Octopus

The scientific Latin term octopus was derived from Ancient Greek ὀκτώπους, 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) for the common octopus.^[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 considered grammatically 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]

Fowler's Modern English Usage states that the only acceptable plural in English is "octopuses", that "octopi" is misconceived, and "octopodes" pedantic;^[14][15][16] the last is nonetheless used frequently enough to be acknowledged by the descriptivist Merriam-Webster 11th Collegiate Dictionary and Webster's New World College Dictionary. The Oxford English Dictionary lists "octopuses", "octopi", and "octopodes", in that order, reflecting frequency of use, calling "octopodes" rare and noting that "octopi" is based on a misunderstanding.^[17] The New Oxford American Dictionary (3rd Edition, 2010) lists "octopuses" as the only acceptable pluralisation, and indicates that "octopodes" is still occasionally used, but that "octopi" is incorrect.^[18]

Discover more about Etymology and pluralisation related topics

Plural form of words ending in -us

Plural form of words ending in -us

In English, the plural form of words ending in -us, especially those derived from Latin, often replaces -us with -i. There are many exceptions, some because the word does not derive from Latin, and others due to custom. Conversely, some non-Latin words ending in -us and Latin words that did not have their Latin plurals with -i form their English plurals with -i. Some words' plurals end in -i even though they are not Latin, or that is not the Latin plural, e.g., octopi is sometimes used as a plural for octopus. Prescriptivists consider these forms incorrect, but descriptivists may simply describe them as a natural evolution of language.

New Latin

New Latin

New Latin is the revival of Literary Latin used, in original, scholarly, and scientific works, first in Italy in the fourteenth and fifteenth centuries' Italian Renaissance, and then across northern Europe after about 1500, as a key feature of the humanist movement. Neo Latin's adoption throughout Europe was coincident with the rise of the printing press and of early modern schooling. Latin was learnt as a spoken language as well as written, as the vehicle of schooling and University education, while vernacular languages were still infrequently used in such settings. As such, Latin dominated early publishing, and made a signficant portion of printed works until the nineteenth century.

Ancient Greek

Ancient Greek

Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek, Dark Ages, the Archaic period, and the Classical period.

Compound (linguistics)

Compound (linguistics)

In linguistics, a compound is a lexeme that consists of more than one stem. Compounding, composition or nominal composition is the process of word formation that creates compound lexemes. Compounding occurs when two or more words or signs are joined to make a longer word or sign. A compound that uses a space rather than a hyphen or concatenation is called an open compound or a spaced compound; the alternative is a closed compound.

Alexander of Tralles

Alexander of Tralles

Alexander of Tralles was one of the most eminent physicians in the Byzantine Empire. His birth date may safely be put in the 6th century AD, for he mentions Aëtius Amidenus, who probably did not write until the end of the 5th or the beginning of the 6th century, and he is himself quoted by Paul of Aegina, who is supposed to have lived in the 7th century; besides which, he is mentioned as a contemporary of Agathias, who set about writing his History in the beginning of the reign of Justin II, about 565.

Plural

Plural

The plural, in many languages, is one of the values of the grammatical category of number. The plural of a noun typically denotes a quantity greater than the default quantity represented by that noun. This default quantity is most commonly one. Therefore, plurals most typically denote two or more of something, although they may also denote fractional, zero or negative amounts. An example of a plural is the English word cats, which corresponds to the singular cat.

Hypercorrection

Hypercorrection

In sociolinguistics, hypercorrection is non-standard use of language that results from the over-application of a perceived rule of language-usage prescription. A speaker or writer who produces a hypercorrection generally believes through a misunderstanding of such rules that the form is more "correct", standard, or otherwise preferable, often combined with a desire to appear formal or educated.

Second declension

Second declension

The second declension is a category of nouns in Latin and Greek with similar case formation. In particular, these nouns are thematic, with an original o in most of their forms. In Classical Latin, the short o of the nominative and accusative singular became u.

Third declension

Third declension

The third declension is a category of nouns in Latin and Greek with broadly similar case formation — diverse stems, but similar endings. Sanskrit also has a corresponding class, in which the so-called basic case endings are applied very regularly.

Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary (OED) is the principal historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a comprehensive resource to scholars and academic researchers, as well as describing usage in its many variations throughout the world.

New Oxford American Dictionary

New Oxford American Dictionary

The New Oxford American Dictionary (NOAD) is a single-volume dictionary of American English compiled by American editors at the Oxford University Press.

Size

A giant Pacific octopus at Echizen Matsushima Aquarium, Japan

The giant 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 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 is bilaterally symmetrical along its dorso-ventral (back to belly) axis; the head and foot are 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, prehensile appendages, 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 hollow mantle is 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 or siphon.^[26][31] The mouth of an octopus, located underneath the arms, has a sharp hard beak.^[30]

Diagram of octopus from side, with gills, funnel, eye, ocellus (eyespot), web, arms, suckers, hectocotylus and ligula labelled.

