There are around 300,000 known ocean species. A species is a particular kind of animal or plant. Ocean species are also called marine species.

Most marine species are part of a food chain. A food chain is a top-to-bottom set of animals and plants. They are connected to each other because those on top eat those below. These food chains come together to form a food web.

Level One: Photoautotrophs

The bottom level of the ocean's food chain is made up of tiny living things. These are called phytoplankton. 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 a process called photosynthesis, they turn that light energy into food.

Level Two: Herbivores

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

There are also larger herbivores. Some examples are turtles and manatees.

Together, herbivores eat a huge amount of plant life. Many end up being eaten themselves. They become food for the carnivorous, or meat-eating, animals. Carnivores make up the food chain's top two levels.

Level Three: Carnivores

The third level of the food chain is made up of small carnivores. It includes fish, such as sardines and herring. These fish eat a great amount of zooplankton. However, they are often eaten, too. They are munched on by bigger fish.

Level Four: Top Predators

Large predators sit at the top of the food chain. They include sea animals, such as sharks, tuna and, dolphins.

Most top predators are very good hunters. They are large and fast.

However, top predators can be eaten too. They are eaten by humans. Some have been nearly wiped put by too much fishing.

The disappearance of top predators can create serious problems. Populations of smaller animals they feed on become too large. These animals then kill off even smaller animals. Or, they eat too much plant life. Then, plant-eating animals no longer have enough food.

Other 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 depend on chemical energy. This energy enters the ocean through openings in the ocean floor. Such openings are called hydrothermal vents. They release heated minerals from deep within Earth.