Written by Bronwyn Allan
The name ‘blue-ringed octopus' does not actually refer to a single species, but rather a genus of species, all with the circular, iridescent blue markings for which they are named. These markings are certainly very beautiful and striking to look at, however they are only displayed when the octopus is about to dispense its deadly poison, so stick to looking at them in photographs!
Size 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.
Blue-ringed octopuses belong to the Phylum Mollusca (the molluscs) which includes snails, slugs and bivalves. They are part of the Class Cephalopodea, a distinctive group of animals so named because their limbs are attached to their head. The name cephalopod comes from the Greek words kephalos for head and podos for foot. Within the cephalopods they belong to Subclass Coleoidea, which includes cuttlefish, squid and octopuses. All octopuses belong to the Order Octopoda, while the blue-ringed octopuses can be found within the Family Octopodidae. They make up the genus Hapalochlaena .
The blue-ringed octopuses range from the Sea of Japan down to the waters of southern Australia; across from the Philippines to Vanuatu. They inhabit depths from intertidal flats down to 50m. They tend to hide in crevices or under rocks during the day, and emerge at night.
One of the limbs of the male octopus, usually the third right arm, is modified for the purpose of mating. This arm has a groove embedded into it and the end is shaped like a spoon. During mating, males insert the spoon-like structure into the oviduct of the female. They then place spermatophores (little "packets" of sperm) into the groove on this arm. These then slide down the arm and into the oviduct of the female. She does not conceive immediately, but rather keeps the sperm until she is ready to lay her eggs. The female takes care of the eggs until they hatch; females usually die once this job is complete.
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.
- Norman, M (2000)Cephalopods: A World GuideHackenheim, ConchBooks