Mostly each other. Phytoplankton and algae like terrestrial plants make food through photosynthesis, then zooplankton and other small animals or filter feeders eat them, then animals like krill eat them, then bigger things eat krill, etc. read more
Fish are animals. Animals in the ocean are made up of herbivores, omnivores and carnivores, just as on land. Some are herbivores, some eat seaweed. There are host of animals that eat plankton, a rather diverse group of organisms that can be either animal or plant. Baleen whales eat krill, which are small crustaceans. read more
Starfish are known to be fierce predators of mollusks, such as oysters, mussels and clams. These organisms are usually slow moving or attached to rocks so hunting them are easy for starfish. Starfish also hunt snails and devour injured fish when it available. Some species of starfish eats detritus or dead sea plants and animals. Other starfish food includes corals, planktons and sponges. read more