The word symbiosis derives from the Greek syn, meaning “with,” and biosis, meaning “living.” Symbiosis thus means “state of living together” and is a situation in which two different species live together under some arrangement that may or may not be beneficial to each organism.
Symbiosis is most commonly understood to mean a mutually beneficial living arrangement, but that is only one of thee types of arrangement that may occur.
* Mutualism is an arrangement that benefits both species.
* Commensalism benefits one partner but the other is neither helped nor harmed by the arrangement.
* Parasitism is a situation in which one member of the pair benefits and the other member is harmed in some way. While some parasites kill their hosts, in most cases the death of the host also results in the death of the parasite. Therefore, over time, the organisms evolve so that they can coexist, although the host is still harmed.
The coral reef habitat contains an extraordinary abundance of life, much of it in some type of symbiotic relationship.
> Mutualism in Coral Reefs <
* Corals and Algae
A type of single-celled algae, zooxanthellae, lives inside the tissues of the corals. The zooxanthellae use photosynthesis to break down carbon dioxide into oxygen and carbohydrates, providing food to the polyps. The polyps then provide food to the algae in the form of their waste products, which the algae then break down into nitrogen and phosphorus to use as energy. Both members of this pair receive vital nutrients as a benefit of their association.
* Cleaning Stations
The “cleaning stations” found in coral reefs are another form of mutualism. In these areas, shrimp and small fish, like the wrasse, remove growths and parasites from larger fish. These larger fish would normally prey on the smaller creatures, but at the cleaning stations, they seem to understand that the cleaners provide a benefit, and they wait patiently for the cleaners to finish their tasks. Without the cleaners, the predators find themselves with potentially deadly parasitic infections or fungal growths. Once again, both parties benefit from the association. The cleaners get a meal and the larger fish avoid parasitic infections.
* Fish and Algae
In a variation on mutualism, called cultivation mutualism, animals grow and harvest fields of another organism. One interesting example of this occurs on some coral reefs. The damselfish, Stegastes nigricans, farms a particular species of the algae Polysiphonia, even weeding out invading species. In a report published in December, 2006, researchers found that one particular Polysiphonia species grows only in the S. nigricans farms and removal of S. nigricans from the farm results in the algae being consumed within days. The algae provides the fish with food and the fish protects the algae from predation.
> Commensalism in Coral Reefs <
* Parrotfish and Rabbitfish
Carnivores on the reef far outnumber the herbivores, leading to constant danger to the herbivores. Some herbivores have developed defenses, like the Goldlined rabbitfish, which has venomous spines on its pelvic fins as well as the ability to change color to avoid detection. Other herbivores, like the parrotfish, have not developed these defenses and use schools of rabbitfish as refuge from predators. The parrotfish benefits from the protection of the rabbitfish, but the rabbitfish is neither helped nor harmed by this arrangement.
* Sponge Crabs and Sea Sponges
Many sea sponges have evolved to produce toxic and distasteful chemicals that protect them from predators. The sponge crab takes advantage of this chemical arsenal by carrying sea sponges on their backs. Predators that avoid the sponge will also avoid the crab. In this relationship, the crab receives obvious benefit in the form of protection, but the sponge does not receive any clear-cut benefit.
> Parasitism in Coral Reefs <
There are two types of parasitism.
* Endoparasitism occurs when the parasite lives inside the host. Endoparasites include bacteria and viruses, as well as many flatworms and leeches.
*Ectoparasitism is a situation in which the parasite attaches itself to the outside of the host. A common example of ectoparasitism is the fish doctor, which is an isopod crustacean. This animal attaches itself to a fish, usually under its fins or in its gills, and sucks the hosts blood. When the host dies, the isopod simply moves to a new host. The isopod benefits from feeding on the host, but the host is fatally harmed.
Many more examples of symbiosis exist within coral reefs, and no doubt many more relationships remain to be discovered.