Common Octopus

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The octopus ( /ˈɒktəpʊs/) is a cephalopod mollusc of the order Octopoda. 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 mantle), 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, but only one group, the blue-ringed octopuses, is known to be deadly to humans.[3]

There are around 300 recognized octopus species, which is over one-third of the total number of known cephalopod species. The term octopus may also be used to refer only to those creatures in the genus Octopus.

Biology
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.[4] 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. A beak, similar in shape to a parrot's beak, is the only hard part of their body. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing frommorays or other predatory fish. The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces. These cirrate species are often free-swimming and live in deep-water habitats, while incirrate octopus species are found in reefs and other shallower seafloor habitats. Octopuses have a relatively short life expectancy, 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. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch. They neglect to eat during the (roughly) one month period spent taking care of their unhatched eggs, but they do not die of starvation. Endocrine secretions from the two optic glands are the cause of genetically programmed death (and if these glands are surgically removed, the octopus may live many months beyond reproduction, until she finally starves).[citation needed] Octopuses have three hearts. Two branchial hearts pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains thecopper-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 draw water into their mantle cavity where it passes through its gills. As mollusks, octopuses have gills that are finely divided and vascularized outgrowths of either the outer or the inner body surface.

Intelligence
Main article: Cephalopod intelligence Octopuses are highly intelligent, likely more so than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists,[5] [6] [7] [8] but maze and problem-solving experiments have shown that they show evidence of a memory system that can store both short- and long-term memory. It is not known precisely what contribution learning makes to adult octopus behavior. Young octopuses learn almost no behaviors from their parents, with whom they have very little contact. An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus'sneurons 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.[9] Unlike vertebrates, the complex motor skills of octopuses are not organized in their brain using an internal somatotopic map of its body, as is the motor system in vertebrates<sup class="reference" id="cite_ref-9" style="line-height: 1em; font-weight: normal; font-style: normal; ">[10] Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the shape and movements of other sea creatures.

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They have been reported to practice observational learning,<sup class="reference" id="cite_ref-10" style="line-height: 1em; font-weight: normal; font-style: normal; ">[11] although the validity of these findings is widely contested on a number of grounds.<sup class="reference" id="cite_ref-ham_4-1" style="line-height: 1em; font-weight: normal; font-style: normal; ">[5] <sup class="reference" id="cite_ref-doug_5-1" style="line-height: 1em; font-weight: normal; font-style: normal; ">[6] Octopuses have also been observed in what some have described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.<sup class="reference" id="cite_ref-11" style="line-height: 1em; font-weight: normal; font-style: normal; ">[12] Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing boats and opened holds to eat crabs.<sup class="reference" id="cite_ref-denizen_6-1" style="line-height: 1em; font-weight: normal; font-style: normal; ">[7]

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">In some countries, octopuses are on the list of experimental animals on which surgery may not be performed withoutanesthesia. In the UK, cephalopods such as octopuses are regarded as honorary vertebrates under the Animals (Scientific Procedures) Act 1986 and other cruelty to animals legislation, extending to them protections not normally afforded to invertebrates.<sup class="reference" id="cite_ref-12" style="line-height: 1em; font-weight: normal; font-style: normal; ">[13]

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">The octopus is the only invertebrate which has been shown to use tools. At least four specimens of the Veined Octopus (Amphioctopus marginatus) have been witnessed retrieving discarded coconut shells, manipulating them, and then reassembling them to use as shelter. This discovery was documented in the journal Current Biology and has also been caught on video.<sup class="reference" id="cite_ref-13" style="line-height: 1em; font-weight: normal; font-style: normal; ">[14] <sup class="reference" id="cite_ref-14" style="line-height: 1em; font-weight: normal; font-style: normal; ">[15]

Defense
<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">An octopus's main (primary) defense is to hide, either not to be seen at all, or not to be detected as an octopus.<sup class="reference" id="cite_ref-behaviour_15-0" style="line-height: 1em; font-weight: normal; font-style: normal; ">[16] 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 the use of ink sacs, camouflage, and autotomising limbs.

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">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 assharks. Ink clouds of some species might serve as pseudomorphs, or decoys that the predator attacks instead.<sup class="reference" id="cite_ref-16" style="line-height: 1em; font-weight: normal; font-style: normal; ">[17] <p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">An octopus's camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectiveness of the epidermis.Chromatophores contain yellow, orange, red, brown, or black pigments; most species have three of these colors, while some have two or four. Other color-changing cells are reflective iridophores, and leucophores (white).<sup class="reference" id="cite_ref-17" style="line-height: 1em; font-weight: normal; font-style: normal; ">[18] This color-changing ability can also be used to communicate with or warn other octopuses. The very venomous blue-ringed octopus becomes bright yellow with blue rings when it is provoked. Octopuses can use muscles in the skin to change the texture of their mantle to achieve a greater camouflage. In some species the mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises. However in some species skin anatomy is limited to relatively patternless shades of one color, and limited skin texture. It is thought that octopuses that are day-active and/or live in complex habitats such as coral reefs have evolved more complex skin than their nocturnal and/or sand-dwelling relatives.<sup class="reference" id="cite_ref-behaviour_15-1" style="line-height: 1em; font-weight: normal; font-style: normal; ">[16]

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">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.

<p style="margin-top: 0.4em; margin-right: 0px; margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;">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, sea snakes, and eels.<sup class="reference" id="cite_ref-18" style="line-height: 1em; font-weight: normal; font-style: normal; ">[19] <sup class="reference" id="cite_ref-19" style="line-height: 1em; font-weight: normal; font-style: normal; ">[20]