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This article is about the order of cephalopod. For other uses, seeOctopus (disambiguation).

Octopus
Temporal range:Middle Jurassic– recent
Common octopus on seabed
Common octopus
(Octopus vulgaris)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
(unranked): Neocoleoidea
Clade: Vampyropoda
Superorder: Octopodiformes
Order: Octopoda
Leach,1818[1]
Suborders

(traditional)

See§ Evolutionfor families

Synonyms
  • Octopoida
    Leach, 1817[2]

Anoctopus(pl.:octopusesoroctopodes[a]) is a soft-bodied, eight-limbedmolluscof theorderOctopoda(/ɒkˈtɒpədə/,ok-TOP-ə-də[3]). The order consists of some 300speciesand is grouped within the classCephalopodawithsquids,cuttlefish,andnautiloids.Like other cephalopods, an octopus isbilaterally symmetricwith two eyes and abeakedmouth at the center point of the eight limbs.[b]The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. Thesiphonis used both forrespirationand forlocomotion,by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of allinvertebrates.

Octopuses inhabit various regions of theocean,includingcoral reefs,pelagicwaters, and theseabed;some live in theintertidal zoneand others atabyssal depths.Most species grow quickly, mature early, and are short-lived. In most species, the male uses a specially adapted arm to deliver a bundle of sperm directly into the female's mantle cavity, after which he becomessenescentand dies, while the female deposits fertilised eggs in a den and cares for them until they hatch, after which she also dies. Strategies to defend themselves against predators include the expulsion ofink,the use ofcamouflageandthreat displays,the ability to jet quickly through the water and hide, and even deceit. All octopuses arevenomous,but only theblue-ringed octopusesare known to be deadly to humans.

Octopuses appear inmythologyas sea monsters like theKrakenof Norway and theAkkorokamuiof theAinu,and possibly theGorgonofancient Greece.A battle with an octopus appears inVictor Hugo's bookToilers of the Sea,inspiring other works such asIan Fleming'sOctopussy.Octopuses appear in Japanese erotic art,shunga.They are eaten and considered a delicacy by humans in many parts of the world, especially theMediterraneanand the Asian seas.

Etymology and pluralisation

See also:Plural form of words ending in -us

Thescientific Latintermoctopuswas derived fromAncient Greekὀκτώπους(oktōpous), acompoundform ofὀκτώ(oktō,'eight') andπούς(pous,'foot'), itself a variant form ofὀκτάπους,a word used for example byAlexander of Tralles(c. 525–c. 605) for the common octopus.[5][6][7]The standardpluralisedform ofoctopusin English isoctopuses;[8]the Ancient Greek pluralὀκτώποδες,octopodes(/ɒkˈtɒpədiːz/), has also been used historically.[9]The alternative pluraloctopiis usually considered grammatically incorrect because itwrongly assumesthatoctopusis a Latinsecond-declension-usnoun or adjective when, in either Greek or Latin, it is athird-declensionnoun.[10][11]

Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the latinate formoctopi,[12]followed by the English formoctopusesin the latter half of the same century. The Hellenic plural is roughly contemporary in usage, although it is also the rarest.[13]

Fowler's Modern English Usagestates that the only acceptable plural in English isoctopuses,thatoctopiis misconceived, andoctopodespedantic;[14][15][16]the last is nonetheless used frequently enough to be acknowledged by thedescriptivistMerriam-Webster 11th Collegiate DictionaryandWebster's New World College Dictionary.TheOxford English Dictionarylistsoctopuses,octopi,andoctopodes,in that order, reflecting frequency of use, callingoctopodesrare and noting thatoctopiis based on a misunderstanding.[17]TheNew Oxford American Dictionary(3rd Edition, 2010) listsoctopusesas the only acceptable pluralisation, and indicates thatoctopodesis still occasionally used, but thatoctopiis incorrect.[18]

Anatomy and physiology

Size

See also:Cephalopod size
Captured specimen of a giant octopus
Agiant Pacific octopusat Echizen Matsushima Aquarium, Japan

Thegiant Pacific octopus(Enteroctopus dofleini)is often cited as the largest known octopus species. Adults usually weigh around 15 kg (33 lb), with an arm span of up to 4.3 m (14 ft).[19]The largest specimen of this species to be scientifically documented was an animal with a live mass of 71 kg (157 lb).[20]Much larger sizes have been claimed for the giant Pacific octopus:[21]one specimen was recorded as 272 kg (600 lb) with an arm span of 9 m (30 ft).[22]A carcass of theseven-arm octopus,Haliphron atlanticus,weighed 61 kg (134 lb) and was estimated to have had a live mass of 75 kg (165 lb).[23][24]The smallest species isOctopus wolfi,which is around 2.5 cm (1 in) and weighs less than 1 g (0.035 oz).[25]

External characteristics

The octopus isbilaterally symmetricalalong its dorso-ventral (back to belly) axis; the head andfootare at one end of an elongated body and function as the anterior (front) of the animal. The head includes the mouth and brain. The foot has evolved into a set of flexible, prehensileappendages,known as "arms", that surround the mouth and are attached to each other near their base by a webbed structure.[26]The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divided into four pairs.[27][26]The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.[28]The bulbous and hollowmantleis fused to the back of the head and is known as the visceral hump; it contains most of the vital organs.[29][30]The mantle cavity has muscular walls and contains the gills; it is connected to the exterior by a funnel orsiphon.[26][31]The mouth of an octopus, located underneath the arms, has a sharp hardbeak.[30]

Schematic of external anatomy
Diagram of octopus from side, with gills, funnel, eye,ocellus(eyespot), web, arms, suckers,hectocotylusandligulalabelled.

