Octopus

Octopus Temporal range: Middle Jurassic – recent PreꞒ Ꞓ O S D C P T J K Pg N
Common octopus (Octopus vulgaris)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
(unranked):	Neocoleoidea
Clade:	Vampyropoda
Superorder:	Octopodiformes
Order:	Octopoda Leach, 1818[1]
Suborders
(traditional) Cirrina Incirrina See § Evolution for families
Synonyms
Octopoida Leach, 1817[2]

An octopus (pl.: octopuses or octopodes^[a]) is a soft-bodied, eight-limbed mollusc of the order Octopoda (/ɒkˈtɒpədə/, ok-TOP-ə-də^[3]). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs.^[b] An octopus can radically deform its shape, enabling it to squeeze through small gaps. They trail their appendages behind them as they swim. The siphon is used for respiration and locomotion (by water jet propulsion). Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse invertebrates.

Octopuses inhabit various

The scientific Latin term octopus was derived from Ancient Greek ὀκτώπους (oktōpous), a compound form of ὀκτώ (oktō, 'eight') and πούς (pous, 'foot'), itself a variant form of ὀκτάπους, a word used for example by Alexander of Tralles (c. 525 – c. 605).^[5][6][7]

The standard pluralised form of octopus in English is octopuses;^[8] the Ancient Greek plural ὀκτώποδες, octopodes (/ɒkˈtɒpədiːz/), has also been used historically.^[9] The alternative plural octopi is usually considered etymologically incorrect because it wrongly assumes that octopus is a Latin second-declension -us noun or adjective when, in either Greek or Latin, it is a third-declension noun.^[10][11] Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the Latinate form octopi,^[12] followed by the English form octopuses in the latter half of the same century. The Hellenic plural is roughly contemporary in usage, although it is also the rarest.^[13]

Anatomy and physiology

Size

See also: Cephalopod size

The

giant Pacific octopus (Enteroctopus dofleini) is often cited as the largest octopus species. Adults usually weigh 10–50 kg (22–110 lb), with an arm span of up to 4.8 m (16 ft).^[19] The largest specimen of this species to be scientifically documented reached a live mass of 71 kg (157 lb).^[20] Much larger sizes have been claimed:^[21] one specimen was recorded as 272 kg (600 lb) with an arm span of 9 m (30 ft).^[22] A carcass of the seven-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 is Octopus wolfi, which is around 2.5 cm (1 in) and weighs less than 1 g (0.035 oz).^[25]

External characteristics

The octopus has an elongated body that is

foot are on the ventral side, but act as the anterior (front). The heads contains both the mouth and the brain.^{[26]: 343–344} The mouth has a sharp chitinous beak and is surrounded by and underneath the foot, which evolved into flexible, prehensile cephalopod limbs, known as "arms", which are attached to each other near their base by a webbed structure.^{[26]: 343–344 [27]: 40–41 [28]: 13–15} The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divide into four pairs.^[29]: 12 The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.^[30] The bulbous and hollow mantle is fused to the back of the head and contains most of the vital organs.^{[28]: 13–15 [27]: 40–41} The mantle also has a cavity with muscular walls and a pair of gills; it is connected to the exterior by a funnel or siphon.^{[26]: 343–344 [31]}

ocellus (eyespot), web, arms, suckers, hectocotylus and ligula

labelled.

The skin consists of a thin epidermis with mucous cells and sensory cells and a fibrous dermis made of collagen and containing various cells that allow colour change.^[26]: 362 Most of the body is made of soft tissue, allowing it to squeeze through tiny gaps; even the larger species can pass through a gap little more than 2.5 cm (1 in) in diameter.^{[27]: 40–41} Lacking skeletal support, the arms work as muscular hydrostats and feature longitudinal, transverse, and circular muscles around a central axial nerve. They can squash and stretch, coil at any place in any direction or stiffen.^[32][33]

The interior surfaces of the arms are covered with circular, adhesive suckers. The suckers allow the octopus to secure itself in place or to handle objects. Each sucker is typically circular and bowl-like and has two distinct parts: an outer disc-shaped

chitinous cuticle lines the outer surface. When a sucker attaches to a surface, the orifice between the two structures is sealed and the infundibulum flattens. Muscle contractions allow for attachment and detachment.^[34][35][32] Each of the eight arms senses and responds to light, allowing the octopus to control its limbs even if its head is obscured.^[36]

