The Ark Files - Monsters of the Deep

Near the bottom of the deepest oceans of the world it is very dark, extremely cold, subject to enormous pressures and there is scarcely any food. The creatures that live here have amazing ways of surviving. So what can you do if you can't see, are freezing, squashed and starving?

Little monsters

Caught in the search lights of a deep-sea explorer craft, the creatures of the deep look like tiny monsters. Because there is so little nutritious food at such depths, they have much less bone and muscle, and this gives them a fragile, almost transparent look. The deeper you go the smaller the fish become - most are less than 5cm long.

Their size and lightness also keeps them from sinking too quickly. At the very bottom there is no current to swim against and the water is very still. This allows some fish such as the rattail, brotulid and the tripod fish to move across or perch on the bottom with only very slender stilt-like fins.

Living in a blackout

Go 1,000 metres under the surface and the light of the sun has been totally blocked out by the mass of the water above. So the animals that live here provide their own light. They need light not so much to see their surroundings, but more to recognise one another and as a means of communication.

Glowing colonies of bacteria are used as batteries which the fish keep in special pouches near their eyes and in other places around their body. The bacteria are capable of shining continuously but if the fish wants to be invisible it can turn off its bacterial torch by raising a small screen of skin in front of it.

Flashlight fish use a pattern of flashing lights to keep their group together and to help males find females. If a predator approaches they all switch off their lights and swim away. Once safe again, they begin flashing at one another somewhere else.

Barreleyes are deep sea bottom feeders with telescope eyes that are directed upwards. The advantage of this is that the fish can keep an eye out for prey and predators without having to move, thus enabling it to stay hidden.

The problem of food

Food is very scarce. Dead bodies drifting down can take up to a week to reach these depths and by then they are so decomposed that they have lost all nutritional content. The need to attract prey is also especially important as there are far fewer creatures here than in the upper regions of the ocean. Encounters are few and full advantage must be taken of them.

This explains why so many deep-sea fish have gigantic, distendable bellies capable of digesting prey much larger than themselves. Jaws and especially teeth are also fearsomely enlarged to make sure they don't miss these scarce meals.

Light-producing organs containing glowing bacteria are also used to catch prey. The bizarre-looking deep-sea angler fish has a long, thin spine on its head which droops over the front of its mouth.

On its tip hangs a glowing green bulb which the fish dangles and waves about like a fisherman's lure. Small fish seem irresistibly attracted to it until suddenly they are sucked into the angler's enormous mouth.

One fish that has worked out a way of getting maximum feeding opportunities is the deep sea viperfish which gets its name from its extremely long, poisonous, snake-like teeth. During the night viperfish can be found near the surface of the ocean, but by day they swim down to feed at depths of almost 3000 metres.

Getting together

It is also hard to find a mate down here where it is so dark and there are so few fish about. This accounts for the strange sexual relationships of many deep-sea angler fish. The male, who is much smaller than the female, attaches himself to the female by his jaws at a convenient place along her body. He then slowly degenerates until he is little more than a bag producing sperm, but he will continue to fertilise her eggs for the rest of her life.

Keeping afloat

The tremendous pressure in the deep oceans would cause problems of buoyancy for deep dwelling fish if it were not for the gas gland or swim bladder. This sac inside the fish acts as a float to counteract gravity and keep a fish from sinking. Gas moves into the sac when the fish moves downwards and passes out when the fish swims up again. This keeps it afloat at different depths in spite of the changes in water pressure.