The skin consists of a thin outerepidermiswith mucous cells and sensory cells and a connective tissuedermisconsisting largely ofcollagenfibres and various cells allowing colour change.[26]Most of the body is made of soft tissue allowing it to lengthen, contract, and contort itself. The octopus can squeeze through tiny gaps; even the larger species can pass through an opening close to 2.5 cm (1 in) in diameter.[30]Lacking skeletal support, the arms work asmuscular hydrostatsand contain longitudinal, transverse and circular muscles around a central axial nerve. They can extend and contract, twist to left or right, bend at any place in any direction or be held rigid.[32][33]

The interior surfaces of the arms are covered with circular, adhesive suckers. The suckers allow the octopus to anchor itself or to manipulate objects. Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called aninfundibulumand a central hollow cavity called anacetabulum,both of which are thick muscles covered in a protective chitinous cuticle. When a sucker attaches to a surface, the orifice between the two structures is sealed. The infundibulum provides adhesion while the acetabulum remains free, and muscle contractions allow for attachment and detachment.[34][35]Each of the eight arms senses and responds to light, allowing the octopus to control the limbs even if its head is obscured.[36]

A stubby round sea-creature with short ear-like fins
A finnedGrimpoteuthisspecies with its atypical octopus body plan

The eyes of the octopus are large and at the top of the head. They are similar in structure to those of a fish, and are enclosed in acartilaginouscapsule fused to the cranium. Thecorneais formed from atranslucentepidermal layer; the slit-shapedpupilforms a hole in theirisjust behind the cornea. The lens is suspended behind the pupil; photoreceptiveretinal cellscover the back of the eye. The pupil can be adjusted in size; a retinal pigment screens incident light in bright conditions.[26]

Some species differ in form from the typical octopus body shape.Basalspecies, theCirrina,have stout gelatinous bodies with webbing that reaches near the tip of their arms, and two largefinsabove the eyes, supported by aninternal shell.Fleshy papillae orcirriare found along the bottom of the arms, and the eyes are more developed.[37][38]

Circulatory system

Octopuses have a closedcirculatory system,in which the blood remains inside blood vessels. Octopuses have three hearts; a systemic or main heart that circulates blood around the body and two branchial or gill hearts that pump it through each of the two gills. The systemic heart becomes inactive when the animal is swimming. Thus the octopus tires quickly and prefers to crawl.[39][40]Octopus blood contains thecopper-rich proteinhaemocyaninto transport oxygen. This makes the blood veryviscousand it requires considerable pressure to pump it around the body; octopuses'blood pressurescan exceed 75 mmHg (10 kPa).[39][40][41]In cold conditions with low oxygen levels, haemocyanin transports oxygen more efficiently thanhaemoglobin.The haemocyanin is dissolved in the plasma instead of being carried within blood cells and gives the blood a bluish colour.[39][40]

The systemic heart has muscular contractile walls and consists of a single ventricle and two atria, one for each side of the body. The blood vessels consist of arteries, capillaries and veins and are lined with a cellularendotheliumwhich is quite unlike that of most otherinvertebrates.The blood circulates through the aorta and capillary system, to the vena cavae, after which the blood is pumped through the gills by the branchial hearts and back to the main heart. Much of the venous system is contractile, which helps circulate the blood.[26]

Respiration

An octopus on the seabed, its siphon protruding near its eye
Octopus with open siphon. The siphon is used for respiration, waste disposal and discharging ink.

Respiration involves drawing water into the mantle cavity through an aperture, passing it through the gills, and expelling it through the siphon. The ingress of water is achieved by contraction of radial muscles in the mantle wall, and flapper valves shut when strong circular muscles force the water out through the siphon.[42]Extensive connective tissue lattices support the respiratory muscles and allow them to expand the respiratory chamber.[43]Thelamellastructure of the gills allows for a high oxygen uptake, up to 65% in water at 20 °C (68 °F).[44]Water flow over the gills correlates with locomotion, and an octopus can propel its body when it expels water out of its siphon.[43][41]

The thin skin of the octopus absorbs additional oxygen. When resting, around 41% of an octopus's oxygen absorption is through the skin. This decreases to 33% when it swims, as more water flows over the gills; skin oxygen uptake also increases. When it is resting after a meal, absorption through the skin can drop to 3% of its total oxygen uptake.[45]

Digestion and excretion

The digestive system of the octopus begins with thebuccal masswhich consists of the mouth with itschitinousbeak, the pharynx,radulaand salivary glands.[46]The radula is a spiked, muscular tongue-like organ with multiple rows of tiny teeth.[30]Food is broken down and is forced into the oesophagus by two lateral extensions of the esophageal side walls in addition to the radula. From there it is transferred to thegastrointestinal tract,which is mostly suspended from the roof of the mantle cavity by numerous membranes. The tract consists of acrop,where the food is stored; a stomach, where food is ground down; acaecumwhere the now sludgy food is sorted into fluids and particles and which plays an important role in absorption; thedigestive gland,where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the accumulated waste is turned into faecal ropes by secretions and blown out of the funnel via the rectum.[46]