The skin consists of a thin outer epidermis with mucous cells and sensory cells, and a connective tissue dermis consisting largely of collagen fibres 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 as muscular hydrostats and 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 an infundibulum and a central hollow cavity called an acetabulum, 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 finned Grimpoteuthis species 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 a cartilaginous capsule fused to the cranium. The cornea is formed from a translucent epidermal layer; the slit-shaped pupil forms a hole in the iris just behind the cornea. The lens is suspended behind the pupil; photoreceptive retinal cells cover 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. Basal species, the Cirrina, have stout gelatinous bodies with webbing that reaches near the tip of their arms, and two large fins above the eyes, supported by an internal shell. Fleshy papillae or cirri are found along the bottom of the arms, and the eyes are more developed.^[37][38]

Circulatory system

Octopuses have a closed circulatory 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 is inactive when the animal is swimming and thus it tires quickly and prefers to crawl.^[39][40] Octopus blood contains the copper-rich protein haemocyanin to transport oxygen. This makes the blood very viscous and it requires considerable pressure to pump it around the body; octopuses' blood pressures can exceed 75 mmHg (10 kPa).^[39][40][41] In cold conditions with low oxygen levels, haemocyanin transports oxygen more efficiently than haemoglobin. 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 cellular endothelium which is quite unlike that of most other invertebrates. 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

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] The lamella structure 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 the buccal mass which consists of the mouth with its chitinous beak, the pharynx, radula and 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 the gastrointestinal tract, which is mostly suspended from the roof of the mantle cavity by numerous membranes. The tract consists of a crop, where the food is stored; a stomach, where food is ground down; a caecum where the now sludgy food is sorted into fluids and particles and which plays an important role in absorption; the digestive 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]

During osmoregulation, fluid is added to the pericardia of the branchial hearts. The octopus has two nephridia (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 the vena cava expands 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]

Nervous system and senses

Octopuses (along with cuttlefish) have the highest brain-to-body mass ratios of all invertebrates;^[48] this is greater than that of many vertebrates.^[49] Octopuses have the same jumping genes that are active in the human brain, implying an evolutionary convergence at molecular level.^[50] The nervous system is complex, only part of which is localised in its brain, which is contained in a cartilaginous capsule.^[51] Two-thirds of an octopus's neurons are in the nerve cords of its arms; these are capable of complex reflex actions without input from the brain.^[52] Unlike vertebrates, the complex motor skills of octopuses are not organised in their brains via internal somatotopic maps of their bodies.^[53]

Eye of common octopus

Like other cephalopods, octopuses have camera-like eyes,^[48] and can distinguish the polarisation of light. Colour vision appears to vary from species to species, for example being present in O. aegina but absent in O. vulgaris.^[54] Opsins in the skin respond to different wavelengths of light and help the animals choose a coloration that camouflages them; the chromatophores in the skin can respond to light independently of the eyes.^[55][56] An alternative hypothesis is that cephalopod eyes in species which only have a single photoreceptor protein may use chromatic aberration to turn monochromatic vision into colour vision, though this sacrifices image quality. This would explain pupils shaped like the letter U, the letter W, or a dumbbell, as well as explaining the need for colourful mating displays.^[57]

Attached to the brain 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. They provide information on the position of the body relative to gravity and can detect angular acceleration. An autonomic response 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.^[58]

Octopuses have an excellent somatosensory system. Their suction cups are equipped with chemoreceptors so they can taste what they touch. Octopus arms move easily because the sensors recognise octopus skin and prevent self-attachment.^[59] Octopuses appear to have poor proprioceptive sense and must observe the arms visually to keep track of their position.^[60][61]

Ink sac

The ink sac of an octopus is located under the digestive gland. A gland attached to the sac produces the ink, 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.^[62] The main pigment in the ink is melanin, which gives it its black colour.^[63] Cirrate octopuses usually lack the ink sac.^[37]

Discover more about Anatomy and physiology related topics

Cephalopod size

Cephalopod size

Cephalopods, which include squids and octopuses, vary enormously in size. The smallest are only about 1 centimetre (0.39 in) long and weigh less than 1 gram (0.035 oz) at maturity, while the largest—the giant and colossal squids—can exceed 10 metres (33 ft) in length and weigh close to half a tonne (1,100 lb), making them the largest living invertebrates. Living species range in mass more than three-billion-fold, or across nine orders of magnitude, from the lightest hatchlings to the heaviest adults. Certain cephalopod species are also noted for having individual body parts of exceptional size. The giant and colossal squids, for example, have the largest known eyes among living animals.

Giant Pacific octopus

Giant Pacific octopus

The giant Pacific octopus, also known as the North Pacific giant octopus, is a large marine cephalopod belonging to the genus Enteroctopus. Its spatial distribution includes the coastal North Pacific, along Mexico, The United States, Canada, Russia, Eastern China, Japan, and the Korean Peninsula. It can be found from the intertidal zone down to 2,000 m (6,600 ft), and is best adapted to cold, oxygen-rich water. It is the largest octopus species, based on a scientific record of a 71-kilogram (157-pound) individual weighed live.

Octopus wolfi

Octopus wolfi

Octopus wolfi, the star-sucker pygmy octopus, is the smallest known octopus. It is found in fairly shallow waters in the western Pacific. It is characterised by a pattern of "papillate fringes" around the edge of the suckers near the arm tip.

Cephalopod limb

Cephalopod limb

All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles.

Mantle (mollusc)

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.

Cephalopod beak

Cephalopod beak

All extant cephalopods have a two-part beak, or rostrum, situated in the buccal mass and surrounded by the muscular head appendages. The dorsal (upper) mandible fits into the ventral (lower) mandible and together they function in a scissor-like fashion. The beak may also be referred to as the mandibles or jaws.

Hectocotylus

Hectocotylus

A hectocotylus is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, analogously to a penis in other animals, or he may wrench it off and present it to the female.

Epidermis

Epidermis

The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water released from the body into the atmosphere through transepidermal water loss.

