Three hearts and a mind in every arm

Start with the heart. Then add two more. An octopus has three functioning hearts, and each has a job. As the National Wildlife Federation explains, two of them push blood through the gills, where it picks up oxygen. The third sends that oxygen-rich blood around the rest of the body.

The blood itself is blue. Instead of the iron-based molecule that colours our blood red, octopuses use a copper-based one, which carries oxygen well in cold water and gives their blood its blue tint.

The largest of them all

There are many kinds of octopus, but the giant Pacific octopus is the largest species in the world. It lives along the northern Pacific, and the Smithsonian’s National Zoo notes that adults in the wild can weigh hundreds of pounds and reach an arm span of up to about 7.6 metres.

They do not start that way. A giant Pacific octopus hatches from an egg only a little larger than a grain of rice.

A mind spread out

Octopuses are famous for intelligence, and their nervous system is unusual. According to Smithsonian Magazine, two-thirds of an octopus’s neurons sit in its arms rather than in its head, which helps explain why the arms can act with a surprising degree of independence.

Keepers who care for them describe octopuses as natural explorers. At the Smithsonian’s National Zoo, researchers have studied how the animals respond to enrichment objects, new things to investigate, as part of a long-running behaviour study.

Skin that speaks

An octopus’s skin contains special cells called chromatophores that let it change colour and pattern — a powerful way to blend into its surroundings.

All octopuses have venom of varying strength, which they deliver with a beak that, as the National Wildlife Federation describes it, is similar to a bird’s.

A short, intense life

For all their complexity, octopuses do not live long. Most species live roughly one to three years, according to the National Wildlife Federation; the giant Pacific octopus can reach about five.

The ending is remarkable. Oceana describes how a female giant Pacific octopus lays up to 100,000 tiny eggs and then devotes the rest of her life to guarding and caring for them.

Why it matters

Octopuses challenge the idea that intelligence must look like ours. An animal with three hearts, blue blood and a mind partly located in its arms reminds us how many different ways there are to be clever — and how much of that variety lives in the ocean. Invertebrates, the group octopuses belong to, make up more than 95 percent of the animal kingdom, as the Smithsonian’s National Zoo points out.