Octopuses are highly intelligent, probably more intelligent 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 that they do have both short- and long-term memory. Their short lifespans probably limit the amount they can ultimately learn. There has been much speculation to the effect that almost all octopus behaviors are independently learned rather than instinct-based, and although this is undoubtedly an interesting idea it remains largely unproven. It can, however, be logically concluded that they learn almost no behaviors from their parents, with whom young octopuses have very little contact.
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 a remarkable amount of autonomy. Octopus arms show a wide variety of complex reflex actions arising on at least three different levels of the nervous system. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the 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.
A common belief is that when stressed, an octopus may begin to eat its own arms. However, limited research conducted in this area has revealed that the cause of this abnormal behavior, called autophagy, may be a virus that attacks the octopus's nervous system. Thus this behavior may be more correctly labeled as a neurological disorder.
Three defensive mechanisms are typical of octopuses: 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 colouring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin colour.
An octopus's camouflage is aided by 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.
When under attack, some octopuses can detach their own limbs, 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; this ability is also used in mating.
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. They have also been observed changing the texture of their mantle in order to achieve a greater camouflage. The mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises.
Octopuses have keen eyesight. Although their slit-shaped pupils might be expected to afflict them with astigmatism, it appears that this is not a problem in the light levels in which an octopus typically hunts. Surprisingly, they do not appear to have colour vision, although they can distinguish the polarization of light. 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 that the pupil slit is always horizontal.
Octopuses also have an excellent sense of touch. The octopus's suckers are equipped with chemoreceptors so that the octopus can taste what it is touching. The arms contain tension sensors so that the octopus knows whether its arms are stretched out. However, the octopus has a very poor proprioceptive sense. The tension receptors are not sufficient for the octopus brain to determine the position of the octopus's body or arms. (It is not clear that 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 that 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
Many species of octopus are eaten as food by human cultures around the world. The arms and sometimes other parts of the body are prepared in various ways, often depending on the species being eaten.
Octopus is a common ingredient in sushi, takoyaki, and Akashiyaki. Some small species are sometimes eaten alive as a novelty and health food (mostly in South Korea).
Though octopuses can be difficult to keep in captivity, some people keep them as pets. Octopuses often escape even from supposedly secure tanks, due to their intelligence and problem solving skills. The variation in size and life span 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 the 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 that it is young with a full life ahead of it.
Octopuses are also quite strong for their size. Octopuses kept as pets have been known to open the covers of their aquariums and survive for a time in the air in order to get to a nearby feeder tank and gorge themselves on the fish there. They have also been known to catch and kill some species of sharks
reference: http://en.wikipedia.org/wiki/Octopus
done by zhan quan
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