FWGNA > Freshwater Gastropods of Mid-Atlantic States
Mid-Atlantic Photobar
Mid-Atlantic States
The present survey is directed toward the freshwater gastropod faunas of Delaware, Maryland, New Jersey, eastern Pennsylvania and the West Virginia panhandle, a total study area of approximately 150,000 km2. Coverage includes most of the Potomac, Susquehanna, and Delaware River drainages, as well as a number of smaller rivers draining directly into the Chesapeake Bay and the Atlantic Ocean, such as the Choptank, Nanticoke, and the Raritan.

Faunally, the Mid-Atlantic states are a region of transition.  The Piedmont and Coastal Plains, which dominated the physiography of the southern Atlantic drainages, narrow through Maryland and New Jersey, as the Central Appalachian ecoregion crowds in from the west.  The Blue Ridge disappears entirely in southern Pennsylvania, as the climate grows more harshly seasonal.  The USDA has assigned northern Pennsylvania and northern New Jersey to “Cold-hardiness Zone 6,” recording average minimum yearly low temperatures below 0 degrees F.  Glaciers extended down into this zone during the coldest stages of the Pleistocene, leaving a scoured topography that today hosts a freshwater gastropod fauna distinct from that of the southern Atlantic drainages.

Patterns of land use transition in parallel with the change in land form.  Large farms with extensive row crops still dominate much of Eastern Maryland, Delaware, and southern New Jersey, as they did most of Virginia and the Carolinas.  But agriculture has largely been displaced by manufacturing and urbanization in the densely-populated heart of the Mid-Atlantic region, where the cities of the “BoWash Corridor” have nearly grown together into a single metropolis.  Land use in western Maryland, the West Virginia panhandle, and central Pennsylvania is predominantly by small farms and livestock, with forested ridges.

FWGMA Study Area The Mid-Atlantic states are the birthplace of American malacology, and might yet today, some 200 years later, still bid fair to be called our home.  For it was in 1812 that Thomas Say and a small society of bright young men founded the Academy of Natural Sciences of Philadelphia, the first natural history museum in the Western Hemisphere.  And writing from Philadelphia in 1817 – 1825, Say published the first works of malacology in the New World, describing many of the most familiar little brown snails we find in our lakes and rivers today, for example Helisoma trivolvis, Lymnaea humilis, Campeloma decisum, and Amnicola limosa.  Say’s mantle was taken up by T. A. Conrad and S. S. Haldeman, then passed to Isaac Lea, G. W. Tryon, H. A. Pilsbry, H. B. Baker, and onward to the present day.

Any North American freshwater gastropod escaping the nets cast from Philadelphia in the early 19th century might well have been scooped up by a second team of fishermen one state south.  The Smithsonian Institution was founded in 1846 and organized a department of mollusks in 1880, curated by W. H. Dall.  Entering the 20th century, the size and scope of the collections at Washington began to rival those of the older institution at Philadelphia, and indeed any in the world.

The Carnegie Museum of Natural History was founded at Pittsburgh in 1895, and although primarily focused on the molluscan fauna of interior drainages, influential early curators H. H. Smith and A. E. Ortmann were not strangers to the rivers and streams east of the Alleghenies.  And the freshwater gastropod holdings of the Delaware Museum of Natural History, founded at Wilmington in 1972, are not negligible by any means, either.

One might imagine that such a concentration of malacological talent and resource as one finds localized in the Mid-Atlantic states might lead to a well-documented freshwater gastropod fauna.  And by nineteenth century standards, that was certainly true.  But most of the region has never seen a systematic inventory of the sort made possible by motorized transportation.

