Octopus, or Devilfish, an eight-armed marine invertebrate (animal without a backbone). Octopuses are found in all oceans, from the water's edge to depths of at least three miles (4,800 m). They range in size from a span of 2 inches (5 cm) to 32 feet (10 m). Octopus meat is considered a delicacy by Mediterranean and Asiatic peoples. There are more then 200 species.

OctopusesOctopuses have acute vision and can detect colors.

Octopuses are basically shy and retiring. Unprovoked attacks on humans are rare, and, contrary to popular belief, octopuses do not wrap their arms around divers and squeeze them to death. Octopuses have poisonous saliva and the bite of some species can be fatal to humans.

An octopus has a beaked mouth, located on the lower side of the body. In the mouth is a radula, a toothed tongue for rasping. On each arm are one or two rows of sensitive suckers, with which the octopus distinguishes different textures and tastes. Above the arms are the brain and the eyes. The brain is highly developed, and the animal learns rapidly. Octopuses have acute vision and can detect colors. Like the human eye, the octopus eye consists of a lens, two fluid-filled chambers, and a light-sensitive retina. Octopuses lack hearing organs.

The globular body of an octopus is encased in a muscular organ called a mantle and contains viscera, an ink sac, and a small piece of shell. A tubular funnel, or siphon, protrudes from beneath the edge of the mantle. The funnel functions as an anus and, when water is forced through it, as a means of propulsion. (Octopuses are also able to "walk" on their arms.)

The skin of an octopus is dotted with cells called chromatophores. Each chromatophore contains one of four pigments: yellow, red, brown, or black. The pigment is visible only when the chromatophore is contracted. An octopus is able to vary in color by contracting only certain chromatophores at any one time. An octopus camouflages itself from its predators and prey by taking on the color of pale water, a sandy bottom, or a dark crevice. When approached by an enemy, it usually turns a deep pink, releases from the ink sac a cloud of dark ink (called sepia), and then becomes pale and darts off. If an octopus is caught by one of its arms, it can free itself by breaking off the arm; a new arm grows in place of the old.

Most octopuses live singly in small rocky crevices or other protected places on the ocean bottom. An octopus catches its prey—primarily shellfish and small fish—with its arms. The octopus then bites the prey, injecting it with poison and digestive enzymes. The enzymes partly liquefy the interior organs, which the octopus then sucks out.

During reproduction, the male octopus uses a specialized arm (the hectocotylus) to place sacs of sperm under the edge of the female's mantle. The female lays more than 100,000 eggs and attaches them in clusters with a gluelike secretion to the crevice wall. On hatching, the young of most octopuses resemble the adults.

The octopusThe octopus is an eight-armed marine invertebrate.
Why Does an Octopus Squirt Ink?

An octopus sometimes squirts a unique substance called ink into the water. It does this when it needs to defend itself from predators such as seals, whales, and sharks.

The Pacific giant octopus, like many other kinds of octopus, may squirt ink to make the water dark. That way, a predator can’t see the octopus escape. At other times, the inky cloud serves as a decoy. The cloud actually looks like the octopus itself. As the predator attacks the decoy, the real octopus makes its getaway!

Squirting ink isn’t the only way an octopus can escape its enemy, though. It can swim quickly away if it has to. It can also change its color. This often confuses the predator and gives the octopus time to flee. Other times, the octopus changes its color to blend in with its surroundings. Then the octopus may not be seen at all.

Which Octopus Is the Deadliest?

The blue-ringed octopus lives off the southern Australian coast. This tiny octopus is only 11/2 to 21/2 inches (4 to 6 centimeters) in diameter. But don’t let its small size fool you. This octopus is the most deadly octopus of all.

Instead of making ink, the blue-ringed octopus makes venom in its saliva. When the blue-ringed octopus bites its victim, it leaves saliva on the wound. The venom in the saliva causes the area around the wound to go numb. Soon, the venom causes the octopus’s victim to stop breathing. The venom is so deadly that it can kill even a human.

This octopus feeds mostly on crabs that live in shallow waters. When it is threatened, its blue rings glow. This warning is usually all that it takes to scare most predators away.

Octopuses are mollusks in the order Octopoda of the class Cephalopoda. Octopuses found along the Atlantic coast of North America include the common octopus, Octopus vulgaris; and the briar octopus, O. briareus. Octopuses found along the Pacific coast of North America include the giant Pacific octopus, O. dofleini; and the orange bigeye octopus, O. californica.