An example of a symbiotic relationship is that which exists between a parasitic worm and the horse within which it lives. In this specific case, the type of symbiotic relationship that is occurring is known as parasitism. One organism, the worm, benefits from the relationship, while the other, the horse, is harmed.
A symbiotic relationship is defined as any close relationship which exists between two different living things in which at least one of the living things derives benefit from the relationship. There are three main types of symbiotic relationships: parasitism, commensalism and mutualism. In parasitism, an example of which is discussed above, one organism lives on or off of another organism. The organism that benefits from the relationship is known as the parasite, while the other organism that is harmed by the relationship is known as the host.
Commensalism is a type of symbiotic relationship in which one organism benefits and the other organism neither benefits or is harmed. An example of commensalism is the dwelling of Goby fish among the spines of sea urchins. The fish benefit in that they have shelter and protection, but the sea urchins experience no advantage or disadvantage because of their presence.
In mutualism, both organisms in the symbiotic relationship experience benefits. An example of mutualism is shrimp that feed on the algae that grow on tropical fish. The shrimp are provided with a source of food, while the fish remain clean and disease-free.