The exception is Pennsylvania, for which R. R. Evans & S. J. Ray published a checklist in 2008, followed by a field survey in 2010.  The 2008 work brought an old and scattered literature together with a modern survey of museum holdings, yielding a remarkable 63 nominal freshwater gastropod species for the Keystone State, of which 43 might be recognized as specifically distinct inhabitants of Atlantic drainages.  The 2010 work reported a field survey of 398 lotic water bodies across the state (no ponds, lakes, or marshes), returning 37 nominal species, 24 of which are specifically distinct inhabitants of the present study area.  Much of the discrepancy in the species richness between the 2008 checklist and the 2010 field survey probably arises from the exclusion of lentic waters from the latter, but not all.  See my essay of 22June10 for more.

The 20th century has seen very scattered surveys of the other Mid-Atlantic states.  The checklist of Richards (1934) contained 44 nominal freshwater gastropod species collected “within a radius of 20 miles of the Capitol Building in the city of Washington,” 30 of which might be considered specifically distinct today.  Pearce & Evans (2008) recovered but 12 of these on Plummers Island, Maryland, approximately 15 km upstream from Washington.  Chapman et al. (2012) reported 13 freshwater gastropod species in a single small pond in northern New Jersey.

But most of the Mid-Atlantic region has never seen a survey of its freshwater gastropod fauna.  Indeed, we have a more complete inventory of the freshwater gastropods of sub-Saharan Africa than we do of Delaware, the eastern shore of Maryland, or the pine barrens of New Jersey, for example.  Here that situation is set aright.

> Methods

The database as analyzed here comprises 3,159 records (9Oct23).  The largest fraction were collected by the natural resource agencies of the several states in connection with their routine water quality monitoring responsibilities.  The Pennsylvania Department of Environmental Protection collects physical, chemical, and biological data from a “Water Quality Network” of approximately 180 sites in August and September every other year (at minimum), approximately 120 of which are located on the eastern (Atlantic-draining) side of the state.  Samples of the macroinvertebrate fauna are taken semi-quantitatively, using a D-frame net for wadeable streams (or other gear, as appropriate) in both riffle and run over reaches of 100 meters, subsampled to a 200-count (PA-DEP 2013).  We visited the PA-DEP lab in Harrisburg, PA in March of 2013 and reviewed all the freshwater gastropods collected in WQN samples over a period of six years, 2006 – 2011.  This yielded a total of 108 records.

Atlantic drainage sample sites

The Maryland Department of Natural Resources has developed two water quality monitoring programs: Core/Trend and MBSS.  A set of approximately 120 “Core/Trend” stations, generally located in the mid-order to larger rivers, has been sampled annually since the 1970s.  More recently the “Maryland Biological Stream Survey” has been developed to focus on smaller streams, and to include some vernal habitats and freshwater tidal marshes.  MBSS sites are randomly chosen from regions cycling around the state.  Details regarding site selection, sampling methods, sorting and reporting are available in Friedman (2009) for Core/Trend and Stranko et al. (2010) or Ashton (2012) for MBSS.

We visited the MD-DNR laboratory in Annapolis in October of 2012 and reviewed all the macroinvertebrate samples taken in connection with the Core/Trend monitoring program 2001 – 2009, and those taken in connection with the MBSS program 2000 – 2006.  We extracted 232 freshwater gastropod records from 92 (fixed) Core/Trend sites and 457 records from the approximately 335 randomly-selected MBSS sites at which freshwater gastropods were collected.

The Delaware Department of Natural Resources and Environmental Control assesses the water quality in its nontidal, perennial streams using standard physical, chemical, and biological criteria (Barbour et al. 1999) on a regional basis: piedmont and coastal plains on alternating years.  Semiquantitative macroinvertebrate samples are taken during fall low-flow conditions using a D-net (approximately 6 m2 per sample), and subsampled to a 200 count.

We visited the Dover laboratories of DNREC in January of 2013 and reviewed macrobenthic samples from three years: 2006 (Piedmont), 2010 (Coastal Plain), and 2011 (Piedmont again).  Approximately 40 – 50 sites were sampled each of these years, yielding a total of 198 freshwater gastropod records.

