<tr><td align="center">
<tr><th bgcolor=pink>Scientific classification
<tr><td>
<tr><td>Kingdom:<td>Animalia
<tr><td>Phylum:<td>Mollusca
<tr><td>Class:<td>Cephalopoda
<tr><td>Subclass:<td>Coleoidea
<tr><td>Superorder:<td>Octopodiformes
<tr><td>Order:<td>Octopoda
</table>
<tr><th bgcolor=pink>Families
<tr><td>
14 in two suborders, see text
</table>
- For other meanings of "octopus", see Octopus (disambiguation).
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 almost entirely soft bodies; they have neither a protective outer shell like the nautilus, nor any internal shell or bones like cuttlefish or squids. A beak, similar in shape to a parrot's beak, is their only hard part. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predating fish.
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 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 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 their eggs hatch.
Octopuses are highly intelligent, by far the most intelligent known invertebrates, 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. An octopus can also learn how to unscrew the lid of a jar with its arms, and the octopus called Einstein at the British Blue Reef Aquarium could open a tin within seconds with two arms, opening it even faster if it was filled with food.
Octopuses may also understand the concept of mirrored images and soon realize there's no use attempting to attack their own images. This seems to suggest that octopuses have some concept of a self; otherwise only monkeys, apes and humans, and possibly some species of dolphins, are smart enough to understand that their mirrored images are not other animals. However, it may also suggest that since an octopus's sense of taste (via its suckers) is highly acute, it quickly realizes there is no real octopus through the mirror, regardless of what it sees.
Octopuses also share some emotions normally associated only with humans and some mammals, 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.
Though octopuses in captivity are rare and expensive, some people keep them as pets. Octopuses often escape, however, even from supposedly secure tanks, due to their intelligence and problem solving skills.
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, thus this behaviour is more correctly labeled as a mental disease.
Plural
A note on the plural form: 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. It is not. It is (Latinized) Greek, from oktopous (ὀκτώπους), gender masculine, whose plural is oktopodes (ὀκτώποδες). If the word were Latin, it would be octopes ('eight-foot') and the plural octopedes, analogous to centipedes and millipedes, as the plural form of pes ('foot') is pedes. In modern, informal Greek, it is called khtapodi (χταπόδι), gender neuter, with plural form khtapodia (χταπόδια). The form octopod (plural octopods) is taken from the collective plural Octopoda, the taxonomic order, but has no classical equivalent.
That said, Merriam-Webster and other dictionaries accept octopi as a plural form. The collective form octopus is also used, but is usually reserved for animals consumed for food. Finally, there is octopussy (and the James Bond movie by that name) which pluralizes to octopussies and which is a blend word of octopus and pussycat, sometimes used as a term of endearment for octopus, but it's completely erroneous.
Classification
References
de:Kraken
es:Pulpo
eo:Polpo
fr:Pieuvre
nl:Octopus
ja:タコ
pt:Polvo
