Octopuses are eight-legged wriggly creatures that live in the sea. They belong to the phylum (group) of animals known as molluscs - this includes a lot of the muscular slimy animals such as slugs, snails, mussels and oysters. Octopuses belong to the class (sub-group) of cephalopods, which also includes squids, cuttlefish and the very rare nautilus.

Octopuses are probably the most intelligent creatures outside of the vertebrate phylum to which we humans belong (which includes fish, amphibians, reptiles, birds and mammals). They are said by some to be as intelligent as dogs.

They also have the remarkable ability to change colour instantly in order to blend in with their surroundings, effectively becoming invisible, or to broadcast messages by flashing coloured signals.

Description

There are a large number of different species of octopus, in a whole range of different sizes, but they are all basically the same shape. They have a rounded body/head known as the mantle. This has a mouth at one end and eyes on either side. Around the mouth are eight muscular arms. Octopuses have no bones or shell, so their arms are completely flexible. They each have a double row of suckers which can be used for gripping. The arms are used to grab things and to walk along the bottom of the sea. Between the base of the arms is a flexible skin known as a web, similar to the wing of a bat or the webbed feet of a waterbird. The octopus can suck water into the web and then expel it quickly by contracting it. This produces a jet of water which can propel the octopus quickly away from danger. Octopuses travel head first with their arms trailing behind them.

Octopuses breathe by extracting oxygen from the water using gills. They have two internal gills. Water is sucked into the gills and blown out in a jet of water through a single tube known as the 'siphon' or 'funnel'. The octopus uses this water jet to propel itself through the sea.

Octopuses can survive for short periods out of water, as long as their gills stay wet. In experimental laboratories, they've been known to climb out of their tanks at night and eat the shellfish from other tanks.

Eyes and Colour

The octopus's eye is an amazing organ, because it has evolved completely independently from the human eye, and yet has a rather similar structure to it. The ancestors of octopuses branched off from the ancestors of vertebrates (from where humans stem) more than 500 million years ago - long before either group had complex eyes. The octopus's eye has a cornea, iris, lens, vitreous humour and retina like our own. Biologists have long held this as an example of how evolution can come up with a particular design more than once because it is the most practical. There are subtle differences, however. The octopus lens is fixed in shape, but moves closer to and further from the retina in order to focus, whereas in our eyes, the lens distance is fixed but the shape of the lens varies. Octopuses have excellent colour vision.

Along with good eyesight, octopuses have a remarkable ability to change the colour of their skin. The basic octopus skin is a neutral grey/white colour. In the skin are thousands of tiny colour organs called chromatophores. These consist of tiny sacs filled with coloured dye. There are red, blue, yellow, brown and black sacs, in different mixtures depending on the species. Muscles around the chromatophore can expand or contract, making the sac appear or disappear. The upshot is that the octopus can consciously control the colour of each part of its skin.

One use of the colour change is for camouflage. Octopuses are virtually invisible when they are hiding, as they match the colour and pattern of their background exactly. They can also adopt striking colours and patterns as warnings or to communicate with other octopuses. Bright red all over means angry, white means frightened, yellow with blue rings means 'I'm poisonous, keep away' and so on. The octopus has such control over its skin that it can put on moving displays of colour - such as rings of white moving up the arms against a background of red.

Follow this link to see a video of a camouflaged octopus suddenly deciding that the camera is getting too close and reverting to its normal colour.

Name and Terminology

The name octopus comes from the Ancient Greek octopous meaning 'eight legs'. The plural of the Greek word is octopodes, but there is no real reason to use this word in English; it's much easier to stick to the normal rules of the language and say 'octopuses'. Don't say 'octopi' - if you want to be pedantic, go the whole hog and insist on 'octopodes'. Interestingly, in modern Greek, the 'c' and 't' have been softened and the initial 'o' dropped, making the almost unpronounceable chthopodhi.

Modern science calls the limbs 'arms' rather than legs, as they are used for gripping as well as standing on. The octopus has eight of them. They are not tentacles; that word is reserved for a different type of limb on a squid (the squid has eight arms and two tentacles).

Internal Details

An octopus has quite a large brain as molluscs go, but its nervous system is very decentralised compared with ours. The brain only contains one third of the nerves. The other two-thirds of the nerves are spread through the body and arms, so that the arms are semi-intelligent and will continue to crawl along and change colour for a while if they are accidentally cut off.

An octopus can regrow an arm if it loses one, but the arm will not regrow an octopus; it will walk around for a bit and then die.

Octopuses (and other molluscs) have blue blood. It's a sign of just how distant they are from us in the tree of life; their blood has evolved completely separately. The chemistry is completely different, being based on copper rather than iron. This gives it a blue colour when it is charged with oxygen, rather than a red one.

To pump the blood around, the octopus has three hearts. One in front of each gill pushes the blood through the gill. The third heart takes the oxygen-enriched blood as it leaves the two gills and pumps it around the body.