Duringosmoregulation,fluid is added to thepericardiaof the branchial hearts. The octopus has twonephridia(equivalent to vertebrate kidneys) which are associated with the branchial hearts; these and their associated ducts connect the pericardial cavities with the mantle cavity. Before reaching the branchial heart, each branch of thevena cavaexpands to form renal appendages which are in direct contact with the thin-walled nephridium. The urine is first formed in the pericardial cavity, and is modified by excretion, chiefly of ammonia, and selective absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity.[26][47]

Acommon octopus(Octopus vulgaris) moving around. Its nervous system allows the arms to move with some autonomy.

Nervous system and senses

Octopuses (along withcuttlefish) have the highestbrain-to-body mass ratiosof all invertebrates;[48]this is greater than that of many vertebrates.[49]Octopuses have the samejumping genesthat are active in the human brain, implying anevolutionary convergenceat molecular level.[50]Thenervous systemis complex, only part of which is localised in its brain, which is contained in a cartilaginous capsule.[51]Two-thirds of an octopus'sneuronsare in the nerve cords of its arms. This allows their arms to perform complexreflexactions without input from the brain.[52]Unlike vertebrates, the complex motor skills of octopuses are not organised in their brains via internalsomatotopic mapsof their bodies.[53]The nervous system of cephalopods is the most complex of all invertebrates.[54][55]The giantnervefibers of the cephalopodmantlehave been widely used for many years as experimental material inneurophysiology;their large diameter (due to lack ofmyelination) makes them relatively easy to study compared with other animals.[56]

Close up of an octopus showing its eye and an arm with suckers
Eye ofcommon octopus

Like other cephalopods, octopuses have camera-like eyes,[48]and can distinguish thepolarisationof light.Colour visionappears to vary from species to species, for example, being present inO. aeginabut absent inO. vulgaris.[57] Opsinsin the skin respond to different wavelengths of light and help the animals choose a colouration that camouflages them; the chromatophores in the skin can respond to light independently of the eyes.[58][59] An alternative hypothesis is thatcephalopod eyesin species that only have a singlephotoreceptor proteinmay usechromatic aberrationto turn monochromatic vision into colour vision, though this sacrifices image quality. This would explain pupils shaped like the letter "U", the letter "W", or adumbbell,as well as the need for colourful mating displays.[60]

Attached to the brain are two organs calledstatocysts(sac-like structures containing a mineralised mass and sensitive hairs), that allow the octopus to sense the orientation of its body. They provide information on the position of the body relative to gravity and can detect angular acceleration. Anautonomicresponse keeps the octopus's eyes oriented so that the pupil is always horizontal.[26]Octopuses may also use the statocyst to hear sound. The common octopus can hear sounds between 400 Hz and 1000 Hz, and hears best at 600 Hz.[61]

Octopuses have an excellentsomatosensory system.Their suction cups are equipped withchemoreceptorsso they cantastewhat they touch. Octopus arms move easily because the sensors recognise octopus skin and prevent self-attachment.[62]Octopuses appear to have poorproprioceptivesense and must observe the arms visually to keep track of their position.[63][64]

Ink sac

Theink sacof an octopus is located under the digestive gland. A gland attached to the sac produces theink,and the sac stores it. The sac is close enough to the funnel for the octopus to shoot out the ink with a water jet. Before it leaves the funnel, the ink passes through glands which mix it with mucus, creating a thick, dark blob which allows the animal to escape from a predator.[65]The main pigment in the ink ismelanin,which gives it its black colour.[66]Cirrate octopuses usually lack the ink sac.[37]

Life cycle

Reproduction

Drawing of a male octopus with one large arm ending in the sexual apparatus
Adult maleTremoctopus violaceuswithhectocotylus

Octopuses aregonochoricand have a single, posteriorly-located gonad which is associated with thecoelom.Thetestisin males and theovaryin females bulges into thegonocoeland thegametesare released here. The gonocoel is connected by thegonoductto themantle cavity,which it enters at thegonopore.[26]Anoptic glandcreates hormones that cause the octopus to mature and age and stimulate gamete production. The gland may be triggered by environmental conditions such as temperature, light and nutrition, which thus control the timing of reproduction and lifespan.[67][68]

When octopuses reproduce, the male uses a specialised arm called ahectocotylusto transferspermatophores(packets of sperm) from the terminal organ of the reproductive tract (the cephalopod "penis" ) into the female's mantle cavity.[69]The hectocotylus inbenthicoctopuses is usually the third right arm, which has a spoon-shaped depression and modified suckers near the tip. In most species, fertilisation occurs in the mantle cavity.[26]

The reproduction of octopuses has been studied in only a few species. One such species is thegiant Pacific octopus,in which courtship is accompanied, especially in the male, by changes in skin texture and colour. The male may cling to the top or side of the female or position himself beside her. There is some speculation that he may first use his hectocotylus to remove any spermatophore or sperm already present in the female. He picks up a spermatophore from his spermatophoric sac with the hectocotylus, inserts it into the female's mantle cavity, and deposits it in the correct location for the species, which in the giant Pacific octopus is the opening of the oviduct. Two spermatophores are transferred in this way; these are about one metre (yard) long, and the empty ends may protrude from the female's mantle.[70]A complex hydraulic mechanism releases the sperm from the spermatophore, and it is stored internally by the female.[26]

