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encysting cercariae

Mark Rigby rigby at eco.umnw.ethz.ch
Wed Jul 16 17:46:30 EST 1997

> Are folks comfortable with the idea that cercariae that
> encyst "on" something are in general ancestral to those that encyst
> "in" a next intermediate host?  Is there a nice, neat evolutionary
> trend from encysting on a plant to encysting on an animal to
> encysting 'in' an animal?  Any discussion (to the group or to me) is
> welcome.

	Hrmmm, well, just to play Devil's advocate, why could it not be that
cercariae that encyst on something were derived from cercariae that
encyst in another host?  Thus, it would seem that there are 2

1)  cercariae that encyst on something gave rise to cercariae that need
another host


2) cercariae that encyst in another host gave rise to those that encyst
on something.

	In the first case, cercariae that encysted on something may have been
frequently been eaten by the wrong host, favoring acquiring that host as
another intermediate host.  In the second case, cercariae encysting on
something may have been derived from cercariae that originally encysted
in something but had such a low specificity for their encystment cues
that they encysted on anything, and those than encysted on something
reached a potential definitve host more often than cercariae that needed
another host.  Both are plausible, though the second option leaves the
question of just how the use of a third intermediate host arose.

	Though my knowledge of trematodes is limited, there are a few
interesting tidbits from the local trematodes to report:  monostome
cercariae here will encyst on just about anything (plastic, glass,
leaves, rocks, etc, even the glass plate that you are using to crush the
snail is not safe) and some Xiphidiocercariae from herabouts, albeit at
a rather low frequency, do not even bother to leave the sporocyst before
turning into a metacercariae.  Both of these would tend to support the
idea that encystment of the cercariae, of some groups, does not seem to
have very strict cues.

Mark Rigby
Experimental Ecology
ETH-Zentrum NW
CH-8092 Zurich
tel:   +41-1-633 6035
fax:   +41-1-632 1271
email:  rigby at eco.umnw.ethz.ch

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