The octopus (pron.:
/ˈɒktəpʊs/; plural: octopuses, octopi, or octopodes)
is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes
and four pairs of arms and, like other cephalopods, they are bilaterally
symmetric. An octopus has a hard beak, with its mouth at the center point of
the arms. Octopuses have no internal or external skeleton (although some
species have a vestigial remnant of a shell inside their mantles), allowing
them to squeeze through tight places. Octopuses are among the most intelligent
and behaviorally flexible of all invertebrates.
Around 300 species are recognized,
which is over one-third of the total number of known cephalopod species. The
term 'octopus' may also be used to refer only to those creatures in the genus Octopus.
Biology
Schematic lateral aspect of octopod
features
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.
Both types of limbs are muscular hydrostats. Unlike most other cephalopods, the
majority of octopuses – those in the suborder most commonly known,
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. A beak, similar in
shape to a parrot's beak, 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. The
octopuses in the less-familiar Cirrina suborder have two fins and an internal
shell, generally reducing their ability to squeeze into small spaces. These
cirrate species are often free-swimming and live in deep-water habitats, while
incirrate octopus species are found in reefs and other shallower seafloor
habitats.
An octopus moving between tide pools
during low tide
Octopuses have a relatively short
life expectancy, and some species live for as little as six months. Larger
species, such as the giant pacific octopus, may live for up to five years under
suitable circumstances. However, reproduction is a cause of death: males can
only live for a few months after mating, and females die shortly after their
eggs hatch. They neglect to eat during the (roughly) one-month period spent
taking care of their unhatched eggs, eventually dying of starvation. In a
scientific experiment, removal of both optic glands after spawning was found to
result in cessation of broodiness, resumption of feeding, increased growth, and
greatly extended lifespans.
Grimpoteuthis discoveryi, a finned octopus of the suborder Cirrina
Octopuses have three hearts. Two
branchial hearts pump blood through each of the two gills, while the third is a
systemic heart that pumps blood through the body. Octopus blood contains the
copper-rich protein hemocyanin for transporting oxygen. Although less efficient
under normal conditions than the iron-rich hemoglobin of vertebrates, in cold
conditions with low oxygen pressure, hemocyanin oxygen transportation is more
efficient than hemoglobin oxygen transportation. The hemocyanin is dissolved in
the plasma instead of being carried within red blood cells, and gives the blood
a bluish color. The octopus draws water into its mantle cavity, where it passes
through its gills. As mollusks, their gills are finely divided and vascularized
outgrowths of either the outer or the inner body surface.
Intelligence
Octopuses are highly intelligent,
likely more so than any other order of invertebrates. The exact extent of their
intelligence and learning capability is much debated among biologists, 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.
An octopus opening a container with
a screw cap
An octopus has a highly complex
nervous system, only part of which is localized in its brain. Two-thirds of an
octopus's neurons are found in the nerve cords of its arms, which have limited
functional autonomy. Octopus arms show a variety of complex reflex actions that
persist even when they have no input from the brain. Unlike vertebrates, the
complex motor skills of octopuses are not organized in their brain using an
internal somatotopic map of its body, using a nonsomatotopic system unique to
large-brained invertebrates. Some octopuses, such as the mimic octopus, will
move their arms in ways that emulate the shape and movements of other sea
creatures.
In laboratory experiments, octopuses
can be readily trained to distinguish between different shapes and patterns.
They have been reported to practice observational learning, although the
validity of these findings is widely contested on a number of grounds.
Octopuses have also been observed in what some have described as play:
repeatedly releasing bottles or toys into a circular current in their aquariums
and then catching them. Octopuses often break out of their aquariums and
sometimes into others in search of food. They have even boarded fishing boats
and opened holds to eat crabs.
In some countries, octopuses are on
the list of experimental animals on which surgery may not be performed without
anesthesia. In the UK, cephalopods such as octopuses are regarded as 'honorary
vertebrates' under the Animals (Scientific Procedures) Act 1986 and other
cruelty to animals legislation, extending to them protections not normally
afforded to invertebrates.
The octopus is the only invertebrate
which 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. This discovery was documented in the journal Current Biology
and has also been caught on video.
Defense
Greater blue-ringed octopus (Hapalochlaena
lunulata)
An octopus's main (primary) defense
is to hide, either not to be seen at all, or not to be detected as an octopus.
