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. They keep buoyant with the help of gas-filled chambers and oil droplets which reduce their density. Zooplankton in turn are food for fish.
Big fish eat smaller fish.
At the 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.