Only they and some very specialized organisms have the ability to manufacture their own food.
Producers form the first link in the food chain.
The food chain represents the transfer of energy from one organism to another. In the ocean, many individual food chains overlap and intersect to form complex food webs . Most marine creatures eat a variety of foods. This flexibility allows them to switch if one link in the chain is depleted. Food webs usually end with a top predator on which nothing else preys.