Octopus

Octopus

The common octopus, Octopus vulgaris
Scientific classification
Kingdom	Animalia
Phylum	Mollusca
Class	Cephalopoda
Order	Octopoda  Leach, 1818
Characteristics
Species	About 300
Arms	Eight, with suckers
Hearts	Three
Lifespan	Typically 1–2 years

The octopus (order Octopoda) is a soft-bodied, eight-limbed mollusc. Around 300 species are recognised, in every ocean and at every depth from the intertidal zone to the abyssal plain. Together with squid, cuttlefish and nautiluses, octopuses make up the class Cephalopoda — the most neurologically complex invertebrates known.

An octopus has no skeleton of any kind. The only hard structure in its body is a parrot-like beak, and an octopus can pass through any opening the beak will fit through. The body is a muscular hydrostat: arms have no bones and no fluid-filled cavity, and change shape by contracting muscles arranged in three directions against each other, the same principle as a human tongue or an elephant's trunk.

Each arm carries two rows of suckers, which grip and also taste: the suckers are lined with chemoreceptors, so an octopus can detect what it is touching by touching it. Around two thirds of its roughly 500 million neurons lie in the arms rather than the brain, and a severed arm continues to react appropriately to stimuli for a time. The brain issues broad commands; the details of a movement are worked out locally.

Octopuses have three hearts. Two branchial hearts pump blood through the gills, and a systemic heart circulates it to the body. The systemic heart stops beating while the animal swims, which is one reason most species prefer to crawl.^[1] Their blood is blue, using copper-based haemocyanin rather than iron-based haemoglobin — more efficient in cold, low-oxygen water.

Octopus skin contains three layers of optical machinery. Chromatophores are elastic sacs of pigment, each pulled open by radial muscles under direct neural control, changing colour in well under a second. Beneath them, iridophores produce structural colours by interference, and leucophores scatter ambient light. The animal also alters the texture of its skin, raising papillae to imitate algae or rock.

The paradox is that octopuses appear to be colour-blind: they have a single photoreceptor type. Proposed explanations include colour discrimination through chromatic aberration in the pupil, and light-sensitive proteins in the skin itself.

Octopuses are the standard example of intelligence evolved on a separate branch from ours; the common ancestor of an octopus and a human was a simple worm-like animal more than 500 million years ago. Documented abilities include opening jars to reach prey, navigating mazes, using coconut shell halves as portable shelter, discriminating between individual humans, and solving the same problem by different routes on different occasions. Captive animals will squirt water at lights they dislike and escape their tanks.

They also have a strong ability to learn by observation and a marked tendency to play, manipulating objects with no evident reward.

Most octopuses live only one to two years, and reproduce once. The male passes a packet of sperm with a modified arm, the hectocotylus. The female lays tens of thousands of eggs, then stops feeding and spends weeks or months tending them, aerating them and driving off predators, dying at about the time they hatch. In 2014 researchers reported a deep-sea octopus brooding a single clutch for four and a half years, the longest brooding period recorded for any animal.

This pattern — short life, single reproduction, no parental contact with the next generation — means that whatever an octopus learns dies with it. Octopus intelligence is, unusually, not cultural.

Threatened octopuses jet away by forcing water through the siphon and release a cloud of melanin-rich ink, which obscures the water and dulls a predator's sense of smell. Some species mimic other animals: the mimic octopus of Indonesia adopts the appearance and swimming style of sea snakes, lionfish and flatfish. All octopuses are venomous, though only the blue-ringed octopuses of the Indo-Pacific, whose saliva contains tetrodotoxin, are dangerous to humans.

• Photosynthesis
• Encyclopedia

1. ^ Wells, M. J. Octopus: Physiology and Behaviour of an Advanced Invertebrate. Chapman and Hall, 1978.