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.
Biological Sciences, University of Lethbridge
LEWIS at HG.ULETH.CA
P.S. Sorry about the garbled apostrophes!