Scientists have identified around 300,000 different marine, or ocean, species. Together, these make up about 15 percent of all known plants and animals on the planet. However, the ocean is so vast that much of it has not yet been carefully explored. A million or more as yet undiscovered species might live in its waters.
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 largely invisible. It is made up of billions of one-celled organisms, called phytoplankton. These tiny organisms fill sunlit upper-ocean waters worldwide. In a way, phytoplankton work like plants. This is because they take in the sun's energy and, through photosynthesis, turn nutrients and carbon dioxide into organic compounds. On the coast, seaweed and seagrasses do the same thing.
Together, these tiny plants play a large role. They are the main producers of the organic carbon that 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, or so small they are invisible to the human eye. These tiny creatures are known as zooplankton. They drift across the ocean's surface, grazing on whatever they come across. There are also larger herbivores, including surgeonfish, parrotfish, green turtles, and manatees.
Together, herbivores eat up a huge amount of ocean plant life. However, many of them are eaten in turn. They become food for the carnivorous, or flesh-eating, animals of the food chain's top two levels.
Level Three: Carnivores
The zooplankton of level two provides food for a large group of small carnivores, such as sardines, herring and menhaden. These small carnivores are very successful hunters. However, they often fall prey to a simple fact of ocean life: Big fish eat smaller fish.
Level Four: Top Predators
Large predators sit at the top, or apex, of the marine food chain. They are a varied group. They include finned animals, such as sharks, tuna, and dolphins; feathered animals, like pelicans and penguins; and ones with flippers, like seals and walruses. These apex predators tend to be large, fast and very good at catching prey. They also have longer life-spans. Usually, they reproduce slowly. Compared to smaller animals, females do not give birth that often.
The marine food chain's top predators are common prey for the most deadly hunters of all: humans. When populations of top predator species shrink due to overfishing, it can take years for them to recover. This is due to their slow rate of reproduction. The loss of these species can create problems throughout the entire food web. For example, populations of the smaller animals they 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.
Alternative Food Chains
The primary or 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 deep-ocean ecosystems that are entirely independent of the sunlight energy that kick-starts the main marine ecosystem. These ecosystems are fueled by chemical energy. It enters the ocean from sources like hydrothermal vents. Hydrothermal vents are openings in the ocean floor. They release heated minerals from deep within Earth, into the ocean.
