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
