Sea anemones and clownfish are highly co-dependent species, and their evolutionary histories have been closely intertwined. As sea anemones have evolved, so too have clownfish, and vice versa. For example, some sea anemones have developed special stinging cells that are harmless to clownfish but can deter other predators. Clownfish, in turn, have evolved a thick layer of mucus that protects them from the sea anemone's stings.
This co-evolutionary relationship has been so successful that some species of clownfish are now found exclusively with certain species of sea anemones. This level of specificity is a testament to the strong selective pressures that have driven the co-evolution of these two species.
Here are some additional examples of co-evolution in the ocean:
* Cleaner fish and their clients: Cleaner fish are small fish that remove parasites and dead skin from larger fish. This symbiotic relationship benefits both species, as the cleaner fish get a source of food and the larger fish get to stay healthy and free of parasites.
* Coral and algae: Coral is a marine invertebrate that builds hard skeletons out of calcium carbonate. Algae live in close association with coral and provide it with nutrients through photosynthesis. The coral, in turn, provides the algae with a protected environment and a source of carbon dioxide.
* Brittle stars and sea urchins: Brittle stars are small, starfish-like animals that live on the ocean floor. They feed on sea urchins, which are spiny, round animals that also live on the ocean floor. Brittle stars have long, flexible arms that they use to grab and eat sea urchins. Sea urchins, in turn, have evolved a hard, spiny exoskeleton that helps to protect them from predators.