Dermis

Dermis

The dermis or corium is a layer of skin between the epidermis and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided into two layers, the superficial area adjacent to the epidermis called the papillary region and a deep thicker area known as the reticular dermis. The dermis is tightly connected to the epidermis through a basement membrane. Structural components of the dermis are collagen, elastic fibers, and extrafibrillar matrix. It also contains mechanoreceptors that provide the sense of touch and thermoreceptors that provide the sense of heat. In addition, hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels, nerves and blood vessels are present in the dermis. Those blood vessels provide nourishment and waste removal for both dermal and epidermal cells.

Collagen

Collagen

Collagen is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of amino acids bound together to form a triple helix of elongated fibril known as a collagen helix. It is mostly found in connective tissue such as cartilage, bones, tendons, ligaments, and skin.

Muscular hydrostat

Muscular hydrostat

A muscular hydrostat is a biological structure found in animals. It is used to manipulate items or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton.

Acetabulum (morphology)

Acetabulum (morphology)

Acetabulum in invertebrate zoology is a saucer-shaped organ of attachment in some annelid worms and flatworms. It is a specialised sucker for parasitic adaptation in trematodes by which the worms are able to attach on the host. In annelids, it is basically a locomotory organ for attaching to a substratum. The name also applies to the suction appendage on the arms of cephalopod molluscs such as squid, octopus, cuttlefish, Nautilus, etc.

Reproduction

Adult male Tremoctopus violaceus with hectocotylus

Octopuses are gonochoric and have a single, posteriorly-located gonad which is associated with the coelom. The testis in males and the ovary in females bulges into the gonocoel and the gametes are released here. The gonocoel is connected by the gonoduct to the mantle cavity, which it enters at the gonopore.^[26] An optic gland creates 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.^[64][65]

When octopuses reproduce, the male uses a specialised 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.^[66] The hectocotylus in benthic octopuses 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 the giant 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.^[67] A complex hydraulic mechanism releases the sperm from the spermatophore, and it is stored internally by the female.^[26]

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.^[67] In colder waters, such as those off Alaska, it may take up to ten months for the eggs to completely develop.^[68]: 74 The female aerates them and keeps them clean; if left untended, many will die.^[69] She does not feed during this time and dies soon after. Males become senescent and die a few weeks after mating.^[64]

The eggs have large yolks; cleavage (division) is superficial and a germinal disc develops at the pole. During gastrulation, 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]

Octopus paralarva, a planktonic hatchling

Most young octopuses hatch as paralarvae and are planktonic for weeks to months, depending on the species and water temperature. They feed on copepods, arthropod larvae and other zooplankton, eventually settling on the ocean floor and developing directly into adults with no distinct metamorphoses that are present in other groups of mollusc larvae.^[26] Octopus species that produce larger eggs – including the southern blue-ringed, Caribbean reef, California two-spot, Eledone moschata^[70] and deep sea octopuses – instead hatch as benthic animals similar to the adults.^{[68]: 74–75}

In the argonaut (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.^[71]

Lifespan

Octopuses have a relatively short lifespan; some species live for as little as six months. The Giant Pacific octopus, one of the two largest species of octopus, may live for as much as five years. Octopus lifespan is limited by reproduction.^[72] 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.^[73] The larger Pacific striped octopus (LPSO) is an exception, as it can reproduce repeatedly over a life of around two years.^[72]

Octopus reproductive organs mature due to the hormonal influence of the optic gland but result in the inactivation of their digestive glands. Unable to feed, the octopus typically dies of starvation.^[73] Experimental removal of both optic glands after spawning was found to result in the cessation of broodiness, 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.^[74]

Discover more about Lifecycle related topics

Hectocotylus

Hectocotylus

A hectocotylus is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, analogously to a penis in other animals, or he may wrench it off and present it to the female.

Gonochorism

Gonochorism

In biology, gonochorism is a sexual system where there are only two sexes and most organisms are either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric.

Coelom

Coelom

The coelom is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.

Ovary

Ovary

The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary found on each side of the body. The ovaries also secrete hormones that play a role in the menstrual cycle and fertility. The ovary progresses through many stages beginning in the prenatal period through menopause. It is also an endocrine gland because of the various hormones that it secretes.

Gamete

Gamete

A gamete is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum— and a male produces the smaller type—called a sperm. Sperm cells or spermatozoa are small and motile due to the flagellum, a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell or ovum is relatively large and non-motile. In short a gamete is an egg cell or a sperm. In animals, ova mature in the ovaries of females and sperm develop in the testes of males. During fertilization, a spermatozoon and ovum unite to form a new diploid organism. Gametes carry half the genetic information of an individual, one ploidy of each type, and are created through meiosis, in which a germ cell undergoes two fissions, resulting in the production of four gametes. In biology, the type of gamete an organism produces determines the classification of its sex.

Gonopore

Gonopore

A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is the opening of the common oviduct, and in the male, it is the opening of the ejaculatory duct.

Optic gland

Optic gland

The optic glands are endocrine organs in the octopus and squid that play a role in sexual development and senescence. They lie between the brain and optic lobes. The optic gland in female octopuses is associated with their maternal behavior of guarding their eggs without feeding. This self-starvation results in the death of the female octopus before her eggs have hatched. In captivity, some starving females also self-mutilate and entangle their arms in an apparent effort to die even faster.

Giant Pacific octopus

Giant Pacific octopus

The giant Pacific octopus, also known as the North Pacific giant octopus, is a large marine cephalopod belonging to the genus Enteroctopus. Its spatial distribution includes the coastal North Pacific, along Mexico, The United States, Canada, Russia, Eastern China, Japan, and the Korean Peninsula. It can be found from the intertidal zone down to 2,000 m (6,600 ft), and is best adapted to cold, oxygen-rich water. It is the largest octopus species, based on a scientific record of a 71-kilogram (157-pound) individual weighed live.

