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Categories of Symbiosis The various forms of symbiosis include the following In some cases, the term symbiosis is used only if the association is obligatory and benefits both organisms. Symbiosis as defined in this article does not restrict the term to only the mutually beneficial interactions (mutualisms). 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). 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 which are potentially harmful for the animal. 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 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 1970s 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. Interactions can change However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. For example, the oxpecker birds of Africa were thought to be living a mutualistic existence by picking parasites off large mammals including buffalo, elephants and zebra. However, it was recently realised that while the birds do this, they sometimes also keep wounds on the animals' skin open, and feed on the exuding blood. Hence the relationship between oxpeckers and their hosts is sometimes mutualistic and sometimes parasitic. In fact, the direction of a symbiotic interaction can change during the lifetime of the symbionts due to developmental changes as well as changes in the biotic/abiotic environment in which the interaction occurs. 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 outcompete those that don't. | ||||||||||
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