Snail flukes: answer

---------- Forwarded message ----------
From: "Matthew Paul Rhoten" <mrhoten at surly_org>
To: <aquatic-plants at actwin_com>
Cc: <alex_schaefer at studio_disney.com>
Subject: Snail flukes: answer
Date: Thu, 14 Nov 1996 21:08:15 -0800

I knew I had read about the snail flukes mentioned in a previous message to
the digest somewhere. It took a little digging, but please see the book
_The Extended Phenotype_, by Richard Dawkins. I quote a paragraph from
pages 212-213. Typos are mine, Britishisms are his, italics are represented
by _underlines_:

"By discussing inorganic shail _shells_, I have retained continuity with
the caddis houses and other non-living artefacts of the previous chapter,
thereby pursuing my policy of sustaining credulity by extending the concept
of the phenotype gradually by insensible degrees. But now it is time to
grasp the living snail firmly by the horns. Flukes of the genus
_Leucochloridium_ invade the horns of snails where they can be seen through
the skin, conspicuously pulsating. This tends to make birds, who are the
next host in the life cycle of the fluke, bite off the tentacles mistaking
them, Wickler (1968) suggests, for insects. What is interesting here is
that the flukes seem also to manipulate the _behaviour_ of the snails.
Whether it is because the snail's eyes are at the ends of the horns, or
whether through some more indirect physiological route, the normal negative
phototqaxis is replaced in infected snails by positive light-seeking. This
carries them up to open sites where they are presumabley more likely to be
eaten by birds, and this benefits the fluke."

The book cited is: Wickler, W. (1968). _Mimicry_. London: Weidenfeld &