All octopuses are venomous, but only the blue-ringed octopuses are known to be deadly to humans. Strategies to defend themselves against predators include the expulsion of ink, the use of camouflage and threat displays, the ability to jet quickly through the water and hide, and even deceit. In most species, the male uses a specially adapted arm to deliver a bundle of sperm directly into the female's mantle cavity, after which he becomes senescent and dies, while the female deposits fertilised eggs in a den and cares for them until they hatch, after which she also dies. Most species grow quickly, mature early, and are short-lived. Octopuses inhabit various regions of the ocean, including coral reefs, pelagic waters, and the seabed some live in the intertidal zone and others at abyssal depths. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates. The siphon is used both for respiration and for locomotion, by expelling a jet of water. They trail their eight appendages behind them as they swim.
The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the center point of the eight limbs. The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids.
An octopus ( PL: octopuses or octopodes ) is a soft-bodied, eight-limbed mollusc of the order Octopoda ( / ɒ k ˈ t ɒ p ə d ə/, ok- TOP-ə-də ).
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