On 10 Feb 1995, derek a. zelmer wrote:
> Due to recent (sometimes violent) discussions I have been involved in, I
> would like to get some opinions on a question of basic philosophy...what
> is a parasite? Are they always overdispersed? Does there have to be a
> nutritional dependence of the adult on the host? Should parasitoids be
> included, or does obligate host death preclude parasitism? etc, etc....I am
> curious as to how narrow or broad the current viewpoints are, for example
> a colleague of mine considers grazing herbivores to be parasitic. I think
> he's daft. Touch gloves and come out swinging.
>> Derek A. Zelmer
>Other resonses to this post have suggested that there are no
all-encompassing definitions of a parasite. I for one am a little uneasy
with that suggestion (after all, how can anyone say they study parasites
if they can't even define what a parasite is). Maybe breaking down bits
of the definition will help. For starters:
Derek asks, "Does there have to be a nutritional dependence of the adult
on the host?"
Not purely nutritional, but...
Yes, for any organism to be considered a parasite it must have a net
negative effect on host resources. I will qualify that by adding that
for any stage (not just the adult) to be considered parasitic, it must
have a net negative effect on host resources. This means that each stage
in a complex life cycle should be considered separately. For digenetic
trematodes:
Adults: parasitic (all? using host resources for eggs)
Eggs/Miracidia: free living (all?)
Sporocyts/Rediae: parasitic (all? using host resources for cerc.)
Cercariae: free living (almost all? some are ingested while
still in the molluscan host)
Unencysted Meta-/Mesocercariae: parasitic (all? using host
resources for movement, etc?)
Encysted Metacercariae (host response): parasitic (all?
host losing resources by building cyst wall)
Encysted Metacercariae ("parasite" response): free living
(all? e.g. encysting on plants [or plastic in the lab])
Read the ( ) as "all parasitic?" or "all free living?" For digenetic
trematodes, I believe that these stages are considered one or the other
but am not 100% sure, hence the ?.
1. Thus, if an organism (or life-cycle stage) causes a net negative
effect on host resources it COULD be a parasite. {If it causes no net
negative effect, it CANNOT be a parasite (e.g. flagellates in a termite
gut use host resources but make cellulose available to the termite and
so there is a positive effect on host resources; not a parasite).}
2. An organism (or life-cycle stage) that ABSOLUTLEY CAN ONLY use one host
at a time to obtain host resources COULD be a parasite. {An organism that
could POSSIBLY obtain resources from two hosts simultaneously CANNOT be a
parasite (a cow could possibly obtain resources from two separate plants
(grasses) in the same mouthful; not a parsite).}
3. Host is defined here as any living organism (and must be living if
acting as a host to parasites).
4. The combination of #1 and #2 with the definition in #3 is an
all-encompassing definition of a parasite.
When I first started writing this respnse, I was planning only to answer
the question about nutrition, but look what it evolved into. As much as
my mind will work right now, I can't think of exceptions to this
definition for what are classically considered parasites. However, I am
pretty sure there will be some - which is why I put this out for your
consideration.
Brian Keas
keasbe9 at wfu.edu
Wake Forest University
Winston-Salem, NC 27109