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 the use of ink sacs, camouflage, 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 an octopus's 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.
Amphioctopus marginatus travels with shells it has collected for protection
An octopus's camouflage is aided by
certain specialized skin cells which can change the apparent color, opacity,
and reflectiveness of the epidermis. Chromatophores contain yellow, orange,
red, brown, or black pigments; most species have three of these colors, while
some have two or four. Other color-changing cells are reflective iridophores,
and leucophores (white). This color-changing ability can also be used to
communicate with or warn other octopuses. The very venomous blue-ringed octopus
becomes bright yellow with blue rings when it is provoked. Octopuses can use
muscles in the skin to change the texture of their mantle to achieve a greater
camouflage. In some species, the mantle can take on the spiky appearance of
seaweed, or the scraggly, bumpy texture of a rock, among other disguises.
However in some species skin anatomy is limited to relatively patternless
shades of one color, and limited skin texture. It is thought that octopuses
that are day-active and/or live in complex habitats such as coral reefs have
evolved more complex skin than their nocturnal and/or sand-dwelling relatives.
When under attack, some octopuses
can perform arm autotomy, in a similar manner to the way skinks and other
lizards detach their tails. The crawling arm serves as a distraction to
would-be predators.
A few species, such as the mimic
octopus, have a fourth defense mechanism. They can combine their highly
flexible bodies with their color-changing ability to accurately mimic other,
more dangerous animals, such as lionfish, sea snakes, and eels.
Reproduction
When octopuses reproduce, the male
uses a specialized arm called a hectocotylus to insert spermatophores (packets
of sperm) into the female's mantle cavity. The hectocotylus in benthic
octopuses is usually the third right arm. Males die within a few months of
mating. In some species, the female octopus can keep the sperm alive inside her
for weeks until her eggs are mature. After they have been fertilized, the
female lays about 200,000 eggs (this figure dramatically varies between
families, genera, species and also individuals).
Sensation
Eye of Octopus vulgaris
Octopuses have keen eyesight. Like
other cephalopods, they can distinguish the polarization of light. Color vision
appears to vary from species to species, being present in O. aegina but
absent in O. vulgaris. Attached to the brain are two special organs, called
statocysts, that allow the octopus to sense the orientation of its body
relative to horizontal. An autonomic response keeps the octopus's eyes oriented
so the pupil slit is always horizontal.
Octopuses also have an excellent
sense of touch. An octopus's suction cups are equipped with chemoreceptors so
the octopus can taste what it is touching. The arms contain tension sensors so
the octopus knows whether its arms are stretched out. However, it has a very
poor proprioceptive sense. The tension receptors are not sufficient for the
brain to determine the position of the octopus's body or arms. (It is not clear
whether the octopus brain would be capable of processing the large amount of
information that this would require; the flexibility of an octopus's arms is
much greater than that of the limbs of vertebrates, which devote large areas of
cerebral cortex to the processing of proprioceptive inputs.) As a result, the
octopus does not possess stereognosis; that is, it does not form a mental image
of the overall shape of the object it is handling. It can detect local texture
variations, but cannot integrate the information into a larger picture.
The neurological autonomy of the
arms means the octopus has great difficulty learning about the detailed effects
of its motions. The brain may issue a high-level command to the arms, but the
nerve cords in the arms execute the details. There is no neurological path for
the brain to receive feedback about just how its command was executed by the
arms; the only way it knows just what motions were made is by observing the
arms visually.
Octopuses swim headfirst, with arms
trailing behind.
Locomotion
Octopuses move about by crawling or
swimming. Their main means of slow travel is crawling, with some swimming. Jet
propulsion is their fastest means of locomotion, followed by swimming and
walking.
They crawl by walking on their arms,
usually on many at once, on both solid and soft surfaces, while supported in
water. In 2005, some octopuses (Adopus aculeatus and Amphioctopus
marginatus under current taxonomy) were found to walk on two arms, while at
the same time resembling plant matter. This form of locomotion allows these
octopuses to move quickly away from a potential predator while possibly not
triggering that predator's search image for octopus (food). A study of this
behavior conducted by the Weymouth Sea Life Centre led to the suggestion that
the two rearmost appendages may be more accurately termed 'legs' rather than
'arms'. Some species of octopuses can crawl out of the water for a short
period, which they may do between tide pools while hunting crustaceans or
gastropods or to escape predators.
