To the FWGNA group:
The fancy landscape at the office park near my home in Charleston
features a narrow pond, perhaps three meters deep and over 100 meters
long, with concrete lining and a natural earth base. It is
inhabited by our three local pulmonate weeds (Physa acuta, Helisoma trivolvis, and Lymnaea columella) as well as by
mosquito fish, tadpoles, and a variety of macrophytes both submerged
and emergent. This pond has long served as the water source for
my laboratory cultures of pulmonates, and as such I have visited it
approximately every two weeks for 6 - 8 years.
About a month ago the pond was drained for cleaning. A crew
subsequently spent quite a few days shoveling leaves, mud, vegetation
and organic debris from the basin, leaving a few shallow pools
scattered on a clean mud bottom. Figure 1
shows the drying pond at its lowest end, with the concrete standpipe
that ordinarily regulates water depth in the background. Figure
2 shows a close up of one of the drying pools with Helisoma trivolvis, both the living
and the dead, much in evidence. And as Figure 3
illustrates, the individual Helisoma
were exceptionally large.
A similar phenomenon also seems to occur occasionally in the Physa acuta population inhabiting
the main pond at Charles Towne Landing State Park. I have many
years of casual observations on this population, which has served as my
source of control snails for experiments on the reproductive biology of
Physa since 1988.
In culture, P. acuta from
this population reach maturity at about 6 mm shell length (seven weeks
post hatch), and have never grown larger than about 12 mm when reaching
the end of their life at 12 - 13 months of age (Wethington & Dillon
1993, 1997). For years I never saw an individual larger than
about 12 mm in the wild. But on a casual visit in March, 2001, I
discovered that the population was comprised almost entirely of
gigantic individuals, 15 - 20 mm in shell length (Figure 4).
What might cause such sporadic cases of gigantism? Growth is
indeterminate in all the basomatophoran pulmonates of which I am
aware. But since growth rate slows dramatically at onset of
reproduction, maximum size effectively becomes a function of size at
maturity. Following Charnov and similarly minded evolutionary
biologists from the 1980s and 1990s, I have suggested that size at
maturation in pulmonates may be a function of survivorship schedule
(Dillon 2000: 140 - 149).
In both my local Helisoma
population and my local Physa
population, gigantism appeared in the fall or winter and was not
associated with parasitism. Both populations seemed stressed by
low water, and probably severe temperatures. But neither
population appeared to be starving, at least in the long term.
And perhaps most strikingly, in neither population were eggs or
juveniles in evidence. It is my hypothesis that gigantism in
pulmonates may be prompted by some environmental perturbation
postponing, perhaps even canceling, reproductive maturity.
Such a perturbation, although perhaps ongoing as gigantic individuals
are observed, must have begun earlier in the season, when maturation
would in an ordinary year have taken place. Its effect would be
to improve survivorship above expected levels, perhaps through an
unusual reduction in predation, parasitism, or disease. Did
something kill the predators that ordinarily live in the ponds with my
gigantic pulmonate populations without adversely affecting the snails
themselves, triggering a growth spurt?
And do systematic malacologists have a history of being fooled by
gigantism? About 300 km up the coast from where I write this
essay is the home of Helisoma
("Planorbella") magnifica, a pulmonate of truly gigantic
proportions (Fig. 5).
Long feared extinct, a couple scattered populations of H. magnifica were re-discovered in
1988 near Wilmington, North Carolina, by Bill Adams and Andy
Gerberich. The subsequent history of this species, tragic and
comic by turns, is a tale best left for another day, and another
teller. I know that a sample of H.
magnifica was held in culture for a number of years, and that at
least occasionally, magnifica
adults bore offspring subsequently identified as H. trivolvis, with which magnifica
co-occurs. The morphology of H.
magnifica does not suggest simple gigantism of H. trivolvis, however, the magnifica shell being much more
boxy and broad than the trivolvis
shells shown in Figure 3. The extent to which the shape of the
pulmonate shell may be influenced by environmental cues is another
topic upon which we might chew in the future.
For what its worth, our colleague Art Bogan and his colleagues (2002)
reported mtDNA sequence divergence in the 4 - 9% range between a sample
of four H. magnifica and
sample of 9 H. trivolvis
collected in Union County, a couple hundred kilometers west of
Wilmington. Given Art's observation that sequence divergence
within conspecific Helisoma
populations ranged over 3%, however, a comparison of magnifica to a trivolvis population with which it
co-occurs would have been much more cogent. But regardless of the
status and fate of H. magnifica,
the existence of great phenotypic plasticity in shell morphology should
remain foremost in the minds of all of us struggling with the taxonomy
and classification of freshwater pulmonates.
Keep in touch,
Rob
References
Adams, W.F. & A. G. Gerbeich (1988) Rediscovery of Planorbella magnifica in
southeastern North Carolina. Nautilus 102: 125-126.
Baker. F. C. (1945) The Molluscan Family Planorbidae.
University of Illinois Press, Urbana.
Bogan, A.E., M. Raley, & J. Levine (2002) Conservation status
of the magnificent ramshorn, Planorbella
magnifica (Pilsbry, 1903), endemic to the lower Cape Fear River
Basin, North Carolina. Abstract, American Malacological Society,
Charleston.
Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.
Cambridge University Press.
Wethington, A. R. & R. T. Dillon (1993) Reproductive
development in the hermaphroditic freshwater snail, Physa, monitored with complementing
albino lines. Proc. R. Soc. Lond. B 252: 109 - 114.
Wethington, A. R. & R. T. Dillon (1997) Selfing, outcrossing,
and mixed mating in the freshwater snail Physa heterostropha: lifetime
fitness and inbreeding depression. Invert. Biol. 116: 192-199.
Figures
Fig. 1.
Section of an ornamental pond in a Charleston office park, drained for
cleaning. The drying pools at foreground contain high
concentrations of Helisoma trivolvis.
As an aside, the similarity between this situation and what I've
imagined for medically-important pulmonates in irrigation systems of
northern Africa is striking.
http://www.cofc.edu/%7Edillonr/FWGSC/Drying_pond.jpg
Fig.
2. High resolution photo of the edge of one of the
pools. Zoom in to see unusually large Helisoma.
http://www.cofc.edu/%7Edillonr/FWGSC/Helisoma_habitat.jpg
Fig. 3.
An individual Helisoma tivolvis
collected fresh dead from the margin of a drying pool 22Nov04, compared
to a typical adult collected in mid-season in 2001.
http://www.cofc.edu/%7Edillonr/FWGSC/Big_trivolvis.jpg
Fig. 4.
Gigantic Physa acuta
collected at Charles Towne Landing State Park in March 2001, compared
to a typical adult from culture.
http://www.cofc.edu/%7Edillonr/FWGSC/Big_acuta.jpg
Fig. 5.
Helisoma ("Planorbella") magnifica
scanned from Plate 96 of Baker (1945). The scale is very
approximate.
http://www.cofc.edu/%7Edillonr/FWGSC/H_magnifica.JPG
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