Which Australian octopus has bright blue rings that appear when it is threatened?

The blue-ringed octopus has bright blue rings that appear when it is threatened.

Blue-ringed octopuses are small cephalopods found in shallow coastal waters and rock pools around Australia and the western Pacific. Their rings are usually subdued while the animal is undisturbed, but become vivid blue when it displays a warning signal.

Their venom contains tetrodotoxin, a powerful neurotoxin that can cause paralysis, including paralysis of the muscles needed for breathing. There is no widely available specific antidote, so emergency treatment relies on immediate respiratory support until the toxin wears off.

The name covers several closely related species rather than one single species. Their small size and attractive appearance can make them seem harmless, but handling any blue-ringed octopus is extremely dangerous. They should be left alone in the water.