The common named blue-ringed octopus comprises the genus Hapalochlaena, which includes many species. All of them share very similar characteristics: small size, pigmentation in the skin forming blue rings and a powerful poison.

The main geographic distribution of the blue-ringed octopus extends from Japan to Australia and the tropical area, although it changes depending on the species. These octopuses are small, 5-10 cm long, although some specimens can reach 15 cm. The average weight is between 25 and 80 grams. They feed primarily on small crabs and shrimp, but can eat even weak or dying fish larger than themselves.

Its name comes from its characteristic blue rings present on the skin, which can be more or less evident depending on whether it is hiding or if it stays alert. Blue-ringed octopuses use three structures present on their skin to change colour: chromatophores, iridophores and leucophores. Chromatophores are pigment-filled sacs that are controlled by the surrounding muscles. Flexing and contracting these muscles can expand or shrink the sacs, changing the overall appearance in milliseconds. Below these, there are iridophores, which generate iridescent colours, like the blue of the rings. And lastly, the leukophores are white reflectors that add brightness and contrast to the overall display.

The venom produced by these octopuses can contain up to 10 different compounds, notably tetrodotoxin, a neurotoxin that can also be found in pufferfish. Tetrodotoxin attacks the nervous system, causing muscular paralysis that, if not attended to, can cause death. There is no antidote for the poison, but there are assisted life support methods by which a person can be kept alive until the complete expulsion of the poison, after 24 hours. This toxin is created by bacteria symbiotic with the octopus that inhabit its salivary glands, it can also be found in pufferfish, tunicates and marine snails among others.