Octopus myths, gently untangled

Myth Octopuses have eight tentacles.

Fact Octopuses have eight arms. Squid and cuttlefish have eight arms plus two longer tentacles for snatching prey. Smithsonian Ocean: Octopuses, squids and relatives

Myth Octopus blood is red, like ours.

Fact Their blood really is blue. It carries oxygen with a copper-based molecule called hemocyanin, while our blood uses iron. Smithsonian Ocean: Octopuses, squids and relatives

Myth An octopus has just one brain in its head.

Fact The common octopus has around 500 million neurons, about as many as a dog. About two thirds of them are in its arms. Natural History Museum, London: Octopuses keep surprising us Each of an octopus’s eight arms has its own ‘mini brain’, and each arm can taste, touch and move without direction from the centre. Natural History Museum, London: Octopuses keep surprising us

Myth The correct plural is octopi.

Fact Merriam-Webster lists three plurals: octopuses, octopi and octopodes. Octopuses is now the most common. Merriam-Webster: octopus ‘Octopi’ is the oldest plural, made on the mistaken idea that the word was Latin. It is really Greek, which is where ‘octopodes’ comes from. Merriam-Webster: octopus

Myth Octopuses are slimy fish.

Fact Octopuses are molluscs: boneless invertebrates related to clams. Monterey Bay Aquarium: Giant Pacific octopus

Myth Only the blue-ringed octopus is venomous.

Fact All octopuses are venomous, but only the blue-ringed octopuses are dangerous to people. Shedd Aquarium: Eight strange and wonderful facts about octopuses

Myth Octopuses can live happily out of water.

Fact A few shore-dwelling octopuses make short trips out of the water at low tide, but they can only last a few minutes before crawling back to the sea. Shedd Aquarium: Eight strange and wonderful facts about octopuses

Myth The paper nautilus is a nautilus.

Fact The paper nautilus, or argonaut, is actually an octopus. The female makes a paper-thin, shell-like egg case. Smithsonian Ocean: Octopuses, squids and relatives Only the chambered nautiluses still have a shell on the outside. Over 10,000 nautiloid species once lived; today just seven remain. Smithsonian Ocean: Octopuses, squids and relatives

Myth The vampire squid is a squid.

Fact Despite its name, the vampire squid is more closely related to octopuses than to squid. It’s harmless, eating drifting particles. Smithsonian Ocean: Octopuses, squids and relatives