An Octopus

Intruduction

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.                                                                                                                                                                             

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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]

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").^[6][7][8] Related to the word "octopus" are the term "Octopoda" (the taxonomic order of cephalopod molluscs that comprises the octopuses) and the adjective "octopoid".^[9]
The standard pluralized form of "octopus" in the English language is "octopuses" /ˈɒktəpʊsɪz/,^[10] although the Ancient Greek plural "octopodes" /ɒkˈtɒpədiːz/, has also been used historically.^[9] The alternative plural "octopi" — which misguidedly assumes it is a Latin "-us"-word — is considered grammatically incorrect.^{[11][12][13][14]} 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)^[15] 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.^[16] 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.^[17]

Biology

Schematic lateral aspect of octopod features

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A common octopus (Octopus vulgaris)

Octopuses are characterized by their eight arms, usually bearing suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish.^[18] Both types of limb are muscular hydrostats.

Octopuses can be divided into two suborders, the Incirrina (or Incirrata) and the Cirrina (or Cirrata). The incirrate octopuses are distinguished from the cirrate octopuses by their absence of "cirri" filaments (found with the suckers), as well as by the lack of paired swimming fins on the head. Unlike most other cephalopods, the majority of octopuses – those in the Incirrina – 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 squid. The beak, similar in shape to a parrot's beak, and made of chitin, is the only hard part of their bodies. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from moray eels or other predatory fish.

Intelligence

Main article: Cephalopod intelligence

Octopuses are highly intelligent, possibly more so than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists,^{[20][21][22][23]} but maze and problem-solving experiments have shown evidence of a memory system that can store both short- and long-term memory. It is not known precisely what contribution learning makes to adult octopus behavior. Young octopuses learn almost no behaviors from their parents, with whom they have very little contact. ^[24]
As stated above, even the octopuses that have the longest lifespan (the Giant Pacific Octopus) simply doesn't live long enough after the young are born to teach them very much. Approximately 6 weeks after mating, the female lays 20,000–100,000 eggs over the course of several days on the inner side of her rocky den. For the next 5–8 months she tends the eggs, carefully cleaning and aerating them until they hatch. The female does not leave her brood, even to eat, and will die within weeks or months after they hatch, gradually becoming weaker as she dies of starvation. The male dies shortly after mating. The typical life span of the octopus is between 3–5 years.
                                                              

                                                                 

Tool use                                            

Amphioctopus marginatus travels with shells it has collected for protection

The octopus has been shown to use tools. At least four specimens of the veined octopus (Amphioctopus marginatus) have been witnessed retrieving discarded coconut shells, manipulating them, and then reassembling them to use as shelter.

Protective legislation

Due to their intelligence, octopuses in some countries are on the list of 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.^[36] In 2012, this legislation was extended to include all cephalopods^[37] in accordance with a general EU directive.^[38]

Defense

Greater blue-ringed octopus (Hapalochlaena lunulata)

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]