Fishy ways to brighten up life in the deep

Far below the clear blue surface of the sea is a world of eternal darkness and extreme cold. And in these murky depths, strange, often ferocious-looking creatures exist. They include angler fish (with its own luminous 'fishing rod' attached to the head above a cavernous mouth), ominous-sounding hatchet fish, huge gulper eels and giant squids.

Many of the fish have lights on their bodies. These serve to attract prey (as in the case of the angler fish) or mates (it's difficult finding one's true love in the dark!) or to confuse predators (certain crustaceans spill out clouds of light while trying to escape, just as octopuses exude jets of ink).

The greatest depth at which fish are known to live is at 7,965 metres in the Puerto Rico Trench in the Caribbean Sea - that's nearly as deep as the height of Mount Everest.

A man's body is unable to stand the pressure but deep-sea fish have adapted by producing equal pressures inside their bodies. They are also 'at home' in the intense cold and die when brought to the warm surface.

Fish have developed other features to help them live in deep water. Below 1,000 m, it is forever night so many have huge eyes to obtain maximum vision. Others, however, are blind and possess different senses to enable them to find their way. Between 200 and 1,000 m there is a twilight zone where a very faint light filters through. Some of the fish which live there have evolved tubular, upward-looking eyes to receive every possible ray of light.

A few scavenge rotting bodies of animals which have died in the surface waters above and have slowly drifted down. Sea cucumbers, brittle-stars and sea spiders also wait patiently for the fine rain of plant and animal detritus falling from the surface. However, most fish predate other deep-sea fish so life here really is all about the survival of the fittest. Lantern fish are exceptional in that they swim up to the surface layers at night to feed.