A female octopus underneath hanging strings of her eggs
Female giant Pacific octopus guarding strings of eggs

About forty days after mating, the female giant Pacific octopus attaches strings of small fertilised eggs (10,000 to 70,000 in total) to rocks in a crevice or under an overhang. Here she guards and cares for them for about five months (160 days) until they hatch.[70]In colder waters, such as those offAlaska,it may take up to ten months for the eggs to completely develop.[71]: 74 The female aerates them and keeps them clean; if left untended, many will die.[72]She does not feed during this time and dies soon after. Males becomesenescentand die a few weeks after mating.[67]

The eggs have large yolks;cleavage(division) is superficial and agerminal discdevelops at the pole. Duringgastrulation,the margins of this grow down and surround the yolk, forming a yolk sac, which eventually forms part of the gut. The dorsal side of the disc grows upward and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upward, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo develops.[26]

A microscopic view of a small round-bodied transparent animal with very short arms
Octopusparalarva,aplanktonichatchling

Most young octopuses hatch asparalarvaeand areplanktonicfor weeks to months, depending on the species and water temperature. They feed oncopepods,arthropodlarvae and otherzooplankton,eventually settling on the ocean floor and developing directly into adults with no distinctmetamorphosesthat are present in other groups ofmollusclarvae.[26]Octopus species that produce larger eggs – including thesouthern blue-ringed,Caribbean reef,California two-spot,Eledone moschata[73]and deep sea octopuses – instead hatch as benthic animals similar to the adults.[71]: 74–75 

In theargonaut(paper nautilus), the female secretes a fine, fluted, papery shell in which the eggs are deposited and in which she also resides while floating in mid-ocean. In this she broods the young, and it also serves as a buoyancy aid allowing her to adjust her depth. The male argonaut is minute by comparison and has no shell.[74]

Lifespan

Octopuses have short lifespans, and some species complete their lifecycles in only six months. TheGiant Pacific octopus,one of the two largest species of octopus, usually lives for three to five years. Octopus lifespan is limited by reproduction.[75]For most octopuses, the last stage of their life is called senescence. It is the breakdown of cellular function without repair or replacement. For males, this typically begins after mating. Senescence may last from weeks to a few months, at most. For females, it begins when they lay a clutch of eggs. Females will spend all their time aerating and protecting their eggs until they are ready to hatch. During senescence, an octopus does not feed and quickly weakens. Lesions begin to form and the octopus literally degenerates. Unable to defend themselves, octopuses often fall prey to predators.[76]This makes most octopuses effectivelysemelparous.Thelarger Pacific striped octopus(LPSO) is an exception, as it can reproduce repeatedly over a life of around two years.[75]

Octopus reproductive organs mature due to thehormonalinfluence of the optic gland but result in the inactivation of their digestive glands. Unable to feed, the octopus typically dies of starvation.[76]Experimental removal of both optic glands after spawning was found to result in the cessation ofbroodiness,the resumption of feeding, increased growth, and greatly extended lifespans. It has been proposed that the naturally short lifespan may be functional to prevent rapid overpopulation.[77]

Distribution and habitat

An octopus nearly hidden in a crack in some coral
Octopus cyaneain Kona, Hawaii

Octopuses live in every ocean, and different species have adapted to differentmarine habitats.As juveniles, common octopuses inhabit shallowtide pools.The Hawaiian day octopus (Octopus cyanea) lives on coral reefs;argonautsdrift inpelagic waters.Abdopus aculeatusmostly lives in near-shoreseagrassbeds. Some species are adapted to the cold, ocean depths. The spoon-armed octopus (Bathypolypus arcticus) is found at depths of 1,000 m (3,300 ft), andVulcanoctopus hydrothermalislives nearhydrothermal ventsat 2,000 m (6,600 ft).[29]Thecirratespecies are often free-swimming and live in deep-water habitats.[38]Although several species are known to live atbathyalandabyssaldepths, there is only a single indisputable record of an octopus in thehadal zone;a species ofGrimpoteuthis(dumbo octopus) photographed at 6,957 m (22,825 ft).[78]No species are known to live in fresh water.[79]

Behaviour and ecology

Most species are solitary when not mating,[80]though a few are known to occur in high densities and with frequent interactions, signaling, mate defending and eviction of individuals from dens. This is likely the result of abundant food supplies combined with limited den sites.[81]The LPSO has been described as particularly social, living in groups of up to 40 individuals.[82][83]Octopuses hide in dens, which are typically crevices in rocky outcrops or other hard structures, though some species burrow into sand or mud. Octopuses are notterritorialbut generally remain in a home range; they may leave in search of food. They cannavigateback to a den without having to retrace their outward route.[84]They are not migratory.[85]

Octopuses bring captured prey to the den, where they can eat it safely. Sometimes the octopus catches more prey than it can eat, and the den is often surrounded by amiddenof dead and uneaten food items. Other creatures, such as fish,crabs,molluscs andechinoderms,often share the den with the octopus, either because they have arrived asscavengers,or because they have survived capture.[86]On rare occasions, octopuseshunt cooperatively with other species,with fish as their partners. They regulate thespecies compositionof the huntinggroup — and the behavior of theirpartners — by punching them.[87]

