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. These arms are a type of muscular hydrostat. Unlike most other cephalopods, the majority of octopuses - those in the suborder most commonly known, Incirrina - have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an 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 predatory fish. The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, making them a little less flexible in being able to squeeze into small spaces.
Octopuses are highly intelligent, probably the most intelligent of any of the invertebrates, with their intelligence supposedly comparable to that of the average housecat. Maze and problem-solving experiments show that they have both short- and long-term memory, although their short lifespans limit the amount they can ultimately learn.
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. They also have specialized skin cells both for color changing (chromatophores) and light reflection and refraction (iridophores and leucophores). They use this ability to blend into the environment to hide, communicate with other octopuses, or warn. The very poisonous Blue-ringed Octopus becomes bright yellow with blue rings when it is provoked. When under attack, some octopuses can autotomise 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. A few species, such as the Mimic Octopus have a fourth defense mechanism. They can combine their highly flexible bodies with their color changing ability to accurately mimic other, more dangerous animals such as lionfish and eels.
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