Alaska

Alaska

Alaska is a U.S. state on the northwest extremity of North America. A semi-exclave of the U.S., it borders British Columbia and the Yukon in Canada to the east, and it shares a western maritime border in the Bering Strait with the Russian Federation's Chukotka Autonomous Okrug. To the north are the Chukchi and Beaufort Seas of the Arctic Ocean, and the Pacific Ocean lies to the south and southwest.

Cleavage (embryo)

Cleavage (embryo)

In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals.

Gastrulation

Gastrulation

Gastrulation is the stage in the early embryonic development of most animals, during which the blastula, or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

Paralarva

Paralarva

Paralarvae are young cephalopods in the planktonic stages between hatchling and subadult. This stage differs from the larval stage of animals that undergo true metamorphosis. Paralarvae have been observed only in members of the orders Octopoda and Teuthida.

Octopus cyanea in Kona, Hawaii

Octopuses live in every ocean, and different species have adapted to different marine habitats. As juveniles, common octopuses inhabit shallow tide pools. The Hawaiian day octopus (Octopus cyanea) lives on coral reefs; argonauts drift in pelagic waters. Abdopus aculeatus mostly lives in near-shore seagrass beds. 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), and Vulcanoctopus hydrothermalis lives near hydrothermal vents at 2,000 m (6,600 ft).^[29] The cirrate species are often free-swimming and live in deep-water habitats.^[38] Although several species are known to live at bathyal and abyssal depths, there is only a single indisputable record of an octopus in the hadal zone; a species of Grimpoteuthis (dumbo octopus) photographed at 6,957 m (22,825 ft).^[75] No species are known to live in fresh water.^[76]

Discover more about Distribution and habitat related topics

Tide pool

Tide pool

A tide pool or rock pool is a shallow pool of seawater that forms on the rocky intertidal shore. These pools typically range from a few inches to a few feet deep and a few feet across. Many of these pools exist as separate bodies of water only at low tide, as seawater gets trapped when the tide recedes. Tides are caused by the gravitational pull of the sun and moon. A tidal cycle is usually about 25 hours and consists of one or two high tides and two low tides.

Octopus cyanea

Octopus cyanea

Octopus cyanea, also known as the big blue octopus or day octopus, is an octopus in the family Octopodidae. It occurs in both the Pacific and Indian Oceans, from Hawaii to the eastern coast of Africa. O. cyanea grows to 16 cm in mantle length with arms to at least 80 cm. This octopus was described initially by the British zoologist John Edward Gray in 1849; the type specimen was collected off Australia and is at the Natural History Museum in London.

Argonaut (animal)

Argonaut (animal)

The argonauts are a group of pelagic octopuses. They are also called paper nautili, referring to the paper-thin eggcase that females secrete. This structure lacks the gas-filled chambers present in chambered nautilus shells and is not a true cephalopod shell, but rather an evolutionary innovation unique to the genus. It is used as a brood chamber, and to trap surface air to maintain buoyancy. It was once speculated that argonauts did not manufacture their eggcases but utilized shells abandoned by other organisms, in the manner of hermit crabs. Experiments by pioneering marine biologist Jeanne Villepreux-Power in the early 19th century disproved this hypothesis, as Villepreux-Power successfully reared argonaut young and observed their shells' development.

Pelagic zone

Pelagic zone

The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word pelagic is derived from Ancient Greek πέλαγος (pélagos) 'open sea'. The pelagic zone can be thought of as an imaginary cylinder or water column between the surface of the sea and the bottom. Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients all change. Somewhat analogous to stratification in the Earth's atmosphere, but depending on how deep the water is, the water column can be divided vertically into up to five different layers.

Abdopus aculeatus

Abdopus aculeatus

Abdopus aculeatus is a small octopus species in the order Octopoda. A. aculeatus has the common name of algae octopus due to its typical resting camouflage, which resembles a gastropod shell overgrown with algae. It is small in size with a mantle around the size of a small orange and arms 25 cm in length, and is adept at mimicking its surroundings.

Seagrass

Seagrass

Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families, all in the order Alismatales. Seagrasses evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago.

Bathypolypus arcticus

Bathypolypus arcticus

Bathypolypus arcticus, the North Atlantic octopus, deep sea octopus or spoonarm octopus is a small species of demersal octopus of the North Atlantic. It is usually found at depths of 200 to 600 m (660–1,970 ft) where the temperature is between 2 and 6 °C (36–43 °F).

Vulcanoctopus hydrothermalis

Vulcanoctopus hydrothermalis

Vulcanoctopus hydrothermalis is a small benthic octopus endemic to hydrothermal vents.

Hydrothermal vent

Hydrothermal vent

A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.

Cirrina

Cirrina

Cirrina or Cirrata is a suborder and one of the two main divisions of octopuses. Cirrate octopuses have a small, internal shell and two fins on their head, while their sister suborder Incirrina has neither. The fins of cirrate octopods are associated with a unique cartilage-like shell in a shell sac. In cross-section, the fins have distinct proximal and distal regions, both of which are covered by a thin surface sheath of muscle.

Hadal zone

Hadal zone

The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around 6 to 11 km below sea level, and exists in long, narrow, topographic V-shaped depressions.

