Octopus

The octopus (/ˈɒktəpʊs/ or /ˈɒktəpəs/; plural: octopuses, octopi, or octopodes; see below) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. It has a beak, with its mouth at the center point of the arms. It has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles),[3] allowing it to squeeze through tight places.[4] Octopuses are among the most intelligent and behaviorally diverse of all invertebrates.

Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. They trail their eight arms behind them as they swim. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.[5]

Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus Octopus.




History

Octopuses were depicted on Cretan and Minoan coins (1650 to 1500 BC and 1450 to 1375 BC) and were painted onto jars in the Mycenaen era (circa 1200 to 1100 BC). Cephalopods have been around for 500 million years, although octopus ancestors were in the Carboniferous seas around 300 million years ago. The oldest octopus fossil, Pohlsepia, can be found at the Field Museum in Chicago.[6]




Etymology and pluralization
The scientific Latin term octopus was derived from Ancient Greek ὀκτώπους (oktōpous), which literally translates to "eight-foot" (ὀκτώ "eight" + πούς "foot").[7][8][9] Related to the word "octopus" are the term "Octopoda" (the taxonomic order of cephalopod molluscs that comprises the octopuses) and the adjective "octopoid".[10] The standard pluralized form of "octopus" in the English language is "octopuses" /ˈɒktəpʊsɪz/,[11] although the Ancient Greek plural "octopodes" /ɒkˈtɒpədiːz/, has also been used historically.[10] The alternative plural "octopi" — which misguidedly assumes it is a Latin "-us"-word — is considered grammatically incorrect.[12][13][14][15] It is nevertheless used enough to make it notable, and was formally acknowledged by the descriptivist Merriam-Webster 11th Collegiate Dictionary and Webster's New World College Dictionary. The Oxford English Dictionary (2008 Draft Revision)[16] lists "octopuses", "octopi", and "octopodes", in that order, labelling "octopodes" as rare and noting that "octopi" derives from the apprehension that octōpus comes from Latin.[17] In contrast, New Oxford American Dictionary (3rd Edition 2010) lists "octopuses" as the only acceptable pluralization, with a usage note indicating "octopodes" as being still occasionally used but "octopi" as being incorrect.








The octopus's primary defense is to hide or to disguise itself through camouflage and mimicry.^[39] Octopuses have several secondary defenses (defenses they use once they have been seen by a predator). The most common secondary defense is fast escape. Other defenses include distraction with the use of ink sacs and autotomising limbs.
Most octopuses can eject a thick, blackish ink in a large cloud to aid in escaping from predators. The main coloring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin color. This ink cloud 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 serve as pseudomorphs, or decoys that the predator attacks instead.^[40]
The octopus's camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectivity of the epidermis. Chromatophores contain yellow, orange, red, brown, or black pigments; most species have three of these colors, while some have two or four. Other color-changing cells are reflective iridophores, and leucophores (white).^[41] This color-changing ability can also be used to communicate with or warn other octopuses. The highly venomous blue-ringed octopus becomes bright yellow with blue rings when it is provoked. Octopuses can use muscles in the skin to change the texture of their mantle to achieve a greater camouflage. In some species, the mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises. However, in some species, skin anatomy is limited to relatively patternless shades of one color, and limited skin texture. It is thought that octopuses that are day-active and/or live in complex habitats such as coral reefs have evolved more complex skin than their nocturnal and/or sand-dwelling relatives.^[39]
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 serves as a distraction to would-be predators. Such severed arms remain sensitive to stimuli and move away from unpleasant sensations.