Getting kind of complicated for Zoology 101!

[ParcBob at aol.com]

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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