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definition

Jeffrey Lotz jlotz at medea.gp.usm.edu
Thu Feb 16 15:16:02 EST 1995


In my opinion the best single definition of a parasite is, "an 
organism that uses another heterospecific organism as its 
habitat".

This would place parasite as a class within a wider class of 
symbiotic organisms and would expand the class to include 
viruses and certain bacteria, fungi, protists, plants, and 
perhaps some vertebrates. In my mind the difference between 
algae living in the bodies of corals and nonphotosynthetic 
protists living in the cytoplasm of ciliates is less important 
than the similarities. Fish living in the tentacles of anemones 
share some important characteristics with lice in hair. To me 
the important characteristic is the endeavor of living in or on 
another organism. 

The wider class of symbiosis, i.e, living together would 
include organisms that are not parasitic. Symbiont would 
include cleaner fish or cattle egrets. I think that symbiosis 
has a broader connotation than parasite and would not equate 
the two. 

Further, I would argue that the standard practice of 
subdividing symbiosis into mutualism, commensalism, and 
parasitism on the basis of cost/benefit analysis misses an 
important point about parasites: they live in or on other 
organisms) and that the contrast between free-living and 
parasitic modes of existence catches that distinction. 

I would not opt to restrict the term parasite only to 
pathogenic symbionts. This goes against common usage and has 
not been adopted by parasitologists. Many parasitologists study 
nonpathogenic organisms.

I would argue that if symbiosis means living together then the 
spectrum of living together is best measured along a continuum 
from free-living to host/parasite rather than along the 
profit/loss interaction spectrum: from mutualism (or 
facilitation) (+ +) to competition  (- -), including along the 
way commensalism (0 +), and pathogenism (=predator-prey)(+ -). 
I prefer the term pathogen to parasite for - + relationships. 

Along this symbiosis spectrum one can have any profit/loss 
relationship placed on top of it. One can find mutualistic 
parasites, commensal parasites and pathogenic parasites.

One can view relationships along two axes: one symbiotic from 0 
(=free-living) - infinity (=parasitic). The other one 
interactions from mutualism (both profit) to competition (both 
lose). Whether one opts to place "parasites" at the end point 
of the habitat relationship or in the middle of the interaction 
continuum is perhaps a matter of taste but I think the habitat 
relationship better captures the essence of parasitology.

I would not view nest parasites as parasites because the nest 
parasite doesn't use another organism as its habitat eventhough 
it may be pathogenic to one and benficial to the other

The significance of size (small vs large) for partners in an 
association doesn't appear to me to be very important. Nor does 
the physiological dependence restriction seem too important.

 


--
Jeffrey M. Lotz                     Phone (601) 872-4247
Gulf Coast Research Lab             Fax   (601) 872-4204
P.O. Box 7000                       Internet: jLotz at medea.gp.usm.edu
Ocean Springs, MS 39566-7000 USA



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