The animals without backbones, invertebrates, which populate the sea, outnumber fish by ten to one. They range from some of the most primitive marine creatures to some of the most intelligent. They display such variety in shape and behavior that their world seems almost alien.
The deep ocean is a hard place to live – dark, cold and food is scarce. Many of its inhabitants therefore make nightly migrations up to shallower water. The six-foot long Humboldt squid sweeps up from the depths off Mexico to hunt in packs on fish shoals.
Coordinating their attack, they herd the fish and drive them onto the reef. They flash their bodies red and white – either to confuse the fish or to signal to each other when they are about to go in for an attack.
Other invertebrates also emerge from the deep, but for different reasons. Giant Australian cuttlefish mate in the shallows. Males challenge each other by flashing zebra patterns at each other. But smaller males have a special strategy. They change color to look like a female and hold their tentacles in imitation of a receptive female. With this disguise they edge towards a female and mate with her, right under the nose of a big male guarding her.
The Australian giant spider crab congregates in the shallows in vast numbers. They moult and then mate, while their new shells are still soft. But the shallows are more dangerous than the deep sea in one way because there are predators here. Sting rays prey on the still-soft crabs.
Marine invertebrates manage to thrive in the most inhospitable places. Conditions under the ice in Antarctica’s Ross Sea are as hostile as the deep sea. Yet the sea bed is carpeted with sea stars, sea urchins and huge nemertean worms. There is almost no food down here but they are ready to take advantage of a rare bonus like a seal carcass, which they cover and devour in their thousands.
The female Pacific giant octopus has come up with a fool proof way of safeguarding her young from their numerous predators. She finds a cave and blocks herself and her eggs in with rocks. She guards her eggs, keeping them oxygenated and free from disease and predators. Over the next six months she slowly starves and her last act of devotion before she dies is blowing water over her eggs to help them hatch.
The warms seas of the tropics might appear perfect for life. But in fact they are as inhospitable as the deep ocean. There are almost no nutrients here so they should be barren of life. But in fact microscopic creatures, polyps, have found a way to live here. They build the planet’s largest living structures, coral reefs, which support a quarter of the world’s marine life, on just one half of one percent of the world’s sea bed.