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Amateur question

john janovy jjanovy at unlinfo.unl.edu
Thu Mar 16 06:57:31 EST 1995

lewis at hg.uleth.ca wrote:
: Mark Siddall and John Janovy both correctly pointed out that the
: Leucochloridium worms in the snailÕs tentacle reside in an enlarged
: sac, called a broodsac, which is part of the branched asexual parent
: parasite, the sporocyst.  The worms were all derived asexually (by
: polyembryony) from a common original mass of germinal tissue in the
: sporocyst.  As the sporocyst grows and enlarges to contain the growing 
: number of worms, it becomes pigmented and can pulsate rhythmically
: stimulated by light.  

: As to the number of worms:  when fully developed, it can contain from 
: 75-100 young worms (in broodsacs of species belonging to the genus
: Neoleucochloridium), up to as many as 300-400 (in species belonging
: to the genus Leucochloridium).  

: It is the enlarged broodsacs which distorts the snailÕs tentacles,
: and which have the color pattern you mentioned.  The snailÕs
: tentacles are virtually transparent (though the illustration
: in Scientific American made it look erroneously like it was the
: tentacle which bears the pattern).  Broodsacs of different species 
: have markedly distinct color patterns.   The one illustrated in Sci. 
: American is Leucochloridium cyanocittae, which is a parasite of 
: marsh-dwelling birds such as the Red-Winged Blackbird.

: As Mark Siddall pointed out,  the asexual multiplication of trematodes
: in snails predates the mimick-a-grub adaptation of Leucochloridium.  
: In fact, there are other closely related genera which have branched
: sporocysts, but which do not show the enlarged and pigmented
: broodsacs (Urogonimus is an example).

: John Janovy wondered why the same phenomenon  (pigmentation and 
: motilityleading to either increased conspicuousness or mimicry of a
: food item)had not evolved independently in other trematode groups.  
: In fact, it has.  One example IÕm familiar withis enlarged (up to 15 
: mm long) and conspicuously pigmented cercariae of the genus 
: Proterometra (family Azygiidae).  These emerge from freshwater 
: Goniobasis snails, and alternately swim to the surface, then settle to 
: the bottom, mimicking a mosquito wriggler.  Young centrarchid fishes 
: gobble Ôem up.

: Interestingly, the biotic potential of both Leucochloridium and
: Proterometra is much less than that of most other trematodes in
: their snail hosts.  The snail-infecting larvae of schistosomes and 
: fasciolids (which have a somewhat more daunting task to reach their 
: final hosts (by swimming and penetrating, or encysting on vegetation
: in hopes of being eaten) often produce tens- to hundreds of thousands 
: of young worms in the snail.  By contrast, Leucochloridium produces
: 2-4 broodsacs containing a total of about 1,200 young worms, and
: Proterometra produces a single cercaria about once every 2-3
: days.  Perhaps the mimicry of a food item represents investing energy
: more efficiently in achieving attractiveness than in producing huge
: numbers of offspring in the hopes that some of them aresuccessful in 
: getting to the next host.

: Paul Lewis
: Biological Sciences, University of Lethbridge

: P.S.  Sorry about the garbled apostrophes!

Paul - THANKS! for the elaboration.  JJ

jjanovy at unlinfo.unl.edu

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