Three hearts and blue blood
The OCTOPUS is about as alien as life on Earth gets. It has three hearts — two pump blood through the gills and one round the body — and its blood is blue, because it carries oxygen using copper rather than iron. Around two thirds of its nerve cells are in its eight arms rather than its head, so each arm can taste, feel and act partly on its own.
Octopuses can change colour and even skin texture in a fraction of a second to match rocks or seaweed, and they are famous escape artists in aquariums, squeezing through any gap larger than their beak. The SQUID is a close relative, and so is the cuttlefish. Most octopuses live only a year or two.
Fathers that give birth, and a star with no brain
In SEAHORSE families it is the male who becomes pregnant. The female lays her eggs into a pouch on his belly, he carries them for a few weeks, and then he gives birth to dozens or even hundreds of tiny seahorses. Seahorses are fish, even though they swim upright and hold on to seaweed with their curling tails.
The STARFISH is not a fish at all, and many scientists prefer the name sea star. It has no brain and no blood, and to eat a mussel or CLAM it prises the shell open a crack and pushes its own stomach out of its body and inside the shell. A JELLYFISH has no brain either, and is around 95 per cent water.
The tiny things that make our air
PLANKTON are the drifting living things of the sea, most too small to see. The plant-like kind make food from sunlight, and in doing so they produce something like half of the oxygen in Earth's atmosphere — every other breath you take. They are also the bottom of the food chain: SHRIMP-like krill eat them, and some of the biggest animals on the planet eat the krill.
A sea TURTLE may swim thousands of kilometres over many years and then return to lay eggs on the same beach where it hatched. The temperature of the sand decides whether each egg becomes a male or a female, with warmer nests producing more females. The MANATEE, a slow, gentle grazer of sea grass, is more closely related to elephants than to seals or whales.
A journey nobody has ever watched
European EELS grow up in rivers and ponds across Europe, sometimes for decades. Then they swim out to the Sargasso Sea, in the western Atlantic, to breed and die, and their young drift back across the ocean. Despite more than a century of searching, nobody has ever seen wild eels spawning there.
The rest of the grid covers the sea's big and small: the DOLPHIN, WHALE, ORCA and NARWHAL, the SHARK, the STINGRAY, the SEAL and WALRUS, the CRAB and LOBSTER, the OYSTER, the URCHIN, the ANEMONE and CORAL, and fish like TUNA and MACKEREL. Our coral reef page covers the busiest places in the ocean, and our oceans page the water itself.