Feeding

An octopus in an open seashell on a sandy surface, surrounding a small crab with the suckers on its arms
Veined octopuseating a crab

Nearly all octopuses are predatory; bottom-dwelling octopuses eat mainlycrustaceans,polychaete worms,and other molluscs such aswhelksandclams;open-ocean octopuses eat mainly prawns, fish and other cephalopods.[88]Major items in the diet of the giant Pacific octopus includebivalve molluscssuch as the cockleClinocardium nuttallii,clams and scallops and crustaceans such as crabs andspider crabs.Prey that it is likely to reject includemoon snailsbecause they are too large andlimpets,rock scallops,chitonsandabalone,because they are too securely fixed to the rock.[86]Small cirrate octopuses such as those of the generaGrimpoteuthisandOpisthoteuthistypically prey on polychaetes,copepods,amphipodsandisopods.[89]

A benthic (bottom-dwelling) octopus typically moves among the rocks and feels through the crevices. The creature may make a jet-propelled pounce on prey and pull it toward the mouth with its arms, the suckers restraining it. Small prey may be completely trapped by the webbed structure. Octopuses usually inject crustaceans like crabs with a paralysingsalivathen dismember them with their beaks.[88][90]Octopuses feed on shelled molluscs either by forcing the valves apart, or by drilling a hole in the shell to inject anerve toxin.[91][90]It used to be thought that the hole was drilled by the radula, but it has now been shown that minute teeth at the tip of the salivary papilla are involved, and an enzyme in the toxic saliva is used to dissolve the calcium carbonate of the shell. It takes about three hours forO. vulgaristo create a 0.6 mm (0.024 in) hole. Once the shell is penetrated, the prey dies almost instantaneously, its muscles relax, and the soft tissues are easy for the octopus to remove. Crabs may also be treated in this way; tough-shelled species are more likely to be drilled, and soft-shelled crabs are torn apart.[92]

Some species have other modes of feeding.Grimpoteuthishas a reduced or non-existent radula and swallows prey whole.[37]In the deep-sea genusStauroteuthis,some of the muscle cells that control the suckers in most species have been replaced withphotophoreswhich are believed to fool prey by directing them to the mouth, making them one of the fewbioluminescentoctopuses.[93]

Locomotion

An octopus swimming with its round body to the front, its arms forming a streamlined tube behind
Octopuses swim with their arms trailing behind.

Octopuses mainly move about by relatively slow crawling with some swimming in a head-first position.Jet propulsionor backward swimming, is their fastest means of locomotion, followed by swimming and crawling.[94]When in no hurry, they usually crawl on either solid or soft surfaces. Several arms are extended forward, some of the suckers adhere to the substrate and the animal hauls itself forward with its powerful arm muscles, while other arms may push rather than pull. As progress is made, other arms move ahead to repeat these actions and the original suckers detach. During crawling, the heart rate nearly doubles, and the animal requires ten or fifteen minutes to recover from relatively minor exercise.[32]

Most octopuses swim by expelling a jet of water from the mantle through the siphon into the sea. The physical principle behind this is that the force required to accelerate the water through the orifice produces a reaction that propels the octopus in the opposite direction.[95]The direction of travel depends on the orientation of the siphon. When swimming, the head is at the front and the siphon is pointed backward but, when jetting, the visceral hump leads, the siphon points at the head and the arms trail behind, with the animal presenting afusiformappearance. In an alternative method of swimming, some species flatten themselves dorso-ventrally, and swim with the arms held out sideways, and this may provide lift and be faster than normal swimming. Jetting is used to escape from danger, but is physiologically inefficient, requiring a mantle pressure so high as to stop the heart from beating, resulting in a progressive oxygen deficit.[94]

Three images in sequence of a two-finned sea creature swimming with an 8-cornered web
Movements of the finned speciesCirroteuthis muelleri

Cirrate octopuses cannot produce jet propulsion and rely on their fins for swimming. They have neutral buoyancy and drift through the water with the fins extended. They can also contract their arms and surrounding web to make sudden moves known as "take-offs". Another form of locomotion is "pumping", which involves symmetrical contractions of muscles in their webs producingperistaltic waves.This moves the body slowly.[37]

In 2005,Adopus aculeatusand veined octopus (Amphioctopus marginatus) were found to walk on two arms, while at the same time mimicking plant matter.[96]This form of locomotion allows these octopuses to move quickly away from a potential predator without being recognised.[94]Some species of octopus can crawl out of the water briefly, which they may do between tide pools.[97][98]"Stilt walking" is used by the veined octopus when carrying stacked coconut shells. The octopus carries the shells underneath it with two arms, and progresses with an ungainly gait supported by its remaining arms held rigid.[99]

Intelligence

Main article:Cephalopod intelligence
A captive octopus with two arms wrapped around the cap of a plastic container
Octopus opening a container by unscrewing its cap

Octopuses are highlyintelligent.[100]Mazeandproblem-solvingexperiments have shown evidence of a memory system that can store bothshort-andlong-term memory.[101]Young octopuses learn nothing from their parents, as adults provide noparental carebeyond tending to their eggs until the young octopuses hatch.[71]: 75 

