There are plants in the seas and oceans as well as on land. Tiny plants and animals are part of a layer of millions of living things near the surface of the oceans. All these things are called plankton. The plankton that are plants use light from the sun to help make their own food. The plankton that are animals eat the plants or each other.
Fish, shellfish, and seabirds feed on the plankton. Then these creatures are eaten by larger animals, such as seals and some types of whales. This makes a food chain in the sea. So the food chains of the sea begin with plankton.