In the midnight zone

The Deep expertly details the cold, harsh and hungry world that exists at the bottom of the sea. Tim Radford is fascinated

The Deep: The Extraordinary Creatures of the Abyss
edited by Claire Nouvian 256pp, Chicago, £23

Land dwellers are confined to a mosaic of habitats somewhere between the first metres of soil and the forest canopy. But sharks, jellyfish, copepods, krill, squid, fish, marine mammals and all the other extraordinary denizens of the oceans have the run of one vast, open-plan apartment block that covers more than 70% of the planet's surface, to an average depth of 3,800 metres.

Put that way, 99% of the planet's living room is ocean. Most of the sunlight that powers plant growth, and thus almost all life on Earth, is absorbed in the first 150 metres or so of water. Below 1,000 metres, there is no light at all and therefore no primary production, so tenants of the midnight zone must eat each other. The creatures at basement level grub along on a steady fall of scraps, carrion, excrement and tiny skeletons from the surface. Every now and then, 40 tonnes or more of dead whale will tumble to the abyssal ooze, to deliver a bonanza of decomposing flesh and oil-rich bone that becomes home first to sleeper sharks and hagfish, then snails, worms and crustaceans, and finally snot worms, clams and mussels with a taste for the sulphides of decay. In the active zones in the ocean ridges, creatures skip the leftovers of photosynthesis and go for a direct thermochemical fix: tubeworms, shrimps, clams and other specialised forms exploit the bacteria that, in turn, derive their energy from superheated, sulphide-rich brines that gush from submarine volcanic vents.

But for the most part, life in the deep ocean is cold, harsh and hungry, and the herbivores that live in the dark must rise towards the solar-powered supper table at dusk, returning each dawn, in their billions, and with them must go the carnivores. Biologists call it the greatest synchronised animal movement on the planet. Some animals swim, paddle, or use water-powered jet propulsion. Siphonophores pump carbon monoxide into their air bladders to float towards the feeding zone. Bathyscaphoid squid fill their buoyancy tanks with ammonium ions, to become lighter than water. Sharks get extra hydrodynamic lift from the surplus of oil in their livers.

The diner may also become dinner. So the creatures of the deep invest a great deal of evolutionary ingenuity in survival, as the stunning pictures in this book make clear. The transparent octopus moves under a cloak of invisibility: it is, much of the time, indistinguishable from the water around it. Most exploit bioluminescence - light generated by natural chemical processes - to find food or puzzle predators.

Blue light is best for sea water. Most marine eyes don't have the pigment that picks up red wavelengths. The dragon fish is an exception: it uses a red beam as a kind of sniper-scope to pick off prey by stealth; and switches on blue headlights with high beams for more distant probes. Some hunters just light up and wait. Bioluminescent bacteria, feeding on decomposing tissue, send a kind of eat-me signal to small animals, so some predators dangle luminous lures that mimic a bacterial beanfeast to entice unsuspecting diners.

Some creatures evade capture with belly lights called photophores that will blur into a light field that exactly matches the colour and intensity of the filtered sunlight from the surface. The bent-tooth bristle-mouth, Cyclothone acclinidens, may be the most abundant vertebrate on the planet, says one contributor to the book, but it uses its camouflage so perfectly most people have never seen one. The silky medusa Colobonema sericeum has tentacles that glow and drop off at moments of danger. By the time the predator realises it has a mouthful of cobweb, its chosen lunch has disappeared into the blackness.

The sea has been home to complex life for 600m years, and probably more. It is still largely unexplored. Humans know the topography of the far side of the moon better than they know their own planet. Trawl nets tell tantalisingly little, because most of the haul disintegrates on the long journey to the surface. The first human eyes to look directly into the abyss did so only about 70 years ago: the first systematic exploration had to wait for the development of submersibles that could withstand the crushing pressures.

The book provides an extraordinary procession of beauties, some of them never photographed before. They include the black-eyed squid Gonatus onyx, tenderly hugging its 2,000 eggs in a pouch that it will tug around for up to nine months; the siphonophore Praya dubia, a poisonous superorganism that grows up to 40 metres long, snaking gently through the deep like a wisp of gleaming rope; and the glowing sucker octopus Stauroteuthis syrtensis, flourishing a skirt like a tutu and string of tiny lanterns on each tentacle, 2 km below the surface. The pictures are backed up by brief essays from some of the big players in ocean research. All tell the story of evolution's marvellous variety. But many warn that this newly observed richness is already at risk, from deep-water trawling, pollution and global warming. We never knew it was there, and soon, perhaps, it won't be.

· To order The Deep for £21 with free UK p&p call Guardian book service on 0870 836 0875

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