Grimpoteuthis

Grimpoteuthis

Grimpoteuthis is a genus of pelagic finned or cirrate octopods known as the dumbo octopuses. The name "dumbo" originates from their resemblance to the title character of Disney's 1941 film Dumbo, having a prominent ear-like fin which extends from the mantle above each eye. There are 17 species recognized in the genus. Prey include crustaceans, bivalves, worms and copepods. The average life span of various Grimpoteuthis species is 3 to 5 years.

Most species are solitary when not mating,^[77] 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.^[78] The LPSO has been described as particularly social, living in groups of up to 40 individuals.^[79][80] 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 not territorial but generally remain in a home range; they may leave in search of food. They can navigate back to a den without having to retrace their outward route.^[81] They are not migratory.^[82]

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 a midden of dead and uneaten food items. Other creatures, such as fish, crabs, molluscs and echinoderms, often share the den with the octopus, either because they have arrived as scavengers, or because they have survived capture.^[83] On rare occasions, octopuses hunt cooperatively with other species, with fish as their partners. They regulate the species composition of the hunting group — and the behavior of their partners — by punching them.^[84]

Feeding

Veined octopus eating a crab

Nearly all octopuses are predatory; bottom-dwelling octopuses eat mainly crustaceans, polychaete worms, and other molluscs such as whelks and clams; open-ocean octopuses eat mainly prawns, fish and other cephalopods.^[85] Major items in the diet of the giant Pacific octopus include bivalve molluscs such as the cockle Clinocardium nuttallii, clams and scallops and crustaceans such as crabs and spider crabs. Prey that it is likely to reject include moon snails because they are too large and limpets, rock scallops, chitons and abalone, because they are too securely fixed to the rock.^[83] Small cirrate octopuses such as those of the genera Grimpoteuthis and Opisthoteuthis typically prey on polychaetes, copepods, amphipods and isopods.^[86]

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 paralysing saliva then dismember them with their beaks.^[85][87] Octopuses feed on shelled molluscs either by forcing the valves apart, or by drilling a hole in the shell to inject a nerve toxin.^[88][87] 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 for O. vulgaris to 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.^[89]

Some species have other modes of feeding. Grimpoteuthis has a reduced or non-existent radula and swallows prey whole.^[37] In the deep-sea genus Stauroteuthis, some of the muscle cells that control the suckers in most species have been replaced with photophores which are believed to fool prey by directing them to the mouth, making them one of the few bioluminescent octopuses.^[90]

Locomotion

Octopuses swim with their arms trailing behind.

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, followed by swimming and crawling.^[91] 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.^[92] 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 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.^[91]

Movements of the finned species Cirroteuthis 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 producing peristaltic waves. This moves the body slowly.^[37]

In 2005, Adopus aculeatus and veined octopus (Amphioctopus marginatus) were found to walk on two arms, while at the same time mimicking plant matter.^[93] This form of locomotion allows these octopuses to move quickly away from a potential predator without being recognised.^[91] Some species of octopus can crawl out of the water briefly, which they may do between tide pools.^[94][95] "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.^[96]

Intelligence

Octopus opening a container by unscrewing its cap

Octopuses are highly intelligent.^[97] Maze and problem-solving experiments have shown evidence of a memory system that can store both short- and long-term memory.^[98] Young octopuses learn nothing from their parents, as adults provide no parental care beyond tending to their eggs until the young octopuses hatch.^[68]: 75

In laboratory experiments, octopuses can readily be trained to distinguish between different shapes and patterns. They have been reported to practise observational learning,^[99] although the validity of these findings is contested.^[97] Octopuses have also been observed in what has been described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.^[100] Octopuses often break out of their aquariums and sometimes into others in search of food.^{[94][101][102]} The veined octopus collects discarded coconut shells, then uses them to build a shelter, an example of tool use.^[96]

Camouflage and colour change

Octopuses use camouflage when hunting and to avoid predators. To do this they use specialised skin cells which change the appearance of the skin by adjusting its colour, opacity, or reflectivity. 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.^[103] This colour-changing ability is also used to communicate with or warn other octopuses.^[104]

Octopuses can create distracting patterns with waves of dark coloration 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.^[104]

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.^[105]

Defence

Warning display of greater blue-ringed octopus (Hapalochlaena lunulata)

Aside from humans, octopuses may be preyed on by fishes, seabirds, sea otters, pinnipeds, cetaceans, and other cephalopods.^[106] Octopuses typically hide or disguise themselves by camouflage and mimicry; some have conspicuous warning coloration (aposematism) or deimatic behaviour.^[104] 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% of Enteroctopus dofleini in one study had scars, with 50% having amputated arms.^[106] 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.^[107] The Atlantic white-spotted octopus (Callistoctopus macropus) turns bright brownish red with oval white spots all over in a high contrast display.^[108] Displays are often reinforced by stretching out the animal's arms, fins or web to make it look as big and threatening as possible.^[109]

Once they have been seen by a predator, they commonly try to escape but can also use distraction with an ink cloud ejected from the ink sac. The ink is thought to reduce the efficiency of olfactory organs, which would aid evasion from predators that employ smell for hunting, such as sharks. Ink clouds of some species might act as pseudomorphs, or decoys that the predator attacks instead.^[110]

When under attack, some octopuses can perform arm autotomy, in a manner similar to the way skinks and other lizards detach their tails. The crawling arm may distract would-be predators. Such severed arms remain sensitive to stimuli and move away from unpleasant sensations.^[111] Octopuses can replace lost limbs.^[112]

Some octopuses, such as the mimic octopus, can combine their highly flexible bodies with their colour-changing ability to mimic other, more dangerous animals, such as lionfish, sea snakes, and eels.^[113][114]

