Is an octopus throwing silt at a neighbor, or just tidying up?

Some of them look aimed. Throws during an interaction hit other octopuses often, and they differed from ordinary throws in several ways at once. They were more vigorous, more likely to use silt instead of shells or algae, and more often paired with a uniform or dark body pattern. Some also left from a different point beneath the arms, and those connected most often.

Octopus tetricus at a site in Jervis Bay, Australia, does something odd. The animal gathers shells, silt or algae into its arms, holds the siphon under the arm web, and fires a jet of water that sends the material sailing off through the sea.

On its own that is unremarkable. A den needs clearing, and pushing debris away from the entrance is ordinary maintenance. The interesting question is whether any of the throws are addressed to somebody, since that would put an octopus on a very short list of animals using projectiles against their own kind.

Godfrey-Smith and colleagues filmed the site and split the throws by context. Those thrown during interactions with other octopuses frequently hit another animal. They were also more vigorous than the rest, more often made with silt rather than shells or algae, and more often accompanied by a uniform or dark body pattern.

A handful were released from a different position beneath the arms, and those were the ones that connected most often, which is about as close to aim as video can take you. The authors stop short of calling it intentional. Projectile use directed at members of the same population had been seen before only in some social mammals, so even a cautious reading of this is strange.