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Scientific classification
14 in two suborders, see text
The octopus is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are 289 different octopus species, which is over one-third the total number of cephalopod species.

Octopuses are characterized by their eight arms, usually with sucker cups on them. Unlike most other cephalopods, octopuses have entirely soft bodies; they have neither a protective outer shell like the nautili, nor an internal shell or bone like cuttlefish or squids.

Three defensive mechanisms are typical of octopuses: ink sacs, camouflage and autonomising limbs. Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators. They also have specialized color changing skin cells called chromatophores which they can use to blend into the environment to hide. They can also use this ability as a warning; the very poisonous Blue-ringed Octopus becomes bright yellow with blue rings when it is provoked. When under attack, octopuses can also detach and autonomise their limbs, in a similar manner to skinks and other lizards. The crawling arm serves as a distraction to would-be predators; this ability is also used in mating.

They have a very 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 if they do not reproduce. However reproduction is a cause of death. Males can only live for a few months after mating, whereas females die shortly after hatching their eggs.

Octopuses are highly intelligent, and are able to learn how to distinguish the difference between colors and shapes. More impressive is that they can remember the shapes and colors and their meanings for up to two years. They can also learn how to unscrew the lid of a jar with its tentacles, and the octopus called Einstein at the british Blue Reef Aquarium could open a tin within seconds with two tentacles, opening it even faster if it was filled with food. They also understand the concept of mirrored images and soon realize there's no use attempting to attack its own image. Octopuses also share some emotions normally associated with humans, such as embarassment, trust, and a great curiosity. In many countries, including the United States and the United Kingdom, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia.

In some cultures, octopuses are caught for food.

A common belief is that when stressed, an octopus may begin to eat itself. However, limited research conducted in this area has revealed that the cause of this abnormal behaviour may be due to a virus that attacks the octopus's nervous system.

Table of contents
1 Plural
2 Classification
3 References


A note on the plural: Fowler states that "the only acceptable plural in English is octopuses", and that octopi is misconceived and octopodes pedantic. Octopi derives from the mistaken notion that octopus is Latin. But it isn't; it is Greek, from oktopous, whose plural is oktopodes. If the word were Latin, it would be octoped and the plural octopedes, analogous to centipedes and millipedes.

That said, Merriam-Webster now accepts octopi as a plural form. The collective form octopus is also used, but is usually reserved for animals consumed for food.