Octopus

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This article is about the order of cephalopod. For other uses, see Octopus (disambiguation).
Octopus
Temporal range: Early Pennsylvanian – Recent[1]
Octopus2.jpg
The common octopus, Octopus vulgaris.
Scientific classification e
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Superorder: Octopodiformes
Order: Octopoda
Leach, 1818[2]
Suborders
Synonyms
  • Octopoida
    Leach, 1817[3]

The octopus (/ˈɒktəpʊs/ or /ˈɒktəpəs/; plural: octopuses, octopi, or octopodes; see below) 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 mantles), allowing them to squeeze through tight places.[4] 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 octopus, is known to be deadly to humans.[5]

Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus Octopus.

Biology

Schematic lateral aspect of octopod features
A common octopus (Octopus vulgaris)

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.[6] 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 between underwater rocks, which is very helpful when they are fleeing from moray eels 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 giant pacific 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, eventually dying of starvation. In a scientific experiment, removal of both optic glands after spawning was found to result in cessation of broodiness, resumption of feeding, increased growth, and greatly extended lifespans.[7]

Grimpoteuthis discoveryi, a finned octopus of the suborder Cirrina

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. The octopus draws water into its mantle cavity, where it passes through its gills. As mollusks, their gills are finely divided and vascularized outgrowths of either the outer or the inner body surface.

Intelligence

Octopuses are highly intelligent, possibly more so than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists,[8][9][10][11] but maze and problem-solving experiments have shown 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'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.[12] 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.[13] Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the shape and movements of other sea creatures.

In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They have been reported to practice observational learning,[14] although the validity of these findings is widely contested on a number of grounds.[8][9] 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.[15] 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.[10]

In some countries, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia. In the UK, until 2013, the common octopus, (Octopus vulgaris), was the only invertebrate protected under the Animals (Scientific Procedures) Act 1986 extending to them protections not normally afforded to invertebrates.[16] In 2013, this legislation was extended to include all cephalopods.

Tool use

Amphioctopus marginatus travels with shells it has collected for protection

The octopus 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.[17][18][19]

Defense

Greater blue-ringed octopus (Hapalochlaena lunulata)

An octopus's main (primary) defense is to hide, either not to be seen at all, or through camouflage and mimicry not to be detected as an octopus.[20] 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.[21]

An octopus's camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectivity 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).[22] This color-changing ability can also be used to communicate with or warn other octopuses. The highly 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.[20]

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.[23]

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.[24][25]

Reproduction

When octopuses reproduce, the male uses a specialized arm called a hectocotylus to transfer spermatophores (packets of sperm) from the terminal organ of the reproductive tract (the cephalopod "penis") into the female's mantle cavity.[26] The hectocotylus in benthic octopuses is usually the third right arm. Males die within a few months of mating. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature. After they have been fertilized, the female lays about 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals).[citation needed]

Senses

Octopuses have keen eyesight. Like other cephalopods, they can distinguish the polarization of light. Color vision appears to vary from species to species, being present in O. aegina but absent in O. vulgaris.[27] 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.[citation needed]

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. The arms contain tension sensors so the octopus knows whether its arms are stretched out. However, it has a very poor proprioceptive sense. The tension receptors are not sufficient for the brain to determine the position of the octopus's body or arms. (It is not clear whether the octopus brain would be capable of processing the large amount of information that this would require; the flexibility of an octopus's arms is much greater than that of the limbs of vertebrates, which devote large areas of cerebral cortex to the processing of proprioceptive inputs.) As a result, the octopus does not possess stereognosis; that is, it does not form a mental image of the overall shape of the object it is handling. It can detect local texture variations, but cannot integrate the information into a larger picture.[28]

The neurological autonomy of the arms means the octopus has great difficulty learning about the detailed effects of its motions. The brain may issue a high-level command to the arms, but the nerve cords in the arms execute the details. There is no neurological path for the brain to receive proprioceptive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually, i.e. exteroception.[28]

Octopuses might use the statocyst (a sac-like structure containing a mineralised mass and sensitive hairs) to register sound. The common octopus can hear sounds between 400 Hz and 1000 Hz, and hears best at a frequency of 600 Hz.[29]

Locomotion

Video of an octopus in its natural habitat
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.[30]

They crawl by walking on their arms, usually on many at once, on both solid and soft surfaces, while supported in water. In 2005, some octopuses (Adopus aculeatus and Amphioctopus marginatus under current taxonomy) were found to walk on two arms, while at the same time resembling plant matter.[31] 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).[30] A study of this behavior conducted by the Weymouth Sea Life Centre led to the suggestion that the two rearmost appendages may be more accurately termed 'legs' rather than 'arms'.[32] Some species of octopuses can crawl out of the water for a short period, which they may do between tide pools while hunting crustaceans or gastropods or to escape predators.[33][34]

Octopuses swim by expelling a jet of water from a contractile mantle, and aiming it via a muscular siphon.

