The octopus is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are 289 different octopus species, which is over one-third the total number of cephalopod species. Octopuses are characterized by their eight arms, usually with sucker cups on them. Unlike most other cephalopods, octopuses have entirely soft bodies; they have neither a protective outer shell like the nautiluses, nor an internal shell or bone like cuttlefish or squids. Two defensive mechanisms are typical of octopuses: ink sacs and camouflage. Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators. They also have specialized color changing skin cells called chromatophores which they can use to blend into the environment to hide. They can also use this ability as a warning; the very poisonous Blue-ringed Octopus becomes bright yellow with blue rings when it is provoked. They are highly intelligent, but have a very short life span. Some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years if they do not reproduce. However reproduction is a cause of death. Males can only live for a few months after mating, whereas females die shortly after hatching their eggs. Octopuses have been known to learn to distinguish the difference between colors and shapes. More impressive is that they can remember the shapes and colors and their meanings for up to two years. In some cultures, octopuses are caught for food.