tentacles very wide, to look as big as possible. "And, of course, it's got these scalloped edges between each arm," says Scheel, adding that the octopus would also stand very tall and turn black.
"It looked to me, for all the world, like Dracula approaching his prey," he says. "In my early notes I was calling this the Nosferatu display."
Sometimes an octopus will even do this while standing on the highest available ground, he adds — a piece of junk that's sticking up out of the seafloor.
Until about 15 years ago, scientists believed that octopuses were pretty much asocial.
"When they interacted, they either mated or ate each other," says Crissy Huffard, a senior research technician at the Monterey Bay Aquarium Research Institute in California. "That was the overriding idea."
But Huffard has done research on another octopus species that shows males display a black-and-white striped pattern when they're in the presence of another individual. "And that tends to send the signal, 'I'm male,' " she says. If the other octopus displays a similar body pattern, the male will be aggressive and fight. If not, then he'll try to mate.
She was interested in the threatening body posture that researchers observed in Australia.
"That's very cool to see," she says. "Octopuses are probably not as completely asocial as originally assumed. Their communication system reflects the fact that they're interacting on a fairly regular basis."