To the FWGNA group:
In my post of November 2004 I examined the phenomenon of gigantism in
pulmonates, taking as a point of departure the population of Helisoma trivolvis inhabiting an
ornamental pond near my Charleston neighborhood. This month I'll
develop that theme in an entirely different direction, focusing not on
the mean shell diameter achieved by populations of Helisoma in the lowcountry, but on
the spire height.
Across the Cooper River east of Charleston lies the suburb of Mt
Pleasant, a bedroom community that has witnessed tremendous growth in
the several generations since prosperity returned to the Carolina
lowcountry. Among the scores of housing developments sprawling
across this former patchwork of swampy forests is the subdivision of
"Wakendaw Lakes."
Figure 1
shows the low earthen dam constructed to retain the largest of the
Wakendaw "Lakes." The nomen "pond" would be more descriptive of
this shallow body of water, perhaps 1 - 2 hectares in extent, weedy and
protected. It hosts the usual pulmonate fauna of the Charleston
area, including a population of Helisoma
trivolvis bearing shells that may tend to be a bit more narrow
and compressed than we think of as typical (Figure 2,
at right).
Excess water from the pond overflows a standpipe and exits through an
open channel perhaps 2 meters wide at the base of the dam, then runs no
more than 3 - 4 meters before disappearing into a culvert under the
road. I gather that there must be substantial groundwater input
to the pond, for the flow in the channel is mild but constant year
round. Residents tell me that the current can be extreme in times
of storm. The engineers who built this small work thoughtfully
lined the entire four meters of channel with granite rip rap stones to
forestall erosion.
Grazing on the stones in the moderate current one can find a population
of Helisoma with the very
peculiar shell morphology shown at left in Figure 2.
The animals are often so obliquely coiled as to violate the definition
of planispiral, effectively retaining the shell morphology normally
associated with juveniles. But several years ago I ran a batch of
allozyme gels comparing the populations above and below the dam, and
was able to confirm that all the Wakendaw animals belong to a single
randomly-breeding population of H.
trivolvis.
Once again, I think the best explanation for this phenomenon lies in
ecophenotypic plasticity. The planispiral shell borne by typical H. trivolvis in lentic waters
enfolds an air bubble, which the snails use to regulate their buoyancy
as they graze on floating or emergent vegetation. The
narrow aperture of typical trivolvis
may function as a defense against smaller predators (such as minnows)
that might seek to snatch the snail's body from inside its shell.
But a narrow, planispiral shell is worse than useless in a lotic
environment. Thus individual H.
trivolvis born on rocks in flowing water might retain a lower,
broader shell to present less drag in the current, and a wide aperture
enfolding a broad foot with which to cling. And again I
emphasize, no genetic divergence need be involved.
As all of us in this group are aware, the taxonomy of freshwater
gastropods both here and around the world rests largely, if not
entirely, on shell morphology. In Florida, the related species Helisoma duryi and H. scalare are characterized by a
broad, "buliniform" shell morphology very much like the H. trivolvis population of the
rocky channel at Wakendaw. Burch reprinted a lovely 1934 figure
of H.A. Pilsbry's suggesting evolutionary relationships among duryi, scalare, and several other species
of Florida planorbids with low, more typical spire heights back to the
Pliocene (Figure
3). I hate to be cynical, but I can think of an easier
explanation.
Keep in touch,
Rob Dillon
Figure 1. The main pond in the Wakendaw Lakes subdivision (Mt.
Pleasant, SC) is shown at upper right. Water exits through the
rocky channel at lower left.
http://www.cofc.edu/~fwgna/FWGSC/Wakd_pond.jpg
Figure 2. Helisoma trivolvis
from the Wakendaw Lakes subdivision
pond in November, 1993. The pair of shells at left were collected
from stones in the lotic environment below the dam shown in Figure 1,
while the pair of shells at right were collected from weeds in the
lentic environment above the dam.
http://www.cofc.edu/~fwgna/FWGSC/Wakd_Helisoma.jpg
Figure 3. Burch's reproduction of Pilsbry's model for the
evolution of Florida Helisoma.
From Pilsbry, H. A. (1934) Review of the Planorbidae of Florida,
with noteson other membes of the family. Proc. Acad. Natl. Sci.
Phila. 86:29-66.
http://www.cofc.edu/~fwgna/FWGSC/Pilsbr_Helisoma.JPG
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