Diet

Bottom-dwelling octopuses eat mainly crabs, polychaete worms, and other molluscs such as whelks and clams. Open-ocean octopuses eat mainly prawns, fish and other cephalopods. They usually inject their prey with a paralysing saliva before dismembering it into small pieces with their beaks.[35] Octopuses feed on shelled molluscs either by using force, or by drilling a hole in the shell, injecting a secretion into the hole, and then extracting the soft body of the mollusc.[36]

Large octopuses have also been known to catch and kill some species of sharks.[37]

Size

See also: Cephalopod size

The giant Pacific octopus, Enteroctopus dofleini, is often cited as the largest octopus species. Adults usually weigh around 15 kg (33 lb), with an arm span of up to 4.3 m (14 ft).[38] The largest specimen of this species to be scientifically documented was an animal with a live mass of 71 kg (156.5 lb).[39] The alternative contender is the seven-arm octopus, Haliphron atlanticus, based on a 61 kg (134 lb) carcass estimated to have a live mass of 75 kg (165 lb).[40][41] However, a number of questionable size records would suggest E. dofleini is the largest of all octopus species by a considerable margin;[42] one such record is of a specimen weighing 272 kg (600 lb) and having an arm span of 9 m (30 ft).[43]

Etymology and pluralization

The term "octopus" is derived,[44][45] through scientific Latin, from ancient Greek ὀκτώπους ("eight-footed") < ὀκτώ ("eight") and πούς ("foot").[46] (Ancient Greek also had the form ὀκτάπους,[47] which would give "octapus" in English; cf. Modern Greek χταπόδι < οκταπόδι < οκταπόδιον < ὀκτάπους.)

The usual plural in English is "octopuses" (pronounced /ˈɒktəpʊsɪz/), but the Greek plural form "octopodes" (pronounced /ɒkˈtɒpədiːz/) is sometimes used, though less frequently than in the past.[48] The form "octopi", as if the word were a Latin second-declension noun, is generally considered incorrect,[44][48][49][50][51][52] but is in fact used, so that it is registered by the descriptivist Merriam-Webster 11th Collegiate Dictionary, which lists "octopuses" and "octopi", in that order, and Webster's New World College Dictionary, which lists plurals in the order: "octopuses", "octopi", and "octopodes". The Oxford English Dictionary (2008 Draft Revision)[53] also lists "octopuses", "octopi", and "octopodes", in that order, labelling "octopodes" as rare and noting that "octopi" derives from the misapprehension that octōpus is a second-declension Latin noun and stating that, if the word were native to Latin, it would be third declension octōpēs (plural: octōpedes) after the pattern of pēs ("foot", plural pedēs).[54] The New Oxford American Dictionary (3rd Edition 2010) lists only "octopuses" as being the acceptable pluralization, with a usage note indicating "octopodes" as being "still occasionally used", and "octopi" as being "incorrect".[55]

Related to the word "octopus" are the term "Octopoda" (the taxonomic order of cephalopod molluscs that comprises the octopuses) and the adjective "octopoid".[48]

Relationship to humans

Minoan clay vase showing an octopus, circa 1500 BCE

Ancient peoples of the Mediterranean were aware of the octopus, as evidenced by certain artworks and designs of prehistory. For example, a stone carving found in the archaeological recovery from Bronze Age Minoan Crete at Knossos (1900 – 1100 BC) has a depiction of a fisherman carrying an octopus.[56]

In classical Greece, Aristotle (384 BC – 322 BC) commented on the colour-changing abilities of the octopus, both for camouflage and for signalling, in his Historia animalium:[57]

The octopus ... seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed.

—Aristotle[57]

Octopuses were often depicted in the art of the Moche people of ancient Peru, who worshipped the sea and its animals.[58]

In mythology

The Gorgon of Greek mythology has been thought to have been inspired by the octopus or squid, the octopus itself representing the severed head of Medusa, the beak as the protruding tongue and fangs, and its tentacles as the snakes.[59]

The Kraken are legendary sea monsters of giant proportions said to dwell off the coasts of Norway and Greenland, usually portrayed in art as a giant octopus attacking ships.

The Hawaiian creation myth relates that the present cosmos is only the last of a series, having arisen in stages from the wreck of the previous universe. In this account, the octopus is the lone survivor of the previous, alien universe.[60]

Akkorokamui is a gigantic octopus-like monster from Ainu folklore, which supposedly lurks in Funka Bay in Hokkaidō and has been sighted in several locations including Taiwan and Korea since the 19th century.[61]

In literature

The octopus has a significant role in Victor Hugo's book Travailleurs de la mer (Toilers of the Sea).[62]

As a metaphor

Due to having numerous arms that emanate from a common center, the octopus is often used as a metaphor for a group or organization which is perceived as being powerful, manipulative or bent on domination. Use of this terminology is invariably negative and employed by the opponents of the groups or institutions so described.[63]

As food

Main article: Octopus (food)

Humans in many cultures eat octopus. The arms and sometimes other body parts are prepared in various ways, often varying by species or geography.

As pets

Though octopuses can be difficult to keep in captivity, some people keep them as pets. They often escape even from supposedly secure tanks, due to their problem-solving skills, mobility and lack of rigid structure.[citation needed]

The variation in size and lifespan among octopus species makes it difficult to know how long a new specimen can naturally be expected to live. That is, a small octopus may be just born or may be an adult, depending on its species. By selecting a well-known species, such as the California two-spot octopus, one can choose a small octopus (around the size of a tennis ball) and be confident it is young with a full life ahead of it.[citation needed]

Classification

See also

References

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External links