On Fri, 17 Feb 1995, Brian E. Keas wrote:
> Perhaps the first part of the definition should be "...an organism (or
> life-cycle stage) DIRECTLY causes..." which would account for your
> example and still define acanthocephalans as parasites. The
> definition would then exclude indirect benefits of parasitic organisms
> (if there are any).
I would think that modification of prey behaviour to make it more
accessible to the predator would be classified as a direct benefit, and
therefore the direct net benefit may still be positive.
> IMHO most of the time parasites will end up costing their hosts
> more than the benefits provided by ingesting intermediate hosts. In
> acanthocephalans, attachment of the worms to the intestine may cause
> an inflammatory response, deposition of collagen, etc. Penetration
> through the intestine which occurs sometimes causes severe pathology.
> Can't forget the energy required to grow from a cystacanth to an adult
> that may be 30 cm long in some species as well as production of
> thousands to tens of thousands of eggs/day per female in an infected
> host. Even in hosts in which little pathology apparently occurs, it has
> been shown in fish that heavy or repeated infections cause a doubling in
> the number of goblet cells. Not surprising then that mucus production is
> increased (possibly doubled?) which represents a large energy drain.
But then, based on the ability of some organisms to avoid parasitized
food based on color and taste (if not the obvious behavioral changes) one
would suspect that there would be selection for individual predators that
avoided parasitized prey, unless, as Croll said, they are all idiots.
Derek A. Zelmer