Octopuses swim by expelling a jet of
water from a contractile mantle, and aiming it via a muscular siphon.
Prey
Bottom-dwelling octopuses eat mainly
crabs, polychaete worms, and other molluscs such as whelks and clams.
Open-ocean octopuses eat mainly prawns, fish and other cephalopods. They
usually inject their prey with a paralysing saliva before dismembering it into
small pieces with their beaks. Octopuses feed on shelled molluscs either by
using force, or by drilling a hole in the shell, injecting a secretion into the
hole, and then extracting the soft body of the mollusc.
Large octopuses have also been known
to catch and kill some species of sharks.
Size
An adult giant Pacific octopus, Enteroctopus
dofleini
The giant Pacific octopus, Enteroctopus
dofleini, is often cited as the largest octopus species. Adults usually
weigh around 15 kg (33 lb), with an arm span of up to 4.3 m
(14 ft). The largest specimen of this species to be scientifically
documented was an animal with a live mass of 71 kg (156.5 lb). The
alternative contender is the seven-arm octopus, Haliphron atlanticus,
based on a 61-kg (134-lb) carcass estimated to have a live mass of 75 kg
(165 lb). However, a number of questionable size records would suggest E.
dofleini is the largest of all octopus species by a considerable margin;
one such record is of a specimen weighing 272 kg (600 lb) and having
an arm span of 9 m (30 ft).
Etymology
and pluralization
The term "octopus" is from
Greek ὀκτάπους
(oktapous, "eight-footed"), with traditional plural forms
"octopuses" (pronounced /ˈɒktəpʊsɪz/) from English grammar and "octopodes"
(pronounced /ɒkˈtɒpədiːz/) from
the Greek. Currently, "octopuses" is the most common form in both the
US and the UK. The term "octopod" (plural: "octopods" or
"octopodes") is taken from the taxonomic order Octopoda, but has no
classical equivalent. The collective plural "octopus" is usually
reserved for animals consumed for food.
Some authorities consider "octopi"
an objectionable hypercorrection, feeling the form arose from the incorrect
assumption that "octopus" is a Latin second-declension form. However,
"octopus" is a scientific Latin third-declension noun with a plural
of octopodes. Nevertheless, the Oxford English Dictionary (2008
Draft Revision) lists "octopuses", "octopi", and
"octopodes" (in that order), labelling "octopodes" 'rare'
and noting that "octopi" derives from the misapprehension that octōpus
is a second-declension Latin noun. The book further maintains that if the word
were native to Latin, it would be third declension octōpēs (plural: octōpedes)
after the pattern of pēs ("foot", plural pedēs). The
original Latin word for octopus and other similar species is polypus,
from Greek polýpous (πολύπους, "many-footed"); again, usually
the inappropriate plural polypī is used instead of polypodēs.
Fowler's Modern English Usage states, 'the only acceptable plural in English is
"octopuses"', that "octopi" is 'misconceived', and
"octopodes" 'pedantic'. Chambers 21st Century Dictionary and
the Compact Oxford Dictionary list only "octopuses", although
the latter notes that "octopodes" is 'still occasionally used'. The
descriptivist Merriam-Webster 11th Collegiate Dictionary lists
"octopuses" and "octopi" in that order; likewise, Webster's
New World College Dictionary lists in order "octopuses",
"octopi", and "octopodes".
In modern Greek, the word is χταπόδι
(khtapódi; plural: χταπόδια, khtapódia), from Byzantine ὀκταπόδιον (oktapódion)
derived from the Classical Greek variant ὀκτάπους (oktápous).
Relationship
to humans
Moche Octopus (200 AD), Larco Museum Collection, Lima, Peru
Vase from a Mycenaean Greek
cemetery at Prosymna, Argos, grave 2, circa 1500 BCE
a hoplite with an octopus image on his shield
Staatliche Antikensammlungen,
Munich, Germany
An ancient Greek black-figure amphora, 530–520 BC
Ancient peoples of the Mediterranean
were aware of the octopus, as evidenced by certain artworks and designs of
prehistory. For example, a stone carving found in the archaeological recovery
from Bronze Age Minoan Crete at Knossos has a depiction of a fisherman carrying
an octopus.
