An octopus is a cephalopod mollusc of the order Octopoda. 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. The beak, similar in shape to a parrot's beak, and made of chitin, is the only hard part of their bodies. Octopuses have a relatively short life expectancy, with some species living 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 live for only a few months after mating, and females die shortly after their eggs hatch. The common octopus can grow to about 4.3 feet long and weigh up to 22 pounds, although averages are much smaller. While the giant pacific octopus grows bigger and lives longer than any other octopus species. The size record is held by a specimen that was 30 feet across and weighed more than 600 pounds. Octopuses are highly intelligent, possibly 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 has a highly complex nervous system, only part of which is localized in its brain. An octopus's primary defense is to hide or to disguise itself through camouflage and mimicry. 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 distraction with the use of ink sacs and autotomized 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. An octopus's camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectivity 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. Octopus vulgaris lives in tropical and semitropical waters in oceans around the world; from the Atlantic, Indian, Pacific Oceans and the Mediterranean Sea. 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. 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.