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Portrait of Octopus

Science · Biology

Octopus

Biology — molluscs, order Octopoda

Octopuses are soft-bodied cephalopod molluscs of the order Octopoda, with eight sucker-lined arms, a beak of chitin as the only hard part of the body, and no internal or external shell. They have three hearts — two pumping blood through the gills, one round the rest of the body — and blood made blue by the copper-based pigment haemocyanin. Roughly two-thirds of their half-billion neurons sit in the arms rather than the central brain, so each arm carries out much of its own processing, and they are the standard example of intelligence evolved along a line entirely separate from the vertebrates. Their skin changes colour and texture in a fraction of a second through chromatophores, iridophores and papillae, despite eyes that appear to be colour-blind.

How to spot them

  • ▸Eight arms, three hearts and blue, copper-based blood
  • ▸Two-thirds of its neurons are in its arms; each arm can act semi-independently
  • ▸Boneless — it can squeeze through any gap larger than its beak
  • ▸Changes colour and skin texture in under a second, yet its eyes carry only one visual pigment
  • ▸Class Cephalopoda, with the squid, cuttlefish and nautilus

Key facts

Three hearts — two branchial, one systemic — and blood turned blue by copper

Haemocyanin carries oxygen more efficiently than haemoglobin in cold, low-oxygen water

Around two-thirds of its half-billion neurons lie in the eight arms rather than in the brain

Each arm has its own nerve cord and can taste by touch

Chromatophores, iridophores and leucophores repaint its skin in milliseconds, though it is probably colour-blind

One visual pigment in the retina; chromatic aberration in the pupil may allow colour to be inferred

The giant Pacific species of this genus is the largest, recorded at over 70 kg

Arm spans over 4 m; one disputed report reached 9 m

The tiny blue-ringed genus Hapalochlaena carries tetrodotoxin, the pufferfish poison

Produced by symbiotic bacteria; there is no antivenom

Paul, at a German aquarium, picked the winner of all seven German matches at the 2010 World Cup (2010)

He chose between two mussel-baited boxes; he also got the final right

Alone among animals, this group edits its own RNA on a huge scale, tuning proteins to the water temperature

Extensive A-to-I recoding in the nervous system, at the apparent cost of a slowly evolving genome

Key ideas

  • Distributed cognition: with most neurons outside the brain, arm movements are largely delegated — a severed arm will still reach for and reject food — making the animal a favourite test case for theories of mind and body.
  • Semelparity by hormone: after mating, the optic glands trigger a cascade that stops the female feeding while she broods her eggs, and she dies as they hatch. Removing the optic glands abolishes the behaviour and greatly extends her life.

Quick-fire facts

  • •Octopus vulgaris (Cuvier, 1797), the common octopus, is the standard laboratory and Mediterranean species
  • •Order Octopoda, class Cephalopoda, phylum Mollusca; roughly 300 species
  • •The largest is the giant Pacific octopus, Enteroctopus dofleini, at up to about 70 kg; the smallest are a few centimetres across
  • •The only hard part is the parrot-like beak, so the animal can pass through any opening wider than it
  • •Blue blood: haemocyanin uses copper where vertebrate haemoglobin uses iron
  • •Most species are semelparous — they breed once and die; the female starves while guarding her eggs
  • •In 2010 the EU recognised cephalopods as the only invertebrates covered by laboratory-animal welfare legislation
  • •Ink is mostly melanin, expelled with mucus to form a decoy blob; it is released from an ink sac into the funnel
  • •The preferred English plural is octopuses; 'octopi' applies a Latin ending to a Greek word, and 'octopodes' is pedantic but etymologically defensible
  • •Aristotle described the animal's colour change and detachable mating arm, the hectocotylus, in his Historia Animalium

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Sources