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OCTOPUS,
carnivorous marine MOLLUSK of the genus Octopus, order
Octopoda, class Cephalopoda, found worldwide in tropical and warm
temperate waters. It is characterized by a soft body with a well-developed
brain and by eight arms bearing two rows of suckers each. As in
the vertebrates, the two large, complex eyes of the octopus are
cameralike in structure, and their vision is acute. The animals
can change the color and texture of their skin rapidly. Much of
their life is spent in hiding, and many species—such as
the common octopus, O. vulgaris, which can grow
to about 1 m (about 3 ft) long—choose a natural hole among
rocks or in a pile of rubble. A small species, however, such as
the pygmy octopus, O. joubini, which is about 2
cm (about 5 in) long, may prefer an empty clamshell having both
valves still connected by a ligament; settling into one half, it
pulls the shell shut with its suckers.
When an octopus emerges to find food such as crustaceans and
bivalve mollusks, it often lures its victim by wiggling the tip
of an arm like a worm; or it glides near and pounces on a crab,
sinking its beak into the shell and injecting a poison that kills;
the poison of a very few species is dangerous to humans. Octopuses
are preyed upon in turn by a number of fishes, including the moray
eel. When an octopus is attacked, it draws water into its mantle
cavity and expels it with great force through a funnel. The result
is a jet-propelled exit, usually behind a cloud of “ink,” a
dark substance the octopus ejects for defense. The ink of some species seems
to have a paralyzing effect on the sensory organs of the predator.
A male interested in mating approaches a female just close
enough to stretch out a modified arm, the hectocotylus, and caress
the female. This arm has a deep groove between the two rows of suckers
and ends in a spoonlike tip. After a period of love play the male
inserts its arm under the mantle of the female, and the spermatophores
travel down the groove on the hectocotylus to the female’s
oviduct. Soon after mating, the female begins to lay eggs, each
enclosed in a transparent capsule, in its lair, producing about
150,000 in two weeks. The female guards them for the next 50 days,
jetting water to aerate and clean them. The young of such species
as the white-spotted octopus, O. macropus, are
only about 3 mm (about 0.12 in) long. They float to the surface
and become part of the plankton for about a month, then sink and
begin their normal life on the bottom. Octopuses generally stay
in one area as adults, but those species with planktonic larvae
are found all over the world because the currents and tides move
them.