The cranium has two

translucent epidermal layer; the slit-shaped pupil forms a hole in the iris just behind the cornea. The lens hangs behind the pupil; photoreceptive retinal cells line the back. The pupil can expand and contract; a retinal pigment screens incident light in bright conditions.^[26]

^{: 360–361}

Some species differ in form from the typical body shape. Basal species, the Cirrina, have gelatinous bodies with two fins located above the eyes, an internal shell and mostly webbed arms that are lined with fleshy papillae or cirri underneath.^[37][38]

Circulatory system

Octopuses have a closed

haemocyanin to transport oxygen. This makes the blood viscous and it requires great pressure to pump it around the body; blood pressures can surpass 75 mmHg (10 kPa).^{[29]: 31–35 [27]: 42–43 [39]} In cold conditions with low oxygen levels, haemocyanin transports oxygen more efficiently than haemoglobin.^[40] The haemocyanin is dissolved in the blood plasma instead of carried within blood cells and gives the blood a bluish colour.^{[29]: 31–35 [27]: 42–43 [28]}

^: 22

The systemic heart has muscular contractile walls and consists of a single ventricle and two atria, which attach it to each of the two gills. The blood vessels consist of arteries, capillaries and veins and are lined with a cellular endothelium unlike that of most other invertebrates. The blood circulates through the aorta and capillary system, to the venae 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]: 358

Respiration

Respiration involves drawing water into the mantle cavity through an aperture, passing it through the gills, and expelling it through the siphon. Ingress is achieved by contraction of radial muscles in the mantle wall, and flapper valves shut when strong, circular muscles expel the water through the siphon.^[41] Extensive connective tissue lattices support the respiratory muscles and allow them to inflate the respiratory chamber.^{[29]: 24–26} The lamella structure of the gills allows for high oxygen uptake, up to 65% in water at 20 °C (68 °F).^[42] Respiration can also play a role in locomotion, as an octopus can propel its body shooting water out of the siphon.^{[29]: 18 [39]}

The thin skin absorbs additional oxygen. When resting, around 41% of oxygen absorption is through the skin, reduced to 33% when the octopus swims, despite the amount of oxygen absorption increasing as water flows over the body. When it is resting after a meal, skin absorption can drop to 3%.^[43]

Digestion and excretion

The digestive system begins with the

caecum where the food is separated into particles and liquids and which absorbs fats; the digestive gland, where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the built-up waste is turned into faecal ropes by secretions and ejected out of the funnel via the rectum.^[29]

^: 75–79

During

vena cava has renal appendages that pass over the thin-walled nephridium before reaching the branchial heart. Urine is created in the pericardial cavity, and is altered by excretion, of mostly ammonia, and absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity.^[26]

^{: 358–359}

Nervous system and senses

Octopuses and their relatives have a more expansive and complex

evolutionary convergence at molecular level.^[51]

Like other cephalopods, octopuses have camera-like eyes.

chromatophores in the skin can respond to light independently of the eyes.^[53][54] An alternative hypothesis is that cephalopod eyes in species that only have a single photoreceptor protein may use chromatic aberration to turn monochromatic vision into colour vision, though this lowers image quality. This would explain pupils shaped like the letter "U", the letter "W", or a dumbbell, as well as the need for colourful mating displays.^[55]

Attached to the optic capsules are two organs called statocysts (sac-like structures containing a mineralised mass and sensitive hairs), that allow the octopus to sense the orientation of its body, relative to both gravity and time (angular acceleration). An autonomic response keeps the octopus's eyes oriented so that the pupil is always horizontal.^{[26]: 360–361} Octopuses may also use the statocyst to hear. The common octopus can hear sounds between 400 Hz and 1000 Hz, and hears best at 600 Hz.^[56]

Octopuses have an excellent

chemoreceptors so they can taste what they touch.^[57] Octopus arms move easily because the sensors recognise octopus skin and prevent self-attachment.^[58] Octopuses appear to have poor proprioceptive sense and must see their arms to keep track of their position.^[59][60]

Ink sac

The

ink, and the sac holds it. The sac is close enough to the funnel for the octopus to shoot out the ink with a water jet. As the animal begins to shoot, the ink passes through glands which mix it with mucus and it leaves the funnel as a thick, dark blob which helps the animal to escape from a predator.^[28]: 107 The main pigment in the ink is melanin, which gives it its black colour.^[61] Cirrate octopuses usually lack the ink sac.^[37]

