The Oxford English Dictionary lists "octopuses", "octopi", and "octopodes", in that order, calling "octopodes" rare and noting that "octopi" is based on a misunderstanding. It is considered to be the largest species. The giant Pacific octopus Enteroctopus dofleini is often cited as the largest known octopus species. 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.
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.
The octopus can squeeze through tiny gaps; even the larger species can pass through an opening close to 2. They can extend and contract, twist to left or right, bend at any place in any direction or be held rigid. 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: 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. The eyes of the octopus are large and are 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 and the slit-shaped pupil forms a hole in the iris and lies just behind.
The lens is suspended behind the pupil and photoreceptive retinal cells cover the back of the eye. The pupil can be adjusted in size and a retinal pigment screens incident light in bright conditions. Members of the suborder 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. Octopuses have three hearts; a systemic heart that circulates blood round the body and two branchial 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. This makes the blood very viscous and it requires considerable pressure to pump it round the body; octopuses' blood pressures can exceed 75 mmHg. The haemocyanin is dissolved in the plasma instead of being carried within blood cells, and gives the blood a bluish colour.
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 auxiliary hearts and back to the main heart.
Much of the venous system is contractile, which helps circulate the blood. 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. 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.
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. Its nervous system allows the arms to move with some autonomy. Nervous system and senses The octopus along with cuttlefish has the highest brain-to-body mass ratios of all invertebrates, and is also greater than many vertebrates.
Colour vision appears to vary from species to species, for example being present in O. 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.
The octopus's suction cups are equipped with chemoreceptors so the octopus can taste what it touches. Octopus arms do not become tangled or stuck to each other because the sensors recognise octopus skin and prevent self-attachment. As a result, the octopus does not possess stereognosis ; that is, it does not form a mental image of the overall shape of the object it is handling.
It can detect local texture variations, but cannot integrate the information into a larger picture. The neurological autonomy of the arms means the octopus has great difficulty learning about the detailed effects of its motions. It has a poor proprioceptive sense, and it knows what exact motions were made only by observing the arms visually.
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. 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. The gland may be triggered by environmental conditions such as temperature, light and nutrition, which thus control the timing of reproduction and lifespan.
In most species, fertilisation occurs in the mantle cavity. 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. Here she guards and cares for them for about five months days until they hatch. Males become senescent and die a few weeks after mating. 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 upwards 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 upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo develops. 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.
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. 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: Octopus reproductive organs mature due to the hormonal influence of the optic gland but result in the inactivation of their digestive glands, typically causing the octopus to eventually die from starvation.
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. This is likely the result of abundant food supplies combined with limited den sites. Octopuses are not territorial but generally remain in a home range; they may leave the area in search of food.
They can use navigation skills to return to a den without having to retrace their outward route. 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.
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.
The creature may make a jet-propelled pounce on prey and pull it towards 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. It takes about three hours for O. 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.
Grimpoteuthis has a reduced or non-existent radula and swallows prey whole. Octopuses mainly move about by relatively slow crawling, with some swimming in a head-first position. Jet propulsion , or backwards swimming, is their fastest means of locomotion, followed by swimming and crawling.
Several arms are extended forwards, some of the suckers adhere to the substrate and the animal hauls itself forwards 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.
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. When swimming, the head is at the front and the siphon is pointed backwards, but when jetting, the visceral hump leads, the siphon points towards 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.
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.