The oceans form by far the largest continuous habitat on earth, and they were almost certainly the environment in which life first evolved. The underwater landscape is made up of mountains and volcanoes, cliffs, deep valleys, and vast, flat plains, many of them far larger than any found on land.
The oceans are so immense — they cover more than three-quarters of the earth’s surface — and difficult to explore that scientific knowledge of ocean wildlife lags behind that of life on land. However, research has shown that life is found at all levels, from the sunlit surface to the deepest trenches over 11 km (7 miles) down.
Some inshore waters are so shallow that if the world’s oceans were lowered by just 75m (245tt), huge areas of sea bed would be exposed. Off Western Europe, for example, the coast would be extended by about 200 km (125 miles), and in parts of Siberia, by more than 700 km (485 miles). These shallow waters owe their existence to continental shelves — the gently sloping plateaux that flank many of the deep ocean basins.
Continental shelves are a key habitat tor sea life, supporting large shoals of fish and a diverse collection of other animals, from lobsters, and crabs to molluscs and burrowing worms. This wealth of life is possible because, in shallow water, sunlight can reach the sea bed, promoting the growth of algae, seagrasses, and countless other organisms that need energy from light to survive. Just like plants on land, these provide animals with a year-round supply of food, as well as with plenty of cover and places to breed.
Some inshore animals, such as lobsters and flatfish, spend most of their lives on the sea bed, while a few live entirely in mid-water or at the surface. Others live in quite different habitats at different stages of their lives. For example, some larvae develop as part of the surface plankton, then move to mid-water or the sea bed as adults.
Inshore waters are also visited by animals from the open sea — most often by passive drifters, such as jellyfish, but also by powerful swimmers, such as whales and sharks. Some come to breed, while others arrive by accident and then return to deeper water. Occasionally, the latter get into difficulties; jellyfish often end up on the shore, and whales can become stranded when their navigation systems guide them into shallow water instead of safely out to sea.
Even in the clearest sea water, light penetrates no further than about 250m (825ft) below the surface. More than any other factor, this has a crucial effect on sea life because it determines what there is to eat. In the brightly lit surface zone, microscopic algae grow by harnessing the energy in sunlight, creating an invisible harvest for planktonic animals. This food is passed on when the plankton are themselves eaten in food chains that eventually end with sharks and other large predators.
Below the point where the light fades and finally disappears, there is no home-grown supply of food at this depth, and further below, animals feed either on each other or on the supply of dead remains that constantly drifts down from the surface. Despite the unimaginable volume of the oceans, few of the worlds animal species — perhaps 5 per cent – live in open water.
Since most of these stay near the surface, where they can take advantage of the relatively plentiful food supply, animal life in the huge mid-water zone is comparatively sparse. In contrast with these central zones, much of the ocean floor abounds with animals.
Those that live on the sea bed are called benthic animals and include species that swiw or crawl over the sea bed as well as those that burrow through the soft sediment for food. Many appear to have changed little over millions of years, for although the water is very cold, and the pressure intense, the deep-sea bed is not subjected to the changeable conditions that can affect the surface. It is thus one of the most stable habitats on earth.
LIFE IN SEA
In the billion or more years since the first animal species evolved, competition for survival in the sea has become ever more intense. Today, the oceans are home to the largest predators on the planet, as well as to vast numbers of microscopic animals that drift by unseen.
As on land, ocean wildlife is affected by local conditions, the most important being the supply of food. For many, survival also depends on being able to defend themselves against attack. In some regions, life is thinly spread, but in others, animals are found in greater numbers than they are anywhere else on earth.
FEEDING IN SEA
The oceans are so huge that, although they contain plenty of nutritious food, marine animals face a challenge in finding enough to eat without expending too much energy in the process. Some concentrate on large prey.
The sperm whale, for example, hunts giant squid at depths of over 1,000m(3,300 ft), although most pursuit-hunters search for food near the surface rather than in the depths. Other large sea animals eat smaller fare, scooping it up in huge amounts, often sieving it out with their gills. This technique, known as filter feeding is used by baleen whales and some of the largest sharks and rays.