In laboratory experiments, octopuses can readily be trained to distinguish between different shapes and patterns. They have been reported to practiseobservational learning,[102]although the validity of these findings is contested.[100]Octopuses have also been observed in what has been described asplay:repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.[103]Octopuses often break out of their aquariums and sometimes into others in search of food.[97][104][105]Theveined octopuscollects discardedcoconutshells, then uses them to build a shelter, an example oftool use.[99]

Camouflage and colour change

Video ofOctopus cyaneamoving and changing its colour, shape, and texture

Octopuses usecamouflagewhen hunting and to avoid predators. To do this, they use specialised skin cells that change the appearance of the skin by adjusting its colour, opacity, or reflectivity.Chromatophorescontain yellow, orange, red, brown, or black pigments; most species have three of these colours, while some have two or four. Other colour-changing cells are reflective iridophores and white leucophores.[106]This colour-changing ability is also used to communicate with or warn other octopuses.[107]

Octopuses can create distracting patterns with waves of dark colouration across the body, a display known as the "passing cloud". Muscles in the skin change the texture of the mantle to achieve greater camouflage. In some species, the mantle can take on the spiky appearance of algae; in others, skin anatomy is limited to relatively uniform shades of one colour with limited skin texture. Octopuses that are diurnal and live in shallow water have evolved more complex skin than their nocturnal and deep-sea counterparts.[107]

A "moving rock" trick involves the octopus mimicking a rock and then inching across the open space with a speed matching that of the surrounding water.[108]

Defence

An octopus among coral displaying conspicuous rings of turquoise outlined in black against a sandy background
Warning displayofgreater blue-ringed octopus(Hapalochlaena lunulata)

Aside from humans, octopuses may be preyed on by fishes,seabirds,sea otters,pinnipeds,cetaceans,and other cephalopods.[109]Octopuses typically hide or disguise themselves by camouflage andmimicry;some have conspicuouswarning coloration (aposematism)ordeimatic behaviour.[107]An octopus may spend 40% of its time hidden away in its den. When the octopus is approached, it may extend an arm to investigate. 66% ofEnteroctopus dofleiniin one study had scars, with 50% having amputated arms.[109]The blue rings of the highly venomous blue-ringed octopus are hidden in muscular skin folds which contract when the animal is threatened, exposing the iridescent warning.[110]TheAtlantic white-spotted octopus(Callistoctopus macropus) turns bright brownish red with oval white spots all over in a high contrast display.[111]Displays are often reinforced by stretching out the animal's arms, fins or web to make it look as big and threatening as possible.[112]

Once they have been seen by a predator, they commonly try to escape but can also create a distraction by ejecting an ink cloud from their ink sac. The ink is thought to reduce the efficiency of olfactory organs, which would aid evasion from predators that employsmellfor hunting, such assharks.Ink clouds of some species might act aspseudomorphs,or decoys that the predator attacks instead.[113]

When under attack, some octopuses can perform armautotomy,in a manner similar to the wayskinksand otherlizardsdetach their tails. The crawling arm may distract would-be predators. Such severed arms remain sensitive to stimuli and move away from unpleasant sensations.[114]Octopuses canreplace lost limbs.[115]

Some octopuses, such as themimic octopus,can combine their highly flexible bodies with their colour-changing ability to mimic other, more dangerous animals, such aslionfish,sea snakes, andeels.[116][117]

Pathogens and parasites

The diseases and parasites that affect octopuses have been little studied, but cephalopods are known to be the intermediate or finalhostsof various parasiticcestodes,nematodesand copepods; 150 species ofprotistanandmetazoanparasites have been recognised.[118]TheDicyemidaeare a family of tiny worms that are found in the renal appendages of many species;[119]it is unclear whether they are parasitic orendosymbionts.Coccidiansin the genusAggregataliving in the gut cause severe disease to the host. Octopuses have aninnate immune system;theirhaemocytesrespond to infection byphagocytosis,encapsulation, infiltration, or cytotoxic activities to destroy or isolate the pathogens. The haemocytes play an important role in the recognition and elimination of foreign bodies and wound repair. Captive animals are more susceptible to pathogens than wild ones.[120]A gram-negative bacterium,Vibrio lentus,can cause skin lesions, exposure of muscle and sometimes death.[121]

Evolution

Further information:Evolution of cephalopods

The scientific name Octopoda was first coined and given as the order of octopuses in 1818 by English biologistWilliam Elford Leach,[122]who classified them as Octopoida the previous year.[2]The Octopoda consists of around 300 known species[123]and were historically divided into two suborders, theIncirrinaand the Cirrina.[38]More recent evidence suggests Cirrina is merely the most basal species, not a uniqueclade.[124]The incirrate octopuses (the majority of species) lack the cirri and paired swimming fins of the cirrates.[38]In addition, the internal shell of incirrates is either present as a pair ofstyletsor absent altogether.[125]

Fossil history and phylogeny

Fossil of crown group coleoid on a slab of Jurassic rock from Germany
The octopuses evolved from theMuensterelloidea(fossil pictured) in theJurassicperiod.[126]