Octopuses have no discernible hearing organs of any sort and do not appear to use sound for communication.

Behaviour

Octopuses hunt, usually at night. They live in lairs, which are any sort of a hole in the sea bed that they can find. They come out of their lair and catch crabs and shellfish, which they kill by biting with their bony beak and injecting poison and meat tenderiser into them. They then drag them back to their lair, where they suck out the meat and drop the shells outside. The easiest way to find an octopus lair is to look for these piles of shells, or 'middens' as they are known.

If an octopus is threatened, it will sometimes turn itself black. It will then suddenly release a blob of black ink which hangs in the water looking much like the body of the octopus. At the same time, the octopus will switch from black to white and will hurry away. The confused predator will mistake the blob of ink for the octopus and not notice the white creature leaving.

Octopuses are curious and intelligent, as befits their role as hunters. Experiments have shown that they can learn how to do simple tasks (such as taking a cork out of a bottle to reach food inside) by watching other octopuses doing it. They can recognise differences between geometrical shapes and remember them for long periods.

Species of Octopus

There are many different species of octopus. Only four will be mentioned here:

Common Octopus Octopus vulgaris

The most common octopus is predictably enough known as the Common Octopus or by its scientific name Octopus vulgaris. About 90cm long when its legs are fully extended, it lives just about everywhere in coastal regions of warm and tropical seas. It is comfortable down to a depth of about 200m, so it won't live where sea is deeper than that.

Giant Pacific Octopus Enteroctopus dofleini

This is the biggest octopus. Despite mythical accounts of ships being attacked by giant octopuses, this creature is not even strong enough to overcome a man. An adult male is typically 10 - 15kg in weight, but they have been known to grow to as much as 270kg, with an 'armspan' of more than nine metres.

The giant octopus prefers cold water and can live at depths of down to 750m.

Blue-Ringed Octopus Hapalochlaena lunulata

The most dangerous octopus is not a giant, but a tiny one that lives in the rock pools of Australia. All octopuses have a poisonous bite, but only the blue-ringed octopus is harmful to humans. This little creature is only a few inches across. It is usually brown or yellow in colour, but will switch to its warning coloration of bright yellow with blue rings if it thinks it is in danger. If it is attacked/poked/stood on, it will bite; the bite apparently doesn't hurt very much and may not even be noticed, but the poison can stop a person's heart and breathing within about two minutes and the victim will almost inevitably die¹.

Blanket Octopus Tremoctopus violaceus Chiaie

This rare and little-studied octopus lives far from land in the western Pacific Ocean. Females are large, growing up to 1.6m across (more than five feet). They have an unusual blanket-like appearance.

Young blanket octopuses have an interesting defensive strategy: they pull poisonous tentacles (to which they are immune) off the Portuguese Man-Of-War² and wave them at predators.

The most unusual thing about the blanket octopus is that the males are tiny. A typical male is only 2.4cm in size and can weigh as little as one 40,000th of the weight of a female. It is not known why the males are so different in size from the females.

The Octopus Fishing Industry

Octopus fishing is traditional in the Mediterranean Sea. In the past, clay pots were lowered on strings onto the sea floor and left overnight. Octopuses are constantly on the lookout for newer and better holes to hide in, so they would climb into the pots. In the morning, the string would be hauled up with the pots full of octopus. A good fisherman could kill the octopus on the spot by biting it in a particular place which severed a nerve going into the brain.

The modern-day octopus fishing industry is very similar, but plastic pots are used instead. Between 20,000 and 100,000 tonnes of common octopus are caught every year throughout the world.

Life Cycle and Reproduction

Most octopuses are short-lived; common octopuses live 12 to 18 months. Even the long-lived Giant Pacific Octopus rarely lives more than a few years. They hatch from eggs and live part of their lives as tiny free-floating plankton. Lots of predators eat the baby octopuses, but the few that survive get big enough that they can settle into a bottom-dwelling lifestyle.

Octopuses only mate once during their life. The male may grasp the female or sit beside her. After much signalling using colour changes to make sure that both are ready for the once-in-a-lifetime event, he uses a special organ on one of his arms to transfer a packet of sperm to the female's mantle cavity.

A few weeks after mating, the males deteriorate and die. This appears to be chemically linked to the mating process. Females lay thousands of eggs, then sit guarding them. (The female common octopus can lay up to half a million eggs). They don't eat at all once their eggs are laid and they die after a few weeks, by which time the eggs are on the point of hatching.

The reproductive strategy of putting 'all your eggs in the one basket' seems strange to us, but it means that the female octopus can devote all of her body's resources to the production of eggs, thus maximising the number of eggs that will survive. This strategy has served the octopus well: they have roamed the seas for more than 200 million years.

Tell me More

Try Dr James B Wood's Cephalopod Page.