The sea covers two-thirds of the Earth’s surface and animals have lived in the ocean far longer than on land. It may be hard to imagine when you gaze out at the waves, but the ocean is full of an amazing variety of life. Sea creatures range from tiny animals you cannot see without a microscope to giant sharks and whales.
Among the most important life forms in the sea are the thousands of tiny plants and animals that drift in surface waters. These are known as plankton, and can be divided into two main groups: phytoplankton and zooplankton. Phytoplankton, which is made up of tiny plants and bacteria, is eaten by the microscopic animals in the zooplankton group. Zooplankton in turn are a vital food for lots of different creatures, from small fish to whales. Even the biggest plankton animals are only a centimetre or so long.
ANIMALS WITHOUT BACKBONES
There are many thousands of species of invertebrates (animals without backbones) living in the sea, more than any other type of creature. They live everywhere from the surface waters and the seashore to the deepest depths. These are the main groups:
These are among the simplest of all animals. Most live in the sea, although there are some sponges in fresh waters. Their larvae (young) are free-swimming, but adult sponges settle in one place on a rock or on the seabed. They live by filtering food particles from the water and many look more like plants than animals.
This group includes coral, jellyfish and sea anemones. Most have a simple, cylinder-shaped body with a mouth and tentacles at one end. Many have stinging cells on their tentacles to help them capture prey. One of the most extraordinary coelenterates is the Portuguese man-of-war. This jellyfish is actually a colony of hundreds of animals living together under a sail-like float, which drifts on the water surface.
Segmented worms (annelids)
These worms have a body made up of a number of sections or segments. There are many different kinds living in the sea. Some, such as paddle worms, swim about in search of food. Others, such as feather-duster worms, stay in one place on the seabed and use tentacles on their heads to filter tiny bits of food from the water.
The three largest groups of mollusks are the gastropods, bivalves and cephalopods. A typical gastropod, like a snail or limpet, has a shell into which it can draw its body to hide from danger (though some gastropods, such as sea slugs, have no shell at all). Bivalves, which include clams, mussels and scallops, have two shells hinged at one side. They mostly range from 1 to 10 centimetres long, although the tropical giant clam can grow to 1.35 metres across. Cephalopods include squid, octopus and cuttlefish. The squid has a long body and ten tentacles, eight of which are equipped with suckers. Inside its body is a thin internal shell called a pen. The octopus has no internal shell and only eight tentacles.
Barnacles, crabs, lobsters and shrimp all have a hard outer shell to protect their soft bodies. They are extremely successful animals and there are vast numbers of them in the world’s oceans. Adult barnacles live fixed to one place such as a rock, the bottom of a ship or even the belly of a whale, but they spend the first part of their lives as free-swimming plankton. Crabs are often seen on land as well as in the sea.
This group of invertebrate animals includes brittle stars, starfish, sea cucumbers, sea urchins and sand dollars. Most move around with the help of tiny structures called tube feet, which are tipped with sucking discs.
Fish are the most common vertebrates (animals with backbones) in the sea. There are about 14,000 different kinds, from the tiny dwarf goby, which lives in the Indian Ocean and is only 8 to 10 millimetres long, to the giant whale shark, which grows to 12 metres. There are three main groups of fish. The most primitive are the jawless fishes such as the hagfish. Instead of jaws, the hagfish has a simple mouth like a slit and eats by pushing its tongue into prey to tear off lumps of flesh. Sharks and rays make up the second group. They are known as cartilaginous fish because they have skeletons made of a gristly material called cartilage instead of bone. Most fish belong to the third group—the bony fish. These fish have a skeleton made of bone and most have overlapping scales covering the skin.
Whales, dolphins and porpoises are mammals, but they have become perfectly adapted to life in the sea. They live, feed, mate and give birth in the water, but they must come to the surface to breathe. The blue whale is not only the biggest whale, at nearly 30 metres long, but also the biggest animal known to have lived on Earth.
Seals, sea lions and the walrus are also sea-living mammals. They spend most of their time in the water, but come to land to bask on rocks, to mate and to give birth.
BIRDS AND REPTILES
Many birds depend on the sea for their food and gannets and brown pelicans both make spectacular dives into the water to catch fish. Others, such as penguins, have become expert swimmers and do not fly at all. They are more at home in the water than on land.
Turtles, such as the green turtle, leatherback and hawksbill, spend all their lives at sea, only coming ashore to lay eggs. The several kinds of sea snakes are some of the most deadly of all poisonous snakes. Fortunately, they never come to land. Only one kind of lizard is found in the sea—the marine iguana, which lives on and around the Galápagos Islands in the Pacific Ocean.