Shrimp and crabs sometimes use anemones for protection and to capture food.

A tapeworm gets food from the dogfish shark, but the shark suffers because of it.

Barnacles attach themselves to sea turtles which allows them to have protection

Imperial shrimp get a free ride, and they have an easy way to get food by using sea cucumbers

The corals gain food from the zooxanthellae, and the zooxanthellae gain protection