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
:LEWIS at HG.ULETH.CA
: P.S. Sorry about the garbled apostrophes!
Paul - THANKS! for the elaboration. JJ
jjanovy at unlinfo.unl.edu