Life cycle

Reproduction

Reproduction has been studied in some species. In the giant Pacific octopus, courtship includes changes in skin texture and colour, mostly in the male. 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 in 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.^[63] A complex hydraulic mechanism releases the sperm from the spermatophore.^{[26]: 363–365}

The eggs have large yolks;

Octopuses have short lifespans, living up to four years.[28]^: 17 The lifecycles of some species finish in less than half a year.^[27]: 152 For most octopuses, the ultimate life stage is senescence. It is the breakdown of cellular function without repair or replacement. It may last from weeks to a few months at most. Males senesce after maturity, while for females, it comes after they lay an egg clutch. During senescence, an octopus does not feed, quickly weakens, and becomes sluggish. Lesions begin to form and the octopus literally degenerates. They may die of starvation or get picked off by predators.^[66] Senescence is triggered by the optic glands and experimental removal of them after spawning was found to extend their lifecycle and activity.^[67]

Octopuses inhabit every ocean, with species adapted to many

Octopuses are mostly solitary[28]^{: 17, 134} though a few are known to live in groups and interact regularly, usually in the context of dominance and reproductive competition. This is likely the result of abundant food supplies combined with fewer den sites.^[70] The Larger Pacific striped octopus has been described as particularly social, living in groups of up to 40.^[71][72] Octopuses hide in dens, which are typically crevices in rocky or other hard structures, including man-made ones. Small species may use abandoned shells and bottles.^{[28]: 69, 74–75} They can navigate to a den without having to retrace their outward route.^[73] They are not migratory.^{[27]: 45–46}

Octopuses bring captured prey to the den to eat. Dens are often surrounded by a

Octopuses are generally predatory and feed on prey such as

Octopuses typically locate prey by feeling through their environment;[28]^: 60 some species hide and ambush their target.^[76]: 54 When prey tries to escape, the octopus jets after it.^[28]: 61 Octopuses may drill into the shells of crustaceans, bivalves and gastropods. It used to be thought that drilling was done 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. This can take hours and once the shell is penetrated, the prey dies almost instantaneously. With crabs, tough-shelled species are more likely to be drilled, and soft-shelled crabs are torn apart.^[78]

Some species have other modes of feeding.

Locomotion

Octopuses mainly move about by relatively slow crawling with some swimming in a head-first position. Jet propulsion or backward swimming, is their fastest means of locomotion, while crawling is slowest.^[80] While crawling, the suckers adhere and detach from the substrate as the animal hauls itself forward with its powerful arm muscles.^[32][80] In 2005, Adopus aculeatus and veined octopus (Amphioctopus marginatus) were found to walk on two arms, while at the same time mimicking plant matter.^[81] This form of locomotion allows these octopuses to move quickly away from a potential predator without being recognised.^[80] Some species of octopus can crawl out of the water briefly, which they may do between tide pools.^{[82][28]: 183} "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 awkward gait supported by its remaining arms, which are stiffened.^[83]

Most octopuses swim by expelling a jet of water from the mantle through the siphon into the sea. 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 a fusiform appearance. In an alternative method of swimming, some species flatten themselves dorso-ventrally, and swim with the arms splayed; 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.^[80]

Cirrate octopuses cannot produce jet propulsion and swim using their fins. Their neutrally buoyant bodies float along while the fins are spread. 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 producing peristaltic waves, moving them slowly.^[37]

Octopuses are highly

Octopuses use camouflage to hunt and to avoid predators. To do this, they use specialised skin cells that change colour. Chromatophores contain 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.^[91] This colour-changing ability is also used to communicate with or warn other octopuses.^{[28]: 90–97} The energy cost of the complete activation of the chromatophore system is high, nearly matching the energy used at rest.^[92]

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 bumpy appearance of algae-covered rocks. Diurnal, shallow water octopuses have more complex skin than their nocturnal and deep-sea counterparts. In the latter species, skin anatomy is limited to one colour or pattern.^{[28]: 89–97}

Octopus' "moving rock" trick involves mimicking a rock and then inching across the open space with a speed matching that of the surrounding water.^[93]

Aside from humans, octopuses are prey for fishes,

The scientific name Octopoda was first given as the order of octopuses in 1818 by English biologist

The Cephalopoda descended from a mollusc resembling the

Cephalopods	Nautiloids Nautilus Coleoids Decabrachia Squids and cuttlefish Vampyropoda Vampyromorphida Octopods 155 mya 276 mya 416 mya
530 mya

The molecular analysis of the octopods shows that the suborder Cirrina (Cirromorphida) and the superfamily Argonautoidea are

paraphyletic

and are broken up; these names are shown in quotation marks and italics on the cladogram.

