Examples of commensalism in the ocean include sea anemones and clownfish, crabs and barnacles, as well as certain shrimp and gobies. Commensalism describes the relationship between two animals in which one benefits from the association and the other derives neither benefit nor harm from the relationship. Commensal relationships differ from parasitic relationships, in which the host is harmed from the relationship.
One of the best known examples of commensalism in the ocean is the relationship that exists between the clownfish and sea anemones. Usually, anemones fire microscopic harpoons into animals that swim through their tentacles. This is the primary mechanism by which these animals obtain their food. However, the clownfish is able to swim through the tentacles without being stung. To achieve this, the clownfish secretes a substance on its body that mimics the anemones. This makes it so that the anemones do not even know that the fish are swimming through their tentacles.
By living in the writhing mass of venomous tentacles, the clownfish derives protection from predators. Additionally, the clownfish survives on the little bits of fish, shrimp and other creatures that are stuck in the tentacles of the anemones. Some scientists suspect that this cleaning function is helpful for the anemones, but this has yet to be demonstrated conclusively.