The Cephalopoda evolved from a mollusc resembling theMonoplacophorain theCambriansome 530 million years ago. The Coleoidea diverged from the nautiloids in theDevoniansome 416 million years ago. In turn, the coleoids (including the squids and octopods) brought their shells inside the body and some 276 million years ago, during thePermian,split into the Vampyropoda and the Decabrachia.[127]The octopuses arose from theMuensterelloideawithin the Vampyropoda in theJurassic.The earliest octopus likely lived near the sea floor (benthictodemersal) in shallow marine environments.[127][128][126]Octopuses consist mostly of soft tissue, and so fossils are relatively rare. As soft-bodied cephalopods, they lack the external shell of most molluscs, including other cephalopods like thenautiloidsand the extinctAmmonoidea.[129]They have eight limbs like otherColeoidea,but lack the extra specialised feeding appendages known astentacleswhich are longer and thinner with suckers only at their club-like ends.[130]The vampire squid (Vampyroteuthis) also lacks tentacles but has sensory filaments.[131]

Thecladogramsare based on Sanchez et al., 2018, who created amolecular phylogenybased onmitochondrialandnuclear DNAmarker sequences.[124]The position of the Eledonidae is from Ibáñez et al., 2020, with a similar methodology.[132]Dates of divergence are from Kröger et al., 2011 and Fuchs et al., 2019.[127][126]

Cephalopods
Nautiloids

NautilusA spiral nautilus in a blue sea

Coleoids
Decabrachia

Squids and cuttlefishA squid

Vampyropoda
Vampyromorphida

A strange blood-red octopus, its arms joined by a web

Octopods

A brown octopus with wriggly arms

155 mya
276 mya
416 mya
530 mya

The molecular analysis of the octopods shows that the suborder Cirrina (Cirromorphida) and the superfamily Argonautoidea areparaphyleticand are broken up; these names are shown in quotation marks and italics on the cladogram.

Octopoda

RNA editing and the genome

Octopuses, like other coleoid cephalopods but unlike morebasalcephalopods or other molluscs, are capable of greaterRNA editing,changing thenucleic acid sequenceof theprimary transcriptof RNA molecules, than any other organisms. Editing is concentrated in the nervous system, and affects proteins involved in neural excitability and neuronal morphology. More than 60% of RNA transcripts for coleoid brains are recoded by editing, compared to less than 1% for a human orfruit fly.Coleoids rely mostly onADARenzymes for RNA editing, which requires largedouble-stranded RNAstructures to flank the editing sites. Both the structures and editing sites are conserved in the coleoid genome and the mutation rates for the sites are severely hampered. Hence, greater transcriptome plasticity has come at the cost of slower genome evolution.[133][134]

The octopus genome is unremarkablybilaterianexcept for large developments of two gene families:protocadherins,which regulate the development of neurons; and theC2H2 zinc-fingertranscription factors. Many genes specific to cephalopods are expressed in the animals' skin, suckers, and nervous system.[48]

Relationship to humans

In art, literature, and mythology

An ancient nearly spherical vase with 2 handles by the top, painted all over with an octopus decoration in black
Minoanclay vase with octopus decoration, c. 1500 BC

Ancient seafaring people were aware of the octopus, as evidenced by artworks and designs. For example, a stone carving found in the archaeological recovery from Bronze AgeMinoan CreteatKnossos(1900–1100 BC) depicts a fisherman carrying an octopus.[135]The terrifyingly powerfulGorgonofGreek mythologymay have been inspired by the octopus or squid, the octopus itself representing the severed head ofMedusa,the beak as the protruding tongue and fangs, and its tentacles as the snakes.[136]TheKrakenare legendary sea monsters of giant proportions said to dwell off the coasts of Norway and Greenland, usually portrayed in art as giant octopuses attacking ships.Linnaeusincluded it in the first edition of his 1735Systema Naturae.[137][138]One translation of the Hawaiiancreation myththeKumuliposuggests that the octopus is the lone survivor of a previous age.[139][140][141]TheAkkorokamuiis a gigantic octopus-likemonsterfromAinufolklore, worshipped inShinto.[142]

A battle with an octopus plays a significant role inVictor Hugo's 1866 bookTravailleurs de la mer(Toilers of the Sea).[143]Ian Fleming's 1966 short story collectionOctopussy and The Living Daylights,and the 1983James Bondfilmwere partly inspired by Hugo's book.[144]Japanese erotic art,shunga,includesukiyo-ewoodblock prints such asKatsushika Hokusai's 1814 printTako to ama(The Dream of the Fisherman's Wife), in which anama diveris sexually intertwined with a large and a small octopus.[145][146]The print is a forerunner oftentacle erotica.[147]The biologistP. Z. Myersnoted in his science blog,Pharyngula,that octopuses appear in "extraordinary" graphic illustrations involving women, tentacles, and bare breasts.[148][149]

Since it has numerous arms emanating from a common centre, the octopus is often used as a symbol for a powerful and manipulative organisation, company, or country.[150]

The Beatles song "Octopus's Garden",on the band's 1969 albumAbbey Road,was written byRingo Starrafter he was told about how octopuses travel along the sea bed picking up stones and shiny objects with which to build gardens.[151]

Danger to humans

Coloured drawing of a huge octopus rising from the sea and attacking a sailing ship's three masts with its spiralling arms
Pen andwashdrawing of an imaginedcolossal octopusattacking a ship, by themalacologistPierre de Montfort,1801

