There are around 300,000 known marine, or ocean, species. A species is a particular kind of plant or animal. Together, these different species make up about 15 percent of the planet's plants and animals.

Most marine species are tied together through the food web. A food web is a system of interconnected food chains. A food chain is a top-to-bottom set of animals and plants. They are linked to each other because those on top eat those below.

Level One: Photoautotrophs

The bottom level of the ocean's food chain is made up of one-celled organisms called phytoplankton. These tiny organisms are microscopic. They are so small they cannot be seen without a microscope.

Billions of phytoplankton live in the upper part of the ocean. They take in the sun's light. Through photosynthesis, they turn the sun's light energy into chemical energy. This chemical energy allows them to survive and grow.

Together, these tiny organisms play a large role. They are the main producers of the carbon all ocean animals need to survive. They also produce more than half of the oxygen we breathe on Earth.

Level Two: Herbivores

The next level of the marine food chain is made up of plant-eaters, or herbivores. Many are microscopic animals known as zooplankton. They drift across the ocean's surface. As they drift, they graze on whatever plants they come across.

Many herbivores are big enough for us to see. They come in a huge range of sizes, though. There are smaller ones, such as surgeonfish and parrotfish, and bigger ones, like green turtles and manatees.

Together, herbivores eat up a huge amount of plant life. However, many of them are eaten in turn. They become food for the carnivorous, or flesh-eating, animals. Carnivores make up the food chain's top two levels.

Level Three: Carnivores

The third level of the food chain consists of a large group of small carnivores. It includes fish, like sardines, herring and menhaden. Such smaller fish eat a great amount of zooplankton. However, they themselves are often eaten.

There is one simple fact of ocean life. Big fish eat smaller fish.

Level Four: Top Predators

Large predators sit at the top of the marine food chain. They are a varied group. Some are finned animals, such as sharks, tuna, and dolphins. Others are feathered animals, like pelicans and penguins. Yet others are animals with flippers, like seals and walruses.

Most top predators are large, fast and very good hunters. They also have longer lifespans. Usually, they reproduce slowly. Females of these species do not give birth that often.

Many of the marine food chain's top predators are eaten too. They are hunted by humans, the most deadly of all hunters. Overfishing by humans can greatly shrink top predator populations. Because such animals reproduce slowly, it can take years for their populations to recover.

The loss of top predator species can create serious problems. These problems ripple through the entire food web. For example, populations of smaller animals that top predators normally feed on can become too large. These smaller animals might then nearly wipe out populations of even smaller animals. Or, they might eat too much plant life. Then, animals that live on plants no longer have enough food.

Alternative Food Chains

The main marine food web is based on sunlight and plants. It includes many of the ocean's species. However, it does not include all of them. There are other, separate deep-ocean ecosystems. These are fueled by chemical energy. This energy enters the ocean through hydrothermal vents. Hydrothermal vents are openings in the ocean floor. They release heated minerals from deep within Earth, into the ocean.