Mututalism is a type of symbiotic relationship in which two organisms live in close proximity and both benefit from the relationship. All symbiotic relationships are not mutualistic; if one organism benefits and the other doesn’t, then it could be a symbiotic relationship, but not a mutualistic one. One of the most well-known examples of mutualism in coral reefs is the clown fish and the anemone.
There are two primary types of mutualism that apply on the coral reef: trophic mutualism and defensive mutualism.
Trophic mutualism occurs when both species receive a similar benefit through transfer of energy and nutrients. This type of mutualism occurs in animal-algal mutualisms, such as with coral polyps and dinoflagellate algae. When a dinoflagellate lives in a coral, it is called a zooxanthellae. The coral uses photosynthesis byproducts of the zooxanthellae as food, and the coral secretes a mucus-like substance that protects the zooxanthellae. The coral also protects the zooxanthellae from organisms that might eat it and the intense ultraviolet light that might kill it.
Defensive mutualism occurs when one species receives food and shelter in return for protecting its partner from predators. For example, with the mutualism between the sea star and the scale worm, the scale worm lives in or near the sea star’s mouth. As the sea star eats, the scale worm gets leftover pieces of food. Conversely, if a predator tries to attack a sea star, the scale worm uses its sharp pincer-like jaws to bite the predator.
In some mutualistic relationships, a species may be so dependent on its partner that it can’t survive with it. This is called obligate mutualism. The animal-algal mutualism that exists between a coral polyp and a zooxanthellae is an example of obligate mutualism. The coral bleaching phenomenon occurs when zooxanthellae are expelled by the coral, in which case eventually the coral will die.
On the other hand, facultative mutualism exists when each species gets benefit from the other, but they aren’t so dependent that they can’t survive. The anemone and clown fish is an example of facultative mutualism. The clown fish brings food to the anemone while the anemone wards off predators with its stinging polyps. However, the clown fish could live in another type of home and the anemone could capture food from the water without being fed by the anemone.
The exact nature of a mutualistic relationship may shift from neutral to positive to negative. These changes occur over time, with changing environmental conditions, or because of changes in the organism communities.
The symbiotic relationship that occurs in a mutualistic partnership, especially in an obligate mutualism, creates a situation where coevolution may occur. Coevolution is a process that occurs when the genetics of one species changes in response to genetic changes in another species. Coevolution helps both species survive.