Octopuses were often depicted in the
art of the Moche people of ancient Peru, who worshipped the sea and its
animals.
In
mythology
The Hawaiian creation myth relates
that the present cosmos is only the last of a series, having arisen in stages
from the wreck of the previous universe. In this account, the octopus is the
lone survivor of the previous, alien universe.
As
a metaphor
Due to having numerous arms that
emanate from a common center, the octopus is often used as a metaphor for a
group or organization which is perceived as being powerful, manipulative or
bent on domination. Use of this terminology is invariably negative and employed
by the opponents of the groups or institutions so described.
As
food
Octopus at Tsukiji fish market
Humans eat octopus in many cultures.
The arms and sometimes other body parts are prepared in various ways, often
varying by species.
Octopus is a common ingredient in
Japanese cuisine, including sushi, takoyaki, and akashiyaki.
In Korea, some small species are
sometimes eaten alive as a novelty food. A live octopus is usually sliced up,
and it is eaten while still squirming.
Octopuses are "tickled"
out of their holes in the Hawaiian Islands with three-pronged polespears
Raw octopus arms
Lightly boiled octopus arm that
turned a bright purple
Octopus is eaten regularly in
Hawaii, since many popular dishes are Asian in origin. Locally known by their
Hawaiian or Japanese names (he'e and tako, respectively), octopus
is also a popular fish bait.
Octopus is a common food in
Mediterranean cuisine and Portuguese cuisine. In Galicia, polbo á feira
(market fair style octopus) is a local delicacy. Restaurants which specialize
or serve this dish are known as pulperías. On the Tunisian island of
Djerba, local people catch octopuses by taking advantage of the animals' habit
of hiding in safe places during the night. In the evening, they put grey
ceramic pots on the sea bed. The morning of the following day they check them
for octopuses sheltered there. A common scene in the Greek islands is octopuses
hanging in the sunlight from a rope, just like laundry from a clothesline. They
are often caught by spear fishing close to the shore. The fisherman brings his
prey to land and tenderizes the flesh by pounding the carcass against a stone
surface. Thus treated, they are hung out to dry, and later will be served
grilled, either hot or chilled in a salad. They are considered a superb meze,
especially alongside ouzo.
According to the USDA Nutrient
Database (2007), cooked octopus contains about 139 kilocalories (Calories) per
three-ounce portion, and is a source of vitamin B3, B12,
potassium, phosphorus, and selenium.
Care must be taken to boil the
octopus properly, to rid it of slime, smell, and residual ink.
As
pets
Though octopuses can be difficult to
keep in captivity, some people keep them as pets. They often escape even from
supposedly secure tanks, due to their problem-solving skills, mobility and lack
of rigid structure.
The variation in size and lifespan
among octopus species makes it difficult to know how long a new specimen can
naturally be expected to live. That is, a small octopus may be just born or may
be an adult, depending on its species. By selecting a well-known species, such
as the California two-spot octopus, one can choose a small octopus (around the
size of a tennis ball) and be confident it is young with a full life ahead of
it.
Classification
Cirrothauma murrayi
Amphitretus pelagicus
- Class Cephalopoda
- Subclass Nautiloidea: nautilus
- Subclass Coleoidea
- Superorder Decapodiformes: squid, cuttlefish
- Superorder Octopodiformes
- Family †Trachyteuthididae
- Order Vampyromorphida: vampire squid
- Order Octopoda
- Genus †Keuppia
- Genus †Palaeoctopus
- Genus †Paleocirroteuthis
- Genus †Pohlsepia
- Genus †Proteroctopus
- Genus †Styletoctopus
- Suborder Cirrina: finned deep-sea octopus
- Family Opisthoteuthidae: umbrella octopus
- Family Cirroteuthidae
- Family Stauroteuthidae
- Suborder Incirrina
- Family Amphitretidae: telescope octopus
- Family Bolitaenidae: gelatinous octopus
- Family Octopodidae: benthic octopus
- Family Vitreledonellidae: glass octopus
- Superfamily Argonautoida
- Family Alloposidae: seven-arm octopus
- Family Argonautidae: argonauts
- Family Ocythoidae: tuberculate pelagic octopus
- Family Tremoctopodidae: blanket octopus
Source :
http://en.wikipedia.org/wiki/Octopus
Octopus
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