The octopus (タコ, Tako) is a cephalopod. Octopuses have two eyes and four pairs of arms and, like other cephalopods, they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms. Octopuses have no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing them to squeeze through tight places. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates.
The octopus inhabits many diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous.
Octopuses are characterized by their eight arms, usually bearing suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish. Both types of limbs are muscular hydrostats. 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 squid. The beak, similar in shape to a parrot's beak, and made of chitin, is the only hard part of their bodies. This enables them to squeeze through very narrow slits, which is very helpful when they are fleeing from predators. The octopuses in the less-familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces.
Octopuses have three hearts. Two branchial hearts pump blood through each of the two gills, while the third is a systemic heart that pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Although less efficient under normal conditions than the iron-rich hemoglobin of vertebrates, in cold conditions with low oxygen pressure, hemocyanin oxygen transportation is more efficient than hemoglobin oxygen transportation. The hemocyanin is dissolved in the plasma instead of being carried within red blood cells, and gives the blood a bluish color.
Octopuses are highly intelligent. 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 limited functional autonomy. Octopus arms show a variety of complex reflex actions that persist even when they have no input from the brain. Unlike vertebrates, the complex motor skills of octopuses are not organized in their brain using an internal somatotopic map of its body, instead using a nonsomatotopic system unique to large-brained invertebrates. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the shape and movements of other sea creatures.
An octopus's main and primary defense is to hide, either not to be seen at all, or through camouflage and mimicry not to be detected as an octopus. Octopuses have several secondary defenses; defenses they use once they have been seen by a predator. The most common secondary defense is fast escape. Other defenses include distraction with the use of ink sacs and autotomising limbs.
Most octopuses can eject a thick, blackish ink in a large cloud to aid in escaping from predators. The main coloring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin color. This ink cloud is thought to reduce the efficiency of olfactory organs, which would aid an octopus's evasion from predators that employ smell for hunting, such as sharks. Ink clouds of some species might serve as pseudomorphs, or decoys that the predator attacks instead.
When under attack, some octopuses can perform arm autotomy, in a similar manner to the way skinks and other lizards detach their tails. The crawling arm serves as a distraction to would-be predators. Such severed arms remain sensitive to stimuli and move away from unpleasant sensations. 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.
Octopuses have keen eyesight. 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 the pupil slit is always horizontal. Octopuses also have an excellent sense of touch. An octopus's suction cups are equipped with chemoreceptors so the octopus can taste what it is touching. Octopuses swim headfirst, with arms trailing behind. Octopuses move about by crawling or swimming. Their main means of slow travel is crawling, with some swimming. Jet propulsion is their fastest means of locomotion, followed by swimming and walking.
They crawl by walking on their arms, usually on many at once, on both solid and soft surfaces, while supported in water. Some octopuses can walk on two arms, while at the same time resembling plant matter. This form of locomotion allows these octopuses to move quickly away from a potential predator while possibly not triggering that predator's search image for octopus (food). A study of this behavior led to the suggestion that the two rearmost appendages may be more accurately termed 'legs' rather than 'arms'. Some species of octopuses can crawl out of the water for a short period, which they may do between tide pools while hunting. Octopuses swim by expelling a jet of water from a contractile mantle and aiming it via a muscular siphon.
The greater octopuses mainly gather in a region known as Myōkuroidōkutsu (妙黒洞窟, lit. Mysterious Dark Grotto), which is a great underwater cave that is incredibly difficult to locate. It is the natural inclination of octopuses to hide, and they have always done so, never creating a summoning contract with anyone or revealing the location of their gathering place. This pattern was only ever broken when Yakan was once attacked out at sea and left to drown. While sinking he discovered an octopus nearby him that was trying to hide. Realizing it had been spotted it began to flee, but it saw a small display of Tasogare's Kokuena technique and mistook it for ink. Feeling inclined to help, the octopus rescued Tasogare by bringing him to Myōkuroidōkutsu.
Known Octopuses (Octopi or Octopodes)
Niseru (似せる, lit. Mimic) is an Indonesian Mimic octopus. While he is neither the eldest nor the largest of the octopuses of Myōkuroidōkutsu, he is regarded as their leader, if not their god. He is the most skilled amongst their species and regarded as the wisest amongst their kind. He is the near-constant companion of Yakan, often either wrapped around his waist like a belt or attached to his back, allowing him to offer his tentacles as extra limbs for Tasogare. The pair also frequently uses the Sage Art: Octopus Man Technique.
Kurāken (クラーケン, lit. Kraken) is the regarded as the king of Myōkuroidōkutsu and is the largest of all the octopuses; a giant Pacific octopus.
Karyubudisu (カリュブディス, lit. Charybdis) is the queen of Myōkuroidōkutsu and the third largest of the octopuses.
Yorumungandu (ヨルムンガンド, lit. Jörmungandr) is the son of Kurāken and Karyubudisu and the prince of Myōkuroidōkutsu. He is the second largest of all the octopuses.
- Yakan is the summoning contract holder for the octopuses of Myōkuroidōkutsu.