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The octopus (Greek , 'eight-legs') is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are 289 different octopus species, which is over one-third of the total number of cephalopod species.
Physiology Octopuses are characterized by their eight arms (not tentacles), usually bearing suction cups. These arms are a type of muscular hydrostat. Unlike most other cephalopods, the majority of octopuses — those in the suborder most commonly known, Incirrata — have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squids. A beak, similar in shape to a parrot's beak, is their only hard part. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predatory fish. The octopuses in the less familiar Cirrata suborder have two fins and an internal shell, generally lessening their ability to squeeze into small spaces. Octopuses have a relatively short life span, and some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years under suitable circumstances. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch, for they neglect to eat during the (roughly) one month period spent taking care of their unhatched eggs. Octopuses have three hearts. Two pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Less efficient than the iron-rich hemoglobin of vertebrates, the hemocyanin is dissolved in the plasma instead of being bound in red blood cells and gives the blood a blue color. Octopuses draw water into their mantle cavity where it passes through its gills. As mollusks, octopuses have gills that are finely divided and vascularized outgrowths of either the outer or the inner body surface. Intelligence Octopuses are highly intelligent, probably more intelligent than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists , but maze and problem-solving experiments have shown that they do have both short- and long-term memory. Their short lifespans probably limit the amount they can ultimately learn. There has been much speculation to the effect that almost all octopus behaviors are independently learned rather than instinct-based, and although this is undoubtedly an interesting idea it remains largely unproven. It can, however, be logically concluded that they learn almost no behaviors from their parents, with whom young octopuses have very little contact. An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus's neurons are found in the nerve cords of its arms, which have a remarkable amount of autonomy. Octopus arms show a wide variety of complex reflex actions arising on at least three different levels of the nervous system. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the movements of other sea creatures. In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They have been reported to practice observational learning , although the validity of these findings is widely contested on a number of grounds . Octopuses have also been observed in what some have described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them . Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing boats and opened holds to eat crabs. In some countries, including the United States and the United Kingdom, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia. In the UK, cephalopods such as octopuses are regarded as honorary vertebrates under cruelty to animals legislation, extending to them protections not normally afforded to invertebrates. A common belief is that when stressed, an octopus may begin to eat its own arms. However, limited research conducted in this area has revealed that the cause of this abnormal behaviour, called autophagy, may be a virus that attacks the octopus's nervous system. Thus this behavior may be more correctly labeled as a neurological disorder . Defense
Reproduction When octopuses reproduce, males use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. The hectocotylus, usually the third right arm, detaches from the male during copulation. Males die within a few months after mating. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature. After they have been fertilized, the female lays roughly 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals). The female hangs these eggs in strings from the ceiling of her lair, or individually attaches them to the substratum depending on the species. The female does not eat during the roughly one-month period spent taking care of the unhatched eggs. After the eggs hatch, the mother dies shortly afterward and the young larval octopuses spend a period of time drifting in clouds of plankton, where they feed on copepods, larval crabs and larval seastars until they are ready to sink down to the bottom of the ocean, where the cycle repeats itself. In some deeper dwelling species, the young do not go through this period. This is a dangerous time for the larval octopuses; as they become part of the plankton cloud they are vulnerable to many plankton eaters. Sensation
Locomotion
Terminology Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses", and that octopi is misconceived and octopodes pedantic. Octopi derives from the mistaken notion that is a second declension Latin noun, which it is not. Rather, it is (Latinized) Greek, from , gender masculine, whose plural is (). If the word were native to Latin, it would be ('eight-foot') and the plural , analogous to and , as the plural form of ('foot') is . In modern, informal Greek, it is called , gender neuter, with plural form . Merriam-Webster and other dictionaries accept octopi as a plural form. The Oxford English Dictionary lists octopuses, octopi, and octopodes in order of descending frequency of use. The term octopod (either plural octopods and octopodes can be found) is taken from the taxonomic order octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food. Finally worth mentioning is Octopussy, a play on words which found its way back from the movie title to a term of endearment for the animals that originally inspired it. Relationship to humans Humans usually capture octopuses as food, however some are kept in captivity at public aquaria and in private establishments as pets. As food Many species of octopus are eaten as food by human cultures around the world. The arms and sometimes other parts of the body are prepared in various ways, often depending on the species being eaten. Octopus is a common ingredient in sushi. Some small species are sometimes eaten alive as a novelty. As pets Though octopuses can be difficult to keep in captivity, some people keep them as pets. Octopuses often escape even from supposedly secure tanks, due to their intelligence and problem solving skills. The variation in size and life span among octopus species makes it difficult to know how long a new specimen can naturally be expected to live. That is, a small octopus may be just born or may be an adult, depending on the species. By selecting a well-known species, such as the California Two-spot Octopus, one can choose a small octopus (around the size of a tennis ball) and be confident that it is young with a full life ahead of it. Octopuses are also quite strong for their size. Octopuses kept as pets have been known to open the covers of their aquariums and survive for a time in the air in order to get to a nearby feeder tank and gorge themselves on the fish there. They have also been known to catch and kill some species of sharks. Classification
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