Octopoda	"Cirromorphida" part Cirroteuthidae Stauroteuthidae "Cirromorphida" part Opisthoteuthidae Cirroctopodidae Octopodida "Argonautoidea" part Tremoctopodidae Alloposidae "Argonautoidea" part Argonautidae Ocythoidae Octopodoidea Eledonidae Bathypolypodidae Enteroctopodidae Octopodidae Megaleledonidae Bolitaenidae Amphitretidae Vitreledonellidae

RNA editing and the genome

Octopuses, like other coleoid cephalopods but unlike more basal cephalopods or other molluscs, are capable of greater RNA editing, changing the nucleic acid sequence of the primary transcript of RNA molecules, than any other organisms. Much editing is done in the nervous system, particularly for excitability and neuronal morphology. Coleoids rely mostly on ADAR enzymes for RNA editing, which requires large, double-stranded RNA structures. The many editing sites are conserved in the coleoid genome and the mutation rates for the sites are hampered. Hence, greater transcriptome plasticity has come at the cost of slower genome evolution.^[115]

The octopus genome is unremarkably

Ancient seafaring people were aware of the octopus, as evidenced by artworks and designs. It was depicted on coins during the Minoan civilization possibly as early as 1650 BCE and on pottery in Mycenaean Greece around between 1200 and 1100 BCE. A Hawaiian creation myth suggests that the octopus is the lone survivor of a previous age. The legendary sea monster, the kraken is conceived as octopus-like.^{[27]: 1, 4–5} Similarly, Medusa was compared to an octopus, with her snake-hair resembling the creature's arms.^[116]: 133 The Akkorokamui is a gigantic octopus-like monster from Ainu folklore, worshipped in Shinto.^[117]

In the Asuka-era Japanese legend Taishokan, a female diver battles an octopus to recover a stolen jewel, which became the inspiration for woodblock printings. Similarly, in the 1973 novel Gravity's Rainbow an octopus named Grigori attacks a woman on the beach. A battle with an octopus plays a significant role in Victor Hugo's 1866 book Travailleurs de la mer (Toilers of the Sea). The octopus continues to be depicted as antagonistic in films such as Wake of the Red Witch (1948).^{[116]: 129–131, 138–139, 145–147}

In

Octopuses

Octopus is eaten in many cultures, such as those on the Mediterranean and Asian coasts.[127] The arms and other body parts are prepared in ways that vary by species and geography. Live octopuses or their wriggling pieces are consumed as san-nakji in Korean cuisine.^[128][129] If not prepared properly, however, the severed arms can choke the diner with their suction cups, causing at least one death in 2010.^[130] Animal welfare groups have objected to the live consumption of octopuses on the basis that they can experience pain.^[131]

In classical Greece,

Octopuses offer many possibilities in biological research; the California two-spot octopus had its genome sequenced, allowing exploration of its molecular adaptations.^[44] Having independently evolved mammal-like intelligence, octopuses were compared by the philosopher Peter Godfrey-Smith, who studied the nature of intelligence,^[138] to hypothetical intelligent extraterrestrials.^[139] Their intelligence and flexible bodies enable them to escape from supposedly secure tanks in public aquariums.^[140]

Due to their intelligence, many argue that octopuses should be given protections when used for experiments.^[141] In the UK from 1993 to 2012, the common octopus (Octopus vulgaris) was the only invertebrate protected under the Animals (Scientific Procedures) Act 1986.^[142] In 2012, this legislation was extended to include all cephalopods^[143] in accordance with a general EU directive.^[144]

Some

• My Octopus Teacher – 2020 documentary film by Pippa Ehrlich and James Reed

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External links

The Wikibook Dichotomous Key has a page on the topic of: Octopoda

Wikimedia Commons has media related to Octopoda.

• Octopuses – Overview at the Encyclopedia of Life
• Octopoda Archived 29 September 2020 at the Wayback Machine at the Tree of Life Web Project
• "Can We Really Be Friends with an Octopus?" at
  Hakai Magazine
  , January 11, 2022