Pathogens and parasites

The diseases and parasites that affect octopuses have been little studied, but cephalopods are known to be the intermediate or final hosts of various parasitic cestodes, nematodes and copepods; 150 species of protistan and metazoan parasites have been recognised.^[115] The Dicyemidae are a family of tiny worms that are found in the renal appendages of many species;^[116] it is unclear whether they are parasitic or endosymbionts. Coccidians in the genus Aggregata living in the gut cause severe disease to the host. Octopuses have an innate immune system; their haemocytes respond to infection by phagocytosis, 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.^[117] A gram-negative bacterium, Vibrio lentus, can cause skin lesions, exposure of muscle and sometimes death.^[118]

Discover more about Behaviour and ecology related topics

Animal navigation

Animal navigation

Animal navigation is the ability of many animals to find their way accurately without maps or instruments. Birds such as the Arctic tern, insects such as the monarch butterfly and fish such as the salmon regularly migrate thousands of miles to and from their breeding grounds, and many other species navigate effectively over shorter distances.

Crab

Crab

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen), usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. They first appeared during the Jurassic Period.

Echinoderm

Echinoderm

An echinoderm is any member of the phylum Echinodermata. The adults are recognisable by their radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest entirely marine phylum. The first definitive echinoderms appeared near the start of the Cambrian.

Amphioctopus marginatus

Amphioctopus marginatus

Amphioctopus marginatus, also known as the coconut octopus and veined octopus, is a medium-sized cephalopod belonging to the genus Amphioctopus. It is found in tropical waters of the western Pacific Ocean. It commonly preys upon shrimp, crabs, and clams, and displays unusual behavior including bipedal walking and tool use.

Crustacean

Crustacean

Crustaceans belong to the subphylum Crustacea,, and form a large, diverse group of arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed group referred to as Pancrustacea. Some crustaceans are more closely related to insects and the other hexapods than they are to certain other crustaceans.

Clam

Clam

Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shells of equal size connected by two adductor muscles and have a powerful burrowing foot. They live in both freshwater and marine environments; in salt water they prefer to burrow down into the mud and the turbidity of the water required varies with species and location; the greatest diversity of these is in North America.

Bivalvia

Bivalvia

Bivalvia, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bivalves have no head and they lack some usual molluscan organs, like the radula and the odontophore. The class includes the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment, where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops and file shells, can swim. The shipworms bore into wood, clay, or stone and live inside these substances.

Clinocardium nuttallii

Clinocardium nuttallii

Clinocardium nuttallii, the basket cockle or Nuttall's cockle, is a species of large edible saltwater clam, a marine bivalve mollusc in the family Cardiidae, the cockles.

Majoidea

Majoidea

The Majoidea are a superfamily of crabs which includes the various spider crabs.

Limpet

Limpet

Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended independently from different ancestral gastropods. This general category of conical shell is known as "patelliform" (dish-shaped). All members of the large and ancient marine clade Patellogastropoda are limpets. Within that clade, the members of the Patellidae family in particular are often referred to as "true limpets".

Crassadoma

Crassadoma

Crassadoma is a genus of rock scallops, marine bivalve molluscs in the family Pectinidae. It is monotypic, the only species being Crassadoma gigantea, the rock scallop, giant rock scallop or purple-hinge rock scallop. Although the small juveniles are free-swimming, they soon become sessile, and are cemented to the substrate. These scallops occur in the eastern Pacific Ocean.

Chiton

Chiton

Chitons are marine molluscs of varying size in the class Polyplacophora, formerly known as Amphineura. About 940 extant and 430 fossil species are recognized.

The scientific name Octopoda was first coined and given as the order of octopuses in 1818 by English biologist William Elford Leach,^[119] who classified them as Octopoida the previous year.^[2] The Octopoda consists of around 300 known species^[120] and were historically divided into two suborders, the Incirrina and the Cirrina.^[38] More recent evidence suggests Cirrina is merely the most basal species, not a unique clade.^[121] 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 of stylets or absent altogether.^[122]

Fossil history and phylogeny

The octopuses evolved from the Muensterelloidea (fossil pictured) in the Jurassic period.^[123]

The Cephalopoda evolved from a mollusc resembling the Monoplacophora in the Cambrian some 530 million years ago. The Coleoidea diverged from the nautiloids in the Devonian some 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 the Permian, split into the Vampyropoda and the Decabrachia.^[124] The octopuses arose from the Muensterelloidea within the Vampyropoda in the Jurassic. The earliest octopus likely lived near the sea floor (benthic to demersal) in shallow marine environments.^{[124][125][123]} 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 the nautiloids and the extinct Ammonoidea.^[126] They have eight limbs like other Coleoidea, but lack the extra specialised feeding appendages known as tentacles which are longer and thinner with suckers only at their club-like ends.^[127] The vampire squid (Vampyroteuthis) also lacks tentacles but has sensory filaments.^[128]

The cladograms are based on Sanchez et al., 2018, who created a molecular phylogeny based on mitochondrial and nuclear DNA marker sequences.^[121] The position of the Eledonidae is from Ibáñez et al., 2020, with a similar methodology.^[129] Dates of divergence are from Kröger et al., 2011 and Fuchs et al., 2019.^[124][123]

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.

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. 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 or fruit fly. Coleoids rely mostly on ADAR enzymes for RNA editing, which requires large double-stranded RNA structures 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.^[130][131]

The octopus genome is unremarkably bilaterian except for large developments of two gene families: protocadherins, which regulate the development of neurons; and the C2H2 zinc-finger transcription factors. Many genes specific to cephalopods are expressed in the animals' skin, suckers, and nervous system.^[48]

Discover more about Evolution related topics

Evolution of cephalopods

Evolution of cephalopods

The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.

