The blue ringed octopus can be found on shallow reefs, in coral rock pools and in tidal pools ranging in depth from 0 - 20 m, from Australia to Japan .
The blue ringed octopus may be found along the coast of Australia and some Pacific Islands in very small places including in shallow coral and rock pools, under rocks, in cracks and crevices on the reef, in tidal pools, inside shells, and in discarded bottles, cans and other detritus on the sea floor also in sandy or muddy stretches of the sea bottom where seaweed is plentiful.
The opening to the blue ringed octopus "den" is usually littered with rocks, shells and hollowed out crustacean shells placed there by the octopus to obscure the entrance.
differs between species, but they range from four to six centimetres long, with arms reaching lengths of seven to 10 centimetres. The group is named for the iridescent blue markings that dot their bodies; however these are usually only seen when the octopus feels threatened and is about to attack. This change in colour is due to pigment cells known as chromatophores. They, along with all other octopuses, have eight arms which are attached around their mouth. These arms have rows of broad, muscular suckers.
The brain of an octopus is shaped like a donut, and is centred around their oesophagus. They have two very well-developed eyes that are similar to those possessed by vertebrates. Octopuses have three hearts, with a central heart and one over each gill. These gills in turn are suspended in a cavity under the body. Seawater enters the octopus through this cavity, due to the pumping action of the mantle, a muscular bag-like structure within which is stored the organs of the octopus. The mantle is not responsible for disposing of the seawater from the body however, rather the water is ejected through a funnel, which can be aimed in different directions. The propulsion of water from this funnel allows the octopus to move rapidly in escape. The funnel can also shoot out ink in some blue-ringed octopuses, which comes from a gland located in the liver.
Although molluscs in general are known for their shells, in the octopuses this shell has been greatly reduced through evolution, and now exists only as two small rods. Another distinctive feature of the octopuses is the colour of their blood: transparent blue. This is due to the respiratory pigment of the octopuses being based on a copper atom; the respiratory pigment of a human is based on an iron atom, which makes our blood red.
The blue-ringed octopus has a nasty surprise for any potential prey or predators. Within its salivary glands live bacteria, which produce the chemical tetrodotoxin. This is a strong, fast-acting toxin that paralyses the target by blocking the nerves from transmitting messages. This toxin can be fatal; it has known to have caused the deaths of at least three people: two in Australia and one in Singapore. Many more people have come close to death as a result of the bite of the blue-ringed octopus. The paralysis that overcomes the victim is only to their voluntary muscles; they remain fully conscious. Death usually occurs as a result of lack of oxygen. Thus, if mouth to mouth resuscitation is given to a victim of a blue-ringed octopus, they should fully recover. The good news for swimmers in the waters where blue-ringed octopuses are found, is that they are retiring creatures and will only bite if they are being harassed and poked.