Introduction to Oceans
Life on Earth originated in the seas, and the oceans continue to be home to an incredibly diverse web of life.
Historically, there are four named oceans: the Atlantic, Pacific, Indian, and Arctic. However, most countries now recognize the Southern (Antarctic) as the fifth ocean. The Pacific, Atlantic, and Indian are known as the three major oceans.
Oceans support the greatest variety of life on earth, from microscopic plankton to giant whales. The deepest parts of the oceans have barely begun to be explored, and new life forms are being discovered every year by deep ocean submersible machines.
The floor of the ocean is called the benthic habitat while the water itself is called the pelagic habitat. Both support a diversity of animal life. Coral reefs, which grow in warm tropical and subtropical seas, are perhaps the richest marine habitat in terms of the diversity of life they shelter.
The oceanic food chain begins with microscopic drifting plants called phytoplankton. Phytoplankton are found close to the surface of the water where there is adequate sunlight for photosynthesis.
Phytoplankton are eaten by tiny floating animals known as zooplankton. Zooplankton include the larvae of crabs, jellyfish, corals and worms, as well as adult animals like tiny shrimps, copepods and euphausiids (krill). They keep buoyant with the help of gas-filled chambers and oil droplets which reduce their density.
Moving up the food chain, zooplankton provide food for fish. Big fish eat smaller fish and at the very top of the food chain are large predatory fish like sharks, mammals like seals, and seabirds. A very large fish, the whale shark, and some very large mammals, the baleen whales, feed directly on zooplankton.
Millions of people on all continents depend on fish for food. That is why it is so important that fish populations are conserved. Overfishing by huge modern fishing fleets is threatening the entire ocean food chain.