To the FWGNA group,
I am once again indebted to Dr. Julian Harrison for calling my
attention to a previously unconfirmed element of the South Carolina
gastropod fauna, the little planorbid
Promenetus exacuous. This brings the list of South
Carolina freshwater gastropods up to 28 taxa.
Julian has collected Promenetus
from lentic environments in two areas of the South Carolina coastal
plain, one near Drayton Hall Plantation on the current outskirts of
Charleston, the other by the Edisto River about 90 km inland. But
the meat of the species’ range is much further north - generally above
the glacial maximum. Promenetus
does occur in coastal regions of North Carolina, increasingly scattered
as one proceeds south. Maps of its distribution in both NC and SC
can be viewed from the brand new Promenetus web page*, written
by our colleague Tim Stewart of Iowa State University.
Julian Harrison also gets the credit for discovery of the limpet Hebetancylus excentricus in South
Carolina. The distribution
of Hebetancylus here is
similar to that of Promenetus
- it occurs in a few scattered coastal plain ponds. But Hebetancylus is a more southern
species, the meat of its North American range extending from Florida to
Texas.
The common factor in the occurrence of both Promenetus and Hebetancylus in South Carolina,
other than that both were called to my attention by Julian Harrison, is
that both occur in habitats frequented by migratory waterfowl.
Charleston and the Carolina lowcountry lie directly beneath the
Atlantic flyway, visited twice annually by millions of ducks, geese,
and other migratory water birds. (An interesting radiotelemetric study
has been published by the Atlantic
Flyway Pintail Project.) All the ponds from which Promenetus and Hebetancylus have been collected
here are certainly visited. In fact, the pond from which Hebetancylus was first reported is
an ornithological research site. Both gastropods are small and
clingy. Although the evidence is circumstantial, I think that Promenetus and Hebetancylus may owe their
existence in South Carolina to dispersal through the air.
Broadening our discussion a bit, the distributions of Planorbula armigera and Valvata bicarinata in North
Carolina* look very similar to that of Promenetus in South Carolina -
scattered coastal habitats associated with waterfowl. It is
tempting also to attribute the single South Carolina population of Biomphalaria to aerial dispersal,
although the import of aquatic vegetation may be a more likely
explanation here.
To those interested in learning more about the aerial dispersal of
mollusca, I would recommend the charming (1965) paper of W. J.
Rees. Rees collected several anecdotes connecting freshwater
gastropods to both birds and insects. The experiments of Malone
(1965 a,b) and Boag (1986) are also interesting in this regard.
References below.
In summary, one often sees the adjective “accidental” used in bird
books to describe the occurrence of particular avian species outside
their normal ranges. I would suggest that “accidental” might just
as aptly be applied to certain occurrences of freshwater gastropods as
well.
References
Boag, D. A. (1986) Dispersal in pond snails: potential
role of waterfowl. Can. J. Zool. 64: 904 - 909.
Malone, C. R. (1965a) Dispersal of aquatic gastropods via the
intestinal tract of water birds. Nautilus 78: 135-139.
Malone, C. R. (1965b) Killdeer (Charadrius
vociferus L) as a means of dispersal for aquatic
gastropods. Ecology 46: 551 - 552.
Rees, W. J. (1965) The aerial dispersal of Mollusca. Proc.
Malacol. Soc. Lond. 36: 269 - 282.
*PS - Pardon our pixel dust! The FWGSC web site is currently in a
state of transition. And watch for a brand new North Carolina web
site, to be integrated with the South Carolina site, coming soon!
Return . . . to the FWGNA home
page.