Symbiosis (pl. symbioses) is a close association between two different types of organisms in a community. It can be defined as:
The living together in permanent or prolonged close association of members of usually two different species, with beneficial or deleterious consequences for at least one of the parties.
— [1]
There are several classes of symbiosis below. Some of them are antonyms to positive connotations in symbiosis.
- Mutualism, a relationship in which members of two different species benefit and neither suffers.'[2] (+ +)
- Commensalism, a relationship in which 'one party gains some benefit, whilst the other suffers no serious disadvantage'[3] (+ 0)
- Parasitism, in which one member of the association benefits while the other is harmed (+ -)
- Amensalism, in which the association is disadvantageous to one member while the other is not affected (− 0). Allelopathy can fall into this category.
- Competition, in which both organisms are harmed (- -) Allelopathy can fall into this category.
- Artificial symbiosis, the mutually beneficial integration between a live part and an artifact.
Symbiosis may be divided into two distinct categories: ectosymbiosis and endosymbiosis. In ectosymbiosis, the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. In endosymbiosis, the symbiont lives within the tissues of the host; either in the intracellular space or extracellularly.
Examples
An example of mutual symbiosis is the relationship between clownfish of the genus Amphiprion (family, Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the clownfish from its predators (a special mucus on the clownfish protects it from the stinging tentacles). Not all anemones that house clownfish are in need of protection, but this does not rule out mutualism, as clownfish often share food with their anemone hosts. A single worm will be halved inside the clownfish's dextrous mouth, and a portion spit out into the anemone's wavering tentacles.
Another example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it. When that happens both the shrimp and goby fish quickly retract into the burrow.
A famous land version of symbiosis is the relationship of the Egyptian Plover bird and the crocodile. In this relationship, the bird is well known for preying on parasites that feed on crocodiles and are potentially harmful to them. To that end, the crocodile openly invites the bird to hunt on his body, even going so far as to open the jaws to allow the bird to enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such proximity to its host.
One of the most spectacular examples of a symbiosis is between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps. This is a mutualistic symbiosis where the worm completely loses its digestive tract and is solely reliant on their internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of the world's oceans.
Symbiosis and evolution
The biologist Lynn Margulis, famous for the work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, as incomplete, and claims evolution is strongly based on co-operation, interaction, and mutual dependence among organisms. According to Margulis and Sagan (1986), "Life did not take over the globe by combat, but by networking." As in humans, organisms that cooperate with others of their own or different species often out-compete those that do not.
References
- Lynn Margulis and Dorion Sagan, Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. Summit Books, New York, 1986. ISBN 0-520-21064-6
- Jan Sapp Evolution by Association, Oxford University Press, 1994. ISBN 0-19-508821-2
External links
- Mycorrhizas – a successful symbiosis Biosafety research into gm-barley