Earth’s rivers, lakes, seas, and oceans teem with life. More than two-thirds of the planet’s surface is covered in water. In deep oceans, most marine life exists in the top 100 m (330 ft) or so, as this is where light and vast quantities of microscopic organisms, called plankton, are most abundant. Plankton is crucial as it forms the basis of marine life diets. All underwater life either feeds directly on plankton or preys on creatures that do.
Fish live in water and have a protective covering of scales on their outer bodies. They breathe by absorbing dissolved oxygen in the water using organs called gills located at the back of their heads. Most fish have an array of blades projecting on their body called fins. These give the fish stability in water and help control the fish’s direction of movement.
Every species of bony fish has a skeleton of bones with a flexible spine, or backbone, running the length of the body. An organ called the swim bladder enables a constant buoyancy, regardless of the changing water pressure. This ocean sunfish is the heaviest bony fish – some weigh as much as 2,000 kg (4,400 lbs).
Skates, rays, and sharks, such as this great white, are all cartilaginous fish. They lack a true backbone, but have a structure made of gristly cartilage. They also lack swim bladders and must maintain their depth by swimming. Most species of shark are predators. Rays are flattened fish that live on the seabed where they hunt smaller fish.
Some mammals spend most or all of their lives in water. These include many species of whale, porpoise, and dolphin, and plant-eating dugongs and manatees. All of these creatures that stay in the water must surface to breathe. Other marine mammals, such as this sea lion, along with walruses and seals, are skilled swimmers but must return to land to breed and reproduce.
Starfish, sea urchins, and star dollars are all echinoderms. Their skeletons are made of chalky plates that are sometimes covered in small spines. All echinoderms’ bodies are divided into five parts. Starfish, for example, all have five arms and five sets of digestive and reproductive organs. If an arm breaks off, it will grow back.
This group of more than 40,000 species of invertebrates ranges from tiny water fleas no more than 0.1 mm in size up to the Japanese spider crab with a leg span in excess of 3.7 m (13 ft). Crustaceans include shrimps, lobsters, and crabs. They all have hard-jointed shells as well as gills, eyes on stalks, and four or more pairs of jointed legs.
Jellyfish, sea anemones, and coral are all cnidarians. These creatures have no brains or central nervous system. They tend to have a bell-shaped or hollow body and a mouth ringed with tentacles. The mouth contains stinging cells, called nematocysts, which can disable prey. Some jellyfish are harmful to humans – for example, an Australian box jellyfish sting can kill within minutes.
Molluscs that lack a complete hard outer shell but have tentacles are known as cephalopods. These include cuttlefish, squid, and species of octopus. An octopus has eight arms, a well-developed brain, and good eyesight. It can react quickly to danger, squirting out a jet of water to propel itself away.
Most molluscs are soft-bodied creatures covered in a shell. These shells are made from calcium carbonate secreted by a body part called the mantle. The giant clam can live for up to 100 years. It can grow to more than 1.2 m (4 ft) across. However, most molluscs (including snails, oysters, and mussels) are much smaller.
There are as many as 10,000 species of sponge. Sponges are simple, invertebrate creatures. They are found mostly in saltwater habitats although some freshwater species exist. Sponges attach to rocks and feed on tiny food particles that flow through openings called ostia.