Symbiosis
The term symbiosis (Greek for living together) can be used to describe various degrees of close relationship between organisms of different species. Sometimes it is used only for cases where both organisms benefit, sometimes it is used more generally to describe all varieties of relatively tight relationships, i.e. even parasitism, but not predation. Some even go so far as to use it to describe predation. [1] It can be used to describe relationships where one organism lives on or in another, or it can be used to describe cases where organisms are related by mutual stereotypic behaviors.
In either case symbiosis is much more common in the living world and much more important than is generally assumed. Almost every organism has many internal parasites. A large percentage of herbivores have mutualistic gut fauna that help them digest plant matter, which is more difficult to digest than animal prey. Coral reefs are the result of mutualisms between coral organisms and various types of algae that live inside them. Most land plants and thus, one might say, the very existence of land ecosystems rely on mutualisms between the plants which fix carbon from the air, and Mycorrhyzal fungi which help in extracting minerals from the ground. In fact the evolution of all eukaryotes (plants, animals, fungi, protists) is believed to have resulted from a symbiosis between various sorts of bacteria: endosymbiotic theory.
Varieties of 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.'[2] (+ 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 members are harmed by the relationship. (- -)
- ectosymbiosis, in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands.
- endosymbiosis, in which the symbiont lives within the tissues of the host; either in the intracellular space or extracellularly.
( the terms in the parentheses refer to whether one or both organisms benefits +, is harmed -, or remains neutral 0)
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 that cooperate with each other or members of different species often out-compete those that do not.
References
- Surindar Paracer and Vernon Ahmadjian, "Symbiosis: An Introduction to Biological Associations" Oxford University Press. 2nd Ed. 2000. ISBN 0-195-11806-5
- 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
Further reading
- Douglas, A.E. (1994) Symbiotic Interactions. Oxford University Press, Oxford.
External links
- Mycorrhizas – a successful symbiosis Biosafety research into gm-barley