Dave Terry (dat at interport.net) wrote:
: I read in Scientific American (Jan 92) about the parasitic worm
: Leucochloridium paradoxum, which spends part of its life cycle in a snail
: and part in a bird. To move from the snail to the bird, large populations
: of these worms migrate to the snail's eyestalks, where they cause the
: eyestalks to swell and change in coloration. Such changes result in the
: snail's coming to resemble caterpillars, which the birds then eat. The
: worms subsequently lay their eggs in the bird's digestive track. It takes
: millions of worms to effect the change in the snail's eyestalks. But can
: entire populations of individuals undergo the same genetic mutation at the
: same time and thus secure an advantage? If not, why would such a
: behavioral mechanism evolve, since the behavior could not confer any
: advantages on a single individual?
: Any instruction on this point would be greatly appreciated.
: Humbly yours,
: Dave Terry
: Production Editor
Dave - I'm not sure there are "millions" of worms in a
__Leucochloridium__ brood sac, more like hundreds, and they are all
progeny of a single original larva produced asexually. Actually in
terms of numbers offspring, __Leucochloridium__ is not unusual among
trematodes, but in all of them, the increased reproductive potential
in the snail is a result of asexual polyembryony. Maybe some of the
snail people will respond, but in my view the important question is
not so much how the famous __Leucochloridium__ brood sac evolved, but
why the same type of phenomenon has evidently not evolved
independently in a number of other trematode groups. Might have
something to do with the fact that __Leuchloridium__ passes its
larval phases in land snails.
John Janovy, Jr.
jjanovy at unLinfo.unL.edu