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An octopus (pl.: octopuses or octopodes)is a soft-bodied, eight-limbed mollusc of the order Octopoda (/ɒkˈtɒpədə/, ok-TOP-ə-də). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs. The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.
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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).The cirrate species are often free-swimming and live in deep-water habitats. 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). No species are known to live in fresh water.
Octopuses use camouflage when hunting and to avoid
predators. To do this, they use specialised skin
cells that 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. This colour-changing ability is also
used to communicate with or warn other octopuses. The energy
cost of the complete activation of the chromatophore system is very
high equally being nearly as much as all the energy used by an octopus
at rest.
Octopuses can create distracting patterns with waves
of dark colouration 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.