A carnivorous marine mollusc with a short, sac-like body and eight arms largely connected by webbing; reaches 5·4 m/18 ft in length, and with a maximum outstretched arm-span of nearly 9 m/30 ft; shell usually absent; contains a funnel, used for swimming by jet propulsion; prey caught using suckers on arms; when alarmed, may eject a cloud of ink. (Class: Cephalopoda. Order: Octopoda.)
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The octopus (Greek Ὀκτάπους, 'eight-legs') is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. In the larger sense, there are 289 different octopus species, which is over one-third of the total number of cephalopod species.
Physiology
Octopuses are characterized by their eight arms (not tentacles), usually bearing suction cups.
Octopuses have a relatively short life span, and some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years under suitable circumstances.
Octopuses have three hearts. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen.
Intelligence
Octopuses are highly intelligent, probably more intelligent than any other order of invertebrates. There has been much speculation to the effect that almost all octopus behaviors are independently learned rather than instinct-based, and although this is undoubtedly an interesting idea it remains largely unproven.
An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus's neurons are found in the nerve cords of its arms, which have a remarkable amount of autonomy. Octopus arms show a wide variety of complex reflex actions arising on at least three different levels of the nervous system. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the movements of other sea creatures.
In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns.
In some countries, including the United Kingdom, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia.
A common belief is that when stressed, an octopus may begin to eat its own arms. However, limited research conducted in this area has revealed that the cause of this abnormal behaviour, called autophagy, may be a virus that attacks the octopus's nervous system.
Defense
Three defensive mechanisms are typical of octopuses: ink sacs, camouflage, and autotomising limbs.
Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators.
An octopus's camouflage is aided by specialized skin cells which can change the apparent color, opacity, and reflectiveness of the epidermis.
When under attack, some octopuses can detach their own limbs, in a similar manner to the way skinks and other lizards detach their tails.
A few species, such as the Mimic Octopus, have a fourth defense mechanism.
Reproduction
When octopuses reproduce, males use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature.
Sensation
Octopuses have keen eyesight. Although their slit-shaped pupils might be expected to afflict them with astigmatism, it appears that this is not a problem in the light levels in which an octopus typically hunts. Attached to the brain are two special organs, called statocysts, that allow the octopus to sense the orientation of its body relative to horizontal. An autonomic response keeps the octopus's eyes oriented so that the pupil slit is always horizontal.
Octopuses also have an excellent sense of touch. The octopus's suckers are equipped with chemoreceptors so that the octopus can taste what it is touching. The arms contain tension sensors so that the octopus knows whether its arms are stretched out. However, the octopus has a very poor proprioceptive sense. The tension receptors are not sufficient for the octopus brain to determine the position of the octopus's body or arms. (It is not clear that the octopus brain would be capable of processing the large amount of information that this would require; the flexibility of an octopus's arms is much greater than that of the limbs of vertebrates, which devote large areas of cerebral cortex to the processing of proprioceptive inputs.) As a result, the octopus does not possess stereognosis;
The neurological autonomy of the arms means that the octopus has great difficulty learning about the detailed effects of its motions.
Locomotion
Octopuses move about by crawling or swimming.
They crawl by walking on their arms, usually on many at once, on solid surfaces, while supported in water.
Terminology
Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses", and that octopi is misconceived and octopodes pedantic.
Merriam-Webster and other dictionaries accept octopi as a plural form.
Finally worth mentioning is Octopussy, a play on words which found its way back from the movie title to a term of endearment for the animals that originally inspired it.
Relationship to humans
Humans usually capture octopuses as food, however some are kept in captivity at public aquaria and in private establishments as pets.
As food
Wikibooks Cookbook has more about this subject: OctopusMany species of octopus are eaten as food by human cultures around the world.
Octopus is a common ingredient in sushi.
As pets
Though octopuses can be difficult to keep in captivity, some people keep them as pets. The variation in size and life span among octopus species makes it difficult to know how long a new specimen can naturally be expected to live. That is, a small octopus may be just born or may be an adult, depending on the species. By selecting a well-known species, such as the California Two-spot Octopus, one can choose a small octopus (around the size of a tennis ball) and be confident that it is young with a full life ahead of it.
Octopuses are also quite strong for their size.
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