The blue-ringed octopus is a small, 6-inch octopus that lives in tide pools in the Pacific, in places from Japan to Australia. It tends to blend into its environment until provoked, when it turns quickly into a bright yellow creature with blue rings. It hunts small crabs, but will bite an attacker, even a human if provoked or stepped on.

Its saliva, which can be instilled through a bite or it can be spit, contains the bacterial poison Tetrodotoxin, to which there is no known antidote. Tetrodotoxin can cause cardiac and respiratory arrest. First aid treatment is CPR, and hospital treatment involves respiratory assistance until the toxin is washed out of the body.

There are two species. Hapalochlaena lunulata is larger and rarer. Hapalochlaena maculosa is smaller. Both are equally venomous.