The systematic collections of four museums were reviewed in connection with the present inventory: the US National Museum in Washington, the Academy of Natural Sciences of Philadephia, the Carnegie Museum of Natural History in Pittsburgh, and the Delaware Museum of Natural History in Wilmington.  We screened the catalogued holdings of each institution by four criteria: (1) freshwater gastropods, (2) collected from Mid-Atlantic drainages as defined here, (3) no earlier than 1955, (4) with locality data of sufficient detail to be plotted.

The identity of every qualifying museum lot was directly confirmed on site before entry into the present database.   Ultimately we obtained 58 records from the USNM, 198 records from the ANSP, 108 records from the DMNH, and 465 records from the CMNH.  The great majority of the CMNH records were collected by Evans & Ray (2010), effectively merging that earlier survey into the present one.

Finally, an original field survey was conducted to supplement the collections already held by museums and natural resource agencies, using simple qualitative searches (Dillon 2006).  Our 2012 and 2013 field seasons focused on lentic habitats, which have tended to be neglected by water quality monitoring agencies, the larger rivers, and freshwater tidal estuaries.  Personal field collections yielded 684 records.  

Click the image above to download a pdf map showing our sample sites.  No “absence stations” are mapped.  If freshwater gastropods were not collected at a site, then no record resulted.  Our entire 3,159 record database is available (as an excel spreadsheet) from the senior author upon request.

The taxonomy employed by the FWGNA project is painstakingly researched, well-reasoned and insightful.  Needless to say, it often differs strikingly from the gastropod taxonomy in common currency among casual users and most natural resource agencies.  First-time visitors looking for information about particular species or genera might profitably begin their searches with a check for synonyms in our alphabetical index.


The 41 species and subspecies of freshwater gastropods we have confirmed from the Atlantic drainages of the middle states are figured in the FWGMA gallery and distinguished on the FWGMA dichotomous key.  Ecological and systematic notes for each species and subspecies are provided on dedicated pages, together with regional distribution maps.  Their distributions and abundances on a continental scale are analyzed in our sections on Biogeography and Synthesis

> Acknowledgements

We thank Mr. Tony Shaw, Ms. Molly Pulket, Mr. Rick Spear, and (especially) Mr. Dan Boger of the PA-DEP in Harrisburg.  We thank Mr. Neal Dziepak and especially Ms. Ellen Friedman of the MD-DNR in Annapolis.  Ms. Ellen Dickey was a gracious host at the DNREC laboratory in Dover.  Special appreciation is due to Mr. Ryan Evans for sharing his Pennsylvania dataset so generously.

The following colleagues graciously hosted us in the museums: Tim Pearce at the CMNH in Pittsburg, Gary Rosenberg, Paul Callomon, Amanda Lawless at the Academy of Natural Sciences of Philadelphia, Liz Shea at the Delaware Museum of Natural History, and Bob Hershler at the US National Museum.  Financial support from the Pennsylvania Shell Club is gratefully acknowledged.

Rick Relyea and Andy Turner were marvelous hosts at the University of Pittsburgh's Pymatuning Laboratory of Ecology during the summer of 2012.  Accommodations were also provided by Russ and Karen Ingersoll, whose unflagging hospitality over the course of many months of chronic brother-in-law problems was vital to the success of this endeavor.

> References

Ashton, M. J. (2012)  How a statewide stream survey can aid in understanding freshwater mussel (Bivalvia: Unionidae) ecology: Examples of utility and limitations from Maryland.  Walkerana 15: 1 – 10.
Barbour, M., J. Gerritsen, B. Snyder, & J. Stribling (1999)  Rapid bioassessment protocols for use in streams and wadeable rivers: Periphyton, benthic macroinvertebrates, and fish, Second edition. Washington, DC, US EPA 841-B-99-002. 
Chapman, E. J., R. S. Prezant & R. Shell (2012)  Temporal variation in molluscan community structure in an urban New Jersey pond.  Northeastern Naturalