Symbiosis
Symbiosis (pl. symbioses) (from the
Greek words
syn = with/plus and
bio = life) is an
interaction between two
organisms living together in more or less intimate association or even the merging of two dissimilar organisms.The term
host is usually used for the larger (macro) of the two members of a symbiosis. The smaller (micro) member is called the
symbiont (plural:
symbionts), or alternately,
symbiote (plural:
symbiotes). When a
microscopic symbiont lives inside the
cells of a host, it is referred to as an
endosymbiont.
The various forms of symbiosis include the following
*
Parasitism, in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other (+ −)
*
Mutualism, in which the association is advantageous to both (+ +)
*
Commensalism, in which one member of the association benefits while the other is not affected (+ 0)
*
Amensalism, in which the association is disadvantageous to one member while the other is not affected (− 0)
*
Neutralism, in which both organisms are unaffected (0 0)
*
Competition, in which both organisms are harmed (- -)
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 either in the intracellular space of the host or extracellularly.
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.
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 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.
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.
*
List of symbiotic relationships*
Syntropy* Lynn Margulis and Dorion Sagan,
Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. Summit Books, New York, 1986. ISBN 0520210646
* Jan Sapp
Evolution by Association, Oxford University Press, 1994. ISBN 0195088212