Octopuses generally avoid humans, butincidents have been verified.For example, a 2.4-metre (8 ft) Pacific octopus, said to be nearly perfectly camouflaged, "lunged" at a diver and "wrangled" over his camera before it let go. Another diver recorded the encounter on video.[152]All species are venomous, but only blue-ringed octopuses have venom that is lethal to humans.[153]Blue-ringed octopuses are among the deadliest animals in the sea, their bites are reported each year across the animals' range from Australia to the eastern Indo-Pacific Ocean. They bite only when provoked or accidentally stepped upon; bites are small and usually painless. The venom appears to be able to penetrate the skin without a puncture, given prolonged contact. It containstetrodotoxin,which causes paralysis by blocking the transmission ofnerve impulsesto the muscles. This causes death by respiratory failure leading tocerebral anoxia.No antidote is known, but if breathing can be kept going artificially, patients recover within 24 hours.[154][155]Bites have been recorded from captive octopuses of other species; they leave swellings which disappear in a day or two.[156]

As a food source

Main article:Octopus as food

Octopusfisheriesexist around the world with total catches varying between 245,320 and 322,999 metric tons from 1986 to 1995.[157]The world catch peaked in 2007 at 380,000 tons, and had fallen by a tenth by 2012.[158]Methods to capture octopuses include pots,traps,trawls,snares, drift fishing, spearing, hooking and hand collection.[157]Octopuses have a food conversion efficiency greater than that of chickens, makingoctopus aquaculturea possibility.[159]Octopuses compete with human fisheries targeting other species, and even rob traps and nets for their catch; they may, themselves, be caught asbycatchif they cannot get away.[160]

Octopus is eaten in many cultures, such as those on the Mediterranean and Asian coasts.[161]The arms and other body parts are prepared in ways that vary by species and geography. Live octopuses or their wriggling pieces are consumed asikizukuriin Japanese cuisine andsan-nakjiin Korean cuisine.[162][163]If not prepared properly, however, the severed arms can still choke the diner with their suction cups, causing at least one death in 2010.[164]Animal welfare groups have objected to the live consumption of octopuses on the basis that they can experience pain.[165]

In science and technology

In classical Greece,Aristotle(384–322 BC)commented onthe colour-changing abilities of the octopus, both for camouflage and forsignalling,in hisHistoria animalium:"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 alsowhen alarmed."[166]Aristotle noted that the octopus had a hectocotyl arm and suggested it might be used in sexual reproduction. This claim was widely disbelieved until the 19th century. It was described in 1829 by the French zoologistGeorges Cuvier,who supposed it to be a parasitic worm, naming it as a new species,Hectocotylus octopodis.[167][168]Other zoologists thought it a spermatophore; the German zoologistHeinrich Müllerbelieved it was "designed" to detach during copulation. In 1856 the Danish zoologistJapetus Steenstrupdemonstrated that it is used to transfer sperm, and only rarely detaches.[169]

Flexiblebiomimetic'Octopus'roboticsarm. The BioRobotics Institute,Scuola Superiore Sant'Anna,Pisa,2011[170]

Octopuses offer manypossibilities in biological research,including their ability to regenerate limbs, change the colour of their skin, behave intelligently with a distributed nervous system, and make use of 168 kinds ofprotocadherins(humans have 58), the proteins that guide the connections neurons make with each other. The California two-spot octopus has had its genome sequenced, allowing exploration of its molecular adaptations.[48]Havingindependently evolvedmammal-like intelligence, octopuses have been compared by the philosopherPeter Godfrey-Smith,who has studied the nature of intelligence,[171]to hypotheticalintelligent extraterrestrials.[172]Their problem-solving skills, along with their mobility and lack of rigid structure enable them to escape from supposedly secure tanks in laboratories andpublic aquariums.[173]

Due to their intelligence, octopuses are listed in some countries asexperimental animalson which surgery may not be performed withoutanesthesia,a protection usually extended only to vertebrates. In the UK from 1993 to 2012, the common octopus (Octopus vulgaris) was the only invertebrate protected under theAnimals (Scientific Procedures) Act 1986.[174]In 2012, this legislation was extended to include all cephalopods[175]in accordance with a generalEUdirective.[176]

Someroboticsresearch is exploringbiomimicryof octopus features. Octopus arms can move and sense largely autonomously without intervention from the animal's central nervous system. In 2015 a team in Italy built soft-bodied robots able to crawl and swim, requiring only minimal computation.[177][178]In 2017 a German company made an arm with a softpneumaticallycontrolledsiliconegripper fitted with two rows of suckers. It is able to grasp objects such as a metal tube, a magazine, or a ball, and to fill a glass by pouring water from a bottle.[179]

See also

Notes

  1. ^See§ Etymology and pluralisationfor variants.
  2. ^"Tentacle"is a commonumbrella termfor cephalopod limbs. Inteuthologicalcontext, octopuses have "arms" with suckers along their entire length while "tentacle" is reserved for appendages with suckers only near the end of the limb, which octopuses lack.[4]

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Bibliography

Further reading

See also

External links

The WikibookDichotomous Keyhas a page on the topic of:Octopoda
Wikimedia Commons has media related toOctopoda.


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