Getting kind of complicated for Zoology 101!
ParcBob at aol.com
ParcBob at aol.com
Sun May 26 13:59:24 EDT 2002
Want it simplified?
Definition:
Symbiosis is a close ecological relationship between the individuals of two
(or more) different species. Sometimes a symbiotic relationship benefits
both species, sometimes one species benefits at the other's expense, and in
other cases neither species benefits. Ecologists use a different term for
each type of symbiotic relationship:
Mutualism -- both species benefit
Commensalism -- one species benefits, the other is unaffected
Parasitism -- one species benefits, the other is harmed
Competition -- neither species benefits
Neutralism -- both species are unaffected
What is Symbiosis?
Symbiosis simply means "living together" and in its narrowest form can mean
two species deriving mutual benefit from the association. However, we are
going view it as it was proposed by de Barry in 1879, as defining a wide
range of associations ranging from phoresis to parasitism.
<A HREF="http://martin.parasitology.mcgill.ca/jimspage/biol/aphodius.htm">Phoresis</A> is a fairly loose association and is a specialised form of
commensalism in which one organism (phoront), usually smaller than the other,
uses the larger organism as a transport host.
<A HREF="http://martin.parasitology.mcgill.ca/jimspage/biol/remora.htm">Commensalism:</A> - "eating together at the same table" - in this association one
member, usually the smaller, derives benefit from the association, whereas
for the other member, the association is neither beneficial or harmful. The
relationship can be that of sharing space, substrate, defense, shelter,
transport or food. Most often these associations are facultative, that is the
commensal will not die if it does not enter into the association. As apposed
to an obligate one.
<A HREF="http://martin.parasitology.mcgill.ca/jimspage/biol/mutual.htm">Mutualism:</A> As the name would suggest this is an association in which both
organisms derive mutual benefit.These associations are frequently very
intimate and obligatory. That is if the two organisms are separated neither
will survive. One author, MacInnis defines mutualism as when each of the
interacting species functions as both a host and a parasite.
ParasitismWhat is a parasite?A simple definition is an organism which lives
at the expense of another. However, this is much to simplistic and a more
acceptable one is:" A parasite is an organism living in or on another living
organism, obtaining from it part or all of its organic nutriment, usually to
the detriment of its host."However, there is no such thing as the right
definition and there have been many attempts to define parasitism
Summary
Like most definitions there are exceptions and often it is difficult to know
into which category an association should be placed. Does one of the
organisms derive benefit or not? or is there some detriment to one of the
associates? The degree of association may not be obvious and some organisms
may well include more than one type of association in their life
cycle.Symbiosis encompass a spectrum of associations ranging from phoretic to
parasitic and an interdependence from facultative to obligate. What is
important is to keep these types of associations in mind when ever you are
looking at interacting biological systems.
Appendix A
Example of a phoretic relationship
Pelodera coarctata is a rhabditit nematode that is commonly found in cow
dung, where it continues a normal free-living life cycle. When conditions in
the dung deteriorate, a special resistant ensheathed third stage dauer larva
is produced. This larva attaches to dung beetles of the genus Aphodius.
Beetles may become covered with larvae, which have a characteristic shape due
to the shrinkage of the sheath. The larvae remain in this dormant state until
the beetle migrates to a fresh dung pat, when they emerge and initiate a
further population of free-living nematodes.
Appendix B
Examples of commensal relationships
The Remora fish (Echeniedea) is a long slender fish which has its dorsal fin
modified as a sucker-like attachment organ. It attaches to the sides of
larger fish and turtles using them as transport hosts but in addition,
obtains food fragments dropped from the host.An example which is perhaps a
bit more close to home, is the amoeba known as Entomoeba gingivalis, which
inhabits the buccal cavity of man feeding on bacteria and food scraps but
causing no harm to the host. This association is obligate because the amoeba
cannot live outside the buccal cavity, in fact it does not have the cystic
stage normally found in the amoebas. It therefore depends on direct
transmission from mouth to mouth.
Appendix C
Examples of mutualism
There is a close association between termites and their intestinal population
of cellulose digesting flagellates. In exchange for digesting the cellulose
for the termite, allowing it to utilizes the fermentation products, the
flagellate is provided with a stable environment with low oxygen tension
(obligate anaerobes). A similar association exists between the ciliates of
ungulates.Looser associations are seen in cleaning fish often seen around
sharks feeding on parasites in the mouth and gills. The Egyptian plover
performs a similar service by cleaning the mouth of crocodiles.Tick birds on
rhinos and ox pecker birds on various antelopes also share a mutual
relationship. In addition to removing ticks and other irritating insects, the
ox peckers often signal the presence of predators to the antelopes.
