To the FWGNA group:
I am pleased to report the rediscovery of Gillia altilis in South
Carolina, the 26th freshwater gastropod taxon confirmed living in the state
today. This discovery, in addition to its value as an item of general good
news, has strengthened a couple of convictions I've held for some time, perhaps
of some broader interest, perhaps now worth sharing.
But first, a bit of biological background. Faithful correspondents may
remember an essay I posted on 26May04 admitting my own long standing confusion
regarding Gillia,
Amnicola, and Somatogyrus, three hydrobiid genera of the southern United
States bearing similarly plain shells. That essay featured a photo comparing the three taxa that may be
worth revisiting. In any case, Gillia is much larger than either Amnicola
or Somatogyrus, and is really quite unmistakable, now that I've held one
in my hand. Figure 1 shows a
7.8 mm individual crawling in a beaker. The collection I made earlier
this month was comprised entirely of adults ranging from 6 - 8 mm in shell
length, twice the size of typical adults from either of the other two local
hydrobiid genera. The shell is intermediate in thickness between the
lightly-shelled Amnicola, more characteristic of lentic environments,
and the heavily-shelled Somatogyrus, an inhabitant of rocky riffles.
Gillia altilis was first described by Lea (1841) using specimens sent
him from the Santee
Canal, an 1800 - 1850 passage between the port of Charleston and the Santee
River 40 miles north. Only remnants of that canal remain today. The
Charleston Museum holds two nineteenth-century lots of Gillia, one
labeled simply "Santee Canal" and the other "Lynch's
Creek."
I rediscovered the species Tuesday, January 11, at the US 52 bridge over the
Lynches River 8 miles south of Florence, where I'd stopped on a whim.
We'd had two weeks of unusually mild weather here in the southeastern US, and
very little rainfall. The Lynches River was very low, and the morning so
warm and bright, I pulled off the road primarily to stretch my legs and enjoy
the fresh air. The water was black (as normal) but clearer than
usual. I found Gillia altilis moderately common on the riprap used
to stabilize the banks directly under the bridge. Individuals were also
grazing on hard-packed clay. Three other freshwater gastropods were also
present: Physa acuta, Goniobasis catenaria catenaria, and Amnicola
limosa. Sometimes it seems to me that hydrobiid populations are
positively associated - the occurrence of one species making a second more
likely.
Here's the first moral I have derived from this experience - the value
of revisiting sample sites. I had previously made collections at this
site at least three or four times in the past under good conditions and had
never found a trace of Gillia. Certainly, I had no previous
observations from the winter, nor had I previously sampled water conditions
quite this low. But I feel certain that Gillia is a perennial, and
as common and conspicuous as the things were earlier this month, it's hard to
believe I missed them entirely four times. I was tempted to subtitle this
essay, "Humbled by hydrobiids again."
I've had similar experiences many times in the past. There's a spot on
the Combahee River at Yemassee about 50 miles south of Charleston, for example,
I've sampled at least annually for eight years. It's the type locality of
Physa hendersoni, which we've been using for experiments on reproductive
isolation in pulmonates. Prior to last March I had catalogued five
pulmonate species from the site, generally collected during the course of my
hunts for P. hendersoni. Last March, however, I found every stick
and rock in the Combahee River covered with Amnicola granum, a species
of which I had no prior record, and the five pulmonates had almost
vanished. Freshwater gastropod populations are flashy. Revisit your
sites.
The second moral of this story is never to underestimate the hidden
potential of crappy rivers. The Lynches River
arises in the lower piedmont of South Carolina, and on its roughly 150 mile
journey to join the Pee Dee River passes through a watershed characterized by
intensive row crop farming. Erosion and sedimentation have certainly been
major problems for over 300 years, and who knows what sort of chemicals they
spray on that cotton. Yet Goniobasis is very common in several of
its upstream tributaries, and (again, as faithful readers may remember) the
only modern record of Lioplax
in SC is also from the Lynches, about 40 miles upstream from the US 52
bridge. To appreciate the biodiversity value of any river, one needs much
more than a glance at a map and a few casual visits.
The web page I posted on 9Mar04 to report the results of my (510 record) survey
of the Freshwater Gastropods of South Carolina is now
badly out of date. In the section on recommendations, I wrote:
"The status of Gillia altilis in South Carolina, its type locality,
is worrisome. Burch & Tottenham (1980) quote the range given by
Walker (1918): 'Atlantic drainage from New Jersey to South Carolina,' although
populations apparently live as far north as Vermont and west to Lake Ontario
(Jokinen 1992). I have seen fairly recent collections of Gillia from the
Waccamaw River of southern North Carolina, but have to date been unable to
confirm its modern occurrence here."
The modern occurrence of Gillia in South Carolina is now no longer in
doubt. It is the continued existence of the species here that becomes the
question. Gillia should shoot right to the top of our state list
of aquatic species of concern.
We'll keep in touch,
Rob
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