An example of mutualism in the ocean is the relationship between coral and a type of algae called zooxanthellae. The relationship is mutualistic because neither organism would be able to survive without the other.
The coral that makes up coral reefs gets its food from microscopic algae. These algae, called zooxanthellae, live inside the coral polyp. In return, coral gives the zooxanthellae a place to live.
Another example of mutualism in the ocean is the relationship between some types of sea anemone and boxer crabs. The crab fights off its enemies by hiding among the stinging tentacles of the anemone. In return, the anemone eats food left behind by the crab.