Octopuses first evolved in the sea and there they’ve stayed and flourished. Across the world’s oceans there are roughly 300 known species ‒ none live in freshwater or permanently on dry land ‒ and they come in a huge variety of shapes and sizes. There are whoppers like the giant Pacific and seven-arm octopuses (which in fact have eight arms, but hide one ‒ we’ll find out why later). Then there are star-sucker pygmy octopuses, which when fully grown could swim around in an egg cup.
Many species live in warm or temperate waters. They ramble over the seabed in sandy shallows, on rocky and coral reefs, or slink through the shadows of giant kelp forests.
Frigid seas are no barrier to octopuses. Around Antarctica, they hunt for clams, drilling into their shells and injecting venom that works at sub-zero temperatures.
Out in open seas, octopuses spend their whole lives swimming. Blanket octopuses have webs of skin between their arms which flutter like sheets hanging on a washing line. Dumbo and flapjack octopuses swim by beating ear-like flaps (octopuses don’t actually have external ears but they can hear, with organs called statocysts). Glass octopuses have transparent bodies and look like they’d shatter if you dropped one.
Most extreme are the octopuses recently spotted near scorching hydrothermal vents, nestled among giant tube worms.
To find out how this diverse crop of modern octopuses evolved, we need to jump back in time to meet some cephalopods that are no longer around.