Appendix D
Parasitism
An assortment of definitions
Parasitism is an association, generally continuous, between two different
organisms, one of which lives at the expense of the otherM. Caullery -
Parasitism and Symbiosis. Sidgwick and Jackson 1952.
Parasitism is a form of symbiosis in which one symbiont, or parasite,
receives advantages to the detriment of the other, or host.Hendersons's
dictionary of biological terms (8th edition) Longman Ltd 1975.
Parasitism is a way of living in which an organsim, the parasite, uses an
organism of a different species, the host, both as a habitat and as a food.
C.P. Reed - Animal Parasitism. Prentice-Hall 1972.
Parasites are those animals which use other living animals as their
environment and source of food, at the same time relinqishing to their hosts,
partially or completely, the task of regulating their relationships with the
external environment.V.A. Dogiel - General Parasitology. Oliver & Boyd 1964.
Parasitism is an intimate and obligatory relationship between two
heterospecific organisms during which the parasite, usually the smaller of
the two partners, is metabolically dependent on the host.T.C. Cheng - General
Parasitolgy. Academic Press 1986.
It is clearly difficult to define parasitism precisely. It is a relationship
between two species populations. The essential features are that the parasite
is physiologically dependent on the host, that it has a higher reproductive
potential than the host, and that it is capable ultimately of killing heavily
infected hosts, and that the infection process tends to produce an
overdispersed distribution of parasites within the host population.C.P.
Kennedy - Ecological Animal Parasitology. Blackwell Scientific 1975.
A parasite is an organism which has a detrimental effect on the intrinsic
growth rate of its host population.R. M. Anderson & R. M. May J. Animal
Ecology 1978.
I resort to Webster's Third International Dictionary for what must be a
generally accepted definition: a parasite is an organism living in or on
another living organism, obtaining from it part or all of its organic
nutriment, commonly exhibiting some degree of adaptive structural
modification, and causing some degree of real damage to its host.P.W. Price -
Evolutionary Biology of Parasites. Princeton Univ. Press. 1980
Here is one of my favorite examples of mutualistic symbiotic behavior:
Bottle Gentian
Gentiana andrewsii
The "bottle gentian" (or "closed gentian") is an unusual woodland flower --
its common name derives from the fact that its petals never open! Mature
flowers look just like large buds. Although these flowers produce a rich
source of pollen and nectar, most insect pollinators are not able to get
inside. Bottle gentians are pollinated almost exclusively by large bumblebees
that are strong enough to force the petals open and crawl inside. This is
an example of a mutualistic association -- the bees benefit by having
exclusive access to a bountiful nectar supply, and the plants benefit by
attracting "loyal" pollinators that improve the chances for cross
pollination.
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I leave you with the main meaning in all of this. theoretical biology has
made little use of this interesting potpourri of sumbiotic relationships.
Makes for good Ecology today and Nature programing. but there is some
beginnings jere to be thought about. Unfortuneatly., the greatest thinker
just died.
bob
++++++++
The phrase "survival of the fittest", ubiquitous in our thoughts about
evolution, is often taken to mean mutually exclusive competition.
Accordingly, mutually beneficial relationships are generally treated as a
curio. But biological evidence suggests that symbiosis is an important
enabling mechanism in evolutionary innovation. In its strongest form,
symbiosis can lead to symbiogenesis: the genesis of new species via the
genetic integration of symbionts [Merezhkovsky 1909]:
> "...different bacteria form consortia that, under ecological pressures,
> associate and undergo metabolic and genetic change such that their tightly
> integrated communities result in individuality at a more complex level of
> organization." [Margulis, 1995].
>
For example, eukaryotes, which include all plants and animals, have a
symbiogenic origin. A more general but closely related phenomenon
characterises other "major transitions in evolution":
> "entities that were capable of independent replication before the transition
> can replicate only as part of a larger whole after it." [Maynard-Smith &
> Szathmary, 1995].
However, despite the recognition of symbiosis in the natural history of
evolution, there is very little understanding about how it can be formalised
and integrated into evolutionary theory. The combination of pre-adapted parts
into a new whole is a fundementally different source of innovation from the
Darwinian view - i.e. gradual accumulation of 'blind' mutations/variations.
Of course, the development of symbiotic groups and their role in evolution
all occurs within the framework of natural selection: heritable variation and
differential reproduction. But, symbiogenesis provides a very different
source of variation from random mutations, and the biological facts about the
origin of innovation in evolution suggest that mutualism, as well as
competition, may play an important role in providing the advantage under
selection.
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