Most filter feeders live on plankton, which is so abundant that it allows them to reach a gigantic size. Drifting animals also feed on plankton, although on a much smaller scale. Comb jellies or sea gooseberries, for example, haul it in with sticky cells that work like fishing nets. On the sea bed, animal life depends almost entirely on the dead organic matter that drifts steadily down from above.
Brittlestars are typical of these scavengers, collecting food particles with their arms. However, there are also predators — bizarre fish, for example — that hunt in the total darkness, on or near the sea bed.
Animal life at the bottom of the sea can be sparse, so these predators cannot afford to miss any opportunity to feed. Many of them therefore have gigantic mouths and elastic stomachs that enable them to swallow prey that is almost as large as themselves.
Very few marine animals — with the exception of deep-diving mammals — are found at all levels in the sea. Instead, most are adapted for life at a particular depth, and have buoyancy devices that help to keep them there.
Surface drifters, such as the violet sea snail, have simple floats: filled with material that is lighter than water they ensure that the animal stays at the surface, even in a heavy swell.
For animals that spend their lives fully immersed, remaining at one level requires more complex apparatus. They have to be neutrally buoyant at their optimal depth and yet able to rise or sink as the need arises. To do this, some use adjustable buoyancy aids hidden inside their bodies.
Bony fish, for example have a gas-filled chamber, called the swim bladder, just below the back bone. If the fish needs to sink it removes some of the gas from the swim bladder by pumping it into the bloodstream; if it needs to rise it pumps it back into the bladder. Cartilaginous fish, such as sharks are actually slightly heavier than sea water: swimming provides the lift that allows them to control their depth.
In the open sea, there is nowhere to hide,which leaves animals highly vulnerable to predators. To survive some rely on camouflage or disguise; others behave in ways that make them difficult to attack.
For slow-moving invertebrates, such as those that make up plankton, one of the most effective disguises is transparency. Planktonic animals are often as clear as glass, which makes them difficult to see even at close quarters, most of these animals are only a few millimeters long, although some tunicates form translucent, tube-shaped colonies that can be over 3m (9 3/4 ft) in length.
Some fish are transparent when they are very small, but then use camouflage of a different kind as they get older. Almost all species that live in brightly lit, open water have dark backs but much paler undersides.
This pattern known as countershading, protects fish in 2 ways; it hides them from predators deeper down by disguising their silhouette against the bright sky and it conceals them from the surface hunters, including seabirds, by making them blend in with the dark water beneath.
LIVING IN GROUPS
On land, animal groups can be very large, but none rivals the size of those that can occur at sea. Fish often live in shoals thousands or even millions strong, while some planktonic animals form swarms that can be over 100km (60 miles) long.
These giant aggregations of underwater life often seem easy targets for predators. Whales gorge on krill with almost nonchalant ease, while other predators make huge inroads into shoals of fish. but, in general animals living in groups are safer than they would be alone: they are more difficult to single out much more difficult to take by surprise.
A few animals, such as dolphins, live in sophisticated social groups, Dolphins use echolocation to locate prey, to warn each other of danger, and to organize themselves during hunting.
Many marine animals — including most of the largest whales — migrate between breeding grounds in the tropics and feeding grounds at higher latitudes. The grey whale probably travels the greatest distances: its lifetime annual migrations total up to 800,000 km (500,000 miles) — twice the distance to the moon.
Tuna are also known to undertake immense journeys: some shoals travel the length of the Mediterranean: in the Pacific one fish tagged in Mexico was recovered off Japan. Turtles show very precise migration patterns, returning to the same stretch of beach year after year to lay their eggs.
This is all the more remarkable because it takes 2 or 3 decades for the animal to mature: during this time, a turtle remembers precisely where it hatched so that when it is ready to breed it can make the long journey back. On a smaller scale, many fish swim inshore to spawn, while other slow-moving animals migrate across the sea bed‘. Caribbean spiny lobsters travel between shallow reefs, where they breed, and deeper water, were they overwinter: they set off in single file. each one following the tail of the animal in front.
Journeys like these are usually annual events, but some animals migrate daily to feed. Planktonic animals often rise to the surface at night, sinking back into the depths by day. Some of the planktons predators copy this pattern, creating a 24-hour cycle involving many animals. These vertical migrations are clearly revealed by shipboard sonar, which shows a reflective layer rising at sunset and sinking at dawn.