Clade

Clade

In biological phylogenetics, a clade, also known as a monophyletic group or natural group, is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. In the taxonomical literature, sometimes the Latin form cladus is used rather than the English form.

Muensterelloidea

Muensterelloidea

Muensterelloidea is a superfamily of stem-octopod cephalopods from the Early Jurassic to Late Cretaceous. Two families are currently identified, Muensterellidae, and Patelloctopodidae. The clade is the ancestral group from which modern octopus arose.

Jurassic

Jurassic

The Jurassic is a geologic period and stratigraphic system that spanned from the end of the Triassic Period 201.4 million years ago (Mya) to the beginning of the Cretaceous Period, approximately 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era and is named after the Jura Mountains, where limestone strata from the period were first identified.

Monoplacophora

Monoplacophora

Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record, and were thought to have become extinct 375 million years ago.

Cambrian

Cambrian

The Cambrian Period is the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux. The period was established as "Cambrian series" by Adam Sedgwick, who named it after Cambria, the Latin name for 'Cymru' (Wales), where Britain's Cambrian rocks are best exposed. Sedgwick identified the layer as part of his task, along with Roderick Murchison, to subdivide the large "Transition Series", although the two geologists disagreed for a while on the appropriate categorization. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian biology surpasses that of some later periods.

Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied.

Permian

Permian

The Permian is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleozoic Era; the following Triassic Period belongs to the Mesozoic Era. The concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the region of Perm in Russia.

Benthic zone

Benthic zone

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "the depths." Organisms living in this zone are called benthos and include microorganisms as well as larger invertebrates, such as crustaceans and polychaetes. Organisms here generally live in close relationship with the substrate and many are permanently attached to the bottom. The benthic boundary layer, which includes the bottom layer of water and the uppermost layer of sediment directly influenced by the overlying water, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud.

Demersal zone

Demersal zone

The demersal zone is the part of the sea or ocean consisting of the part of the water column near to the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone.

Ammonoidea

Ammonoidea

Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appeared during the Devonian, with the last species vanishing during the Cretaceous–Paleogene extinction event.

Coleoidea

Coleoidea

Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less". Unlike its extant sister group, Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal cuttlebone, gladius, or shell that is used for buoyancy or support. Some species have lost their cuttlebone altogether, while in some it has been replaced by a chitinous support structure. A unique trait of the group is the ability to edit their own RNA.

Minoan clay vase with octopus decoration, c. 1500 BC

In culture

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 Age Minoan Crete at Knossos (1900–1100 BC) depicts a fisherman carrying an octopus.^[132] The terrifyingly powerful Gorgon of Greek mythology may 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.^[133] The Kraken are 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. Linnaeus included it in the first edition of his 1735 Systema Naturae.^[134][135] One translation of the Hawaiian creation myth the Kumulipo suggests that the octopus is the lone survivor of a previous age.^{[136][137][138]} The Akkorokamui is a gigantic octopus-like monster from Ainu folklore, worshipped in Shinto.^[139]

A battle with an octopus plays a significant role in Victor Hugo's 1866 book Travailleurs de la mer (Toilers of the Sea).^[140] Ian Fleming's 1966 short story collection Octopussy and The Living Daylights, and the 1983 James Bond film were partly inspired by Hugo's book.^[141] Japanese erotic art, shunga, includes ukiyo-e woodblock prints such as Katsushika Hokusai's 1814 print Tako to ama (The Dream of the Fisherman's Wife), in which an ama diver is sexually intertwined with a large and a small octopus.^[142][143] The print is a forerunner of tentacle erotica.^[144] The biologist P. Z. Myers noted in his science blog, Pharyngula, that octopuses appear in "extraordinary" graphic illustrations involving women, tentacles, and bare breasts.^[145][146]

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.^[147]

Danger

Pen and wash drawing of an imagined colossal octopus attacking a ship, by the malacologist Pierre de Montfort, 1801

Octopuses generally avoid humans, but incidents 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.^[148] All species are venomous, but only blue-ringed octopuses have venom that is lethal to humans.^[149] 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 contains tetrodotoxin, which causes paralysis by blocking the transmission of nerve impulses to the muscles. This causes death by respiratory failure leading to cerebral anoxia. No antidote is known, but if breathing can be kept going artificially, patients recover within 24 hours.^[150][151] Bites have been recorded from captive octopuses of other species; they leave swellings which disappear in a day or two.^[152]

Fisheries

Octopus fisheries exist around the world with total catches varying between 245,320 and 322,999 metric tons from 1986 to 1995.^[153] The world catch peaked in 2007 at 380,000 tons, and had fallen by a tenth by 2012.^[154] Methods to capture octopuses include pots, traps, trawls, snares, drift fishing, spearing, hooking and hand collection.^[153] Octopus is eaten in many cultures, such as on the Mediterranean and Asian coasts.^[155] The arms and sometimes other body parts are prepared in various ways, often varying by species or geography. Live octopuses are eaten in several countries around the world, including the US.^[156][157] Animal welfare groups have objected to this practice on the basis that octopuses can experience pain.^[158] Octopuses have a food conversion efficiency greater than that of chickens, making octopus aquaculture a possibility.^[159] Octopuses compete with human fisheries targeting other species, and even rob traps and nets for their catch; they may, themselves, be caught as bycatch if they cannot get away.^[160]

In science and technology

In classical Greece, Aristotle (384–322 BC) commented on the colour-changing abilities of the octopus, both for camouflage and for signalling, in his Historia 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 also when alarmed."^[161] 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 zoologist Georges Cuvier, who supposed it to be a parasitic worm, naming it as a new species, Hectocotylus octopodis.^[162][163] Other zoologists thought it a spermatophore; the German zoologist Heinrich Müller believed it was "designed" to detach during copulation. In 1856 the Danish zoologist Japetus Steenstrup demonstrated that it is used to transfer sperm, and only rarely detaches.^[164]

Flexible biomimetic 'Octopus' robotics arm. The BioRobotics Institute, Scuola Superiore Sant'Anna, Pisa, 2011^[165]

Octopuses offer many possibilities 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 of protocadherins (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] Having independently evolved mammal-like intelligence, octopuses have been compared by the philosopher Peter Godfrey-Smith, who has studied the nature of intelligence,^[166] to hypothetical intelligent extraterrestrials.^[167] Their problem-solving skills, along with their mobility and lack of rigid structure enable them to escape from supposedly secure tanks in laboratories and public aquariums.^[168]

Due to their intelligence, octopuses are listed in some countries as experimental animals on which surgery may not be performed without anesthesia, 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 the Animals (Scientific Procedures) Act 1986.^[169] In 2012, this legislation was extended to include all cephalopods^[170] in accordance with a general EU directive.^[171]

Some robotics research is exploring biomimicry of 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.^[172][173] In 2017 a German company made an arm with a soft pneumatically controlled silicone gripper 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.^[174]

Discover more about Relationship to humans related topics

Minoan civilization

Minoan civilization

The Minoan civilization was a Bronze Age Aegean civilization on the island of Crete and other Aegean Islands, whose earliest beginnings date to c. 3500 BC, with the complex urban civilization beginning around 2000 BC, and then declining from c. 1450 BC until it ended around 1100 BC, during the early Greek Dark Ages, part of a wider bronze age collapse around the Mediterranean. It represents the first advanced civilization in Europe, leaving behind a number of massive building complexes, sophisticated art, and writing systems. Its economy benefited from a network of trade around much of the Mediterranean.

Knossos

Knossos

Knossos is the largest Bronze Age archaeological site on Crete and has been called Europe's oldest city.

Gorgon

Gorgon

A Gorgon is a creature in Greek mythology. Gorgons occur in the earliest examples of Greek literature. While descriptions of Gorgons vary, the term most commonly refers to three sisters who are described as having hair made of living, venomous snakes and horrifying visages that turned those who beheld them to stone. Traditionally, two of the Gorgons, Stheno and Euryale, were immortal, but their sister Medusa was not and was slain by the demigod and hero Perseus.

Greek mythology

Greek mythology

A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities of deities, heroes, and mythological creatures, and the origins and significance of the ancient Greeks' own cult and ritual practices. Modern scholars study the myths to shed light on the religious and political institutions of ancient Greece, and to better understand the nature of myth-making itself.

Medusa

Medusa

In Greek mythology, Medusa, also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those who gazed into her eyes would turn to stone. Most sources describe her as the daughter of Phorcys and Ceto, although the author Hyginus makes her the daughter of Gorgon and Ceto.

Kraken

Kraken

The kraken is a legendary sea monster of enormous size said to appear off the coasts of Norway.

Carl Linnaeus

Carl Linnaeus

Carl Linnaeus, also known after his ennoblement in 1761 as Carl von Linné, was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the modern system of naming organisms. He is known as the "father of modern taxonomy". Many of his writings were in Latin; his name is rendered in Latin as Carolus Linnæus and, after his 1761 ennoblement, as Carolus a Linné.

Creation myth

Creation myth

A creation myth or cosmogonic myth is a type of cosmogony, a symbolic narrative of how the world began and how people first came to inhabit it. While in popular usage the term myth often refers to false or fanciful stories, members of cultures often ascribe varying degrees of truth to their creation myths. In the society in which it is told, a creation myth is usually regarded as conveying profound truths – metaphorically, symbolically, historically, or literally. They are commonly, although not always, considered cosmogonical myths – that is, they describe the ordering of the cosmos from a state of chaos or amorphousness.

Kumulipo

Kumulipo

In Hawaiian religion, the Kumulipo is the creation chant, first recorded by Westerners in the 18th century. It also includes a genealogy of the members of Hawaiian royalty and was created in honor of Kalaninuiamamao and passed down orally to his daughter Alapaiwahine.

Akkorokamui

Akkorokamui

The Akkorokamui is a gigantic octopus-like monster from Ainu folklore, similar to the Nordic Kraken, which supposedly lurks in Uchiura Bay in Hokkaido. It is said that its enormous body can reach sizes of up to 120 metres in length. Its name can be translated as "tentacle-holding kamuy."

Ainu people

Ainu people

The Ainu are the indigenous people of the lands surrounding the Sea of Okhotsk, including Hokkaido Island, Northeast Honshu Island, Sakhalin Island, the Kuril Islands, the Kamchatka Peninsula and Khabarovsk Krai, before the arrival of the Yamato Japanese and Russians. These regions are referred to as Ezo (蝦夷) in historical Japanese texts.

Ian Fleming

Ian Fleming

Ian Lancaster Fleming was a British writer, best known for his postwar James Bond series of spy novels. Fleming came from a wealthy family connected to the merchant bank Robert Fleming & Co., and his father was the Member of Parliament (MP) for Henley from 1910 until his death on the Western Front in 1917. Educated at Eton, Sandhurst, and, briefly, the universities of Munich and Geneva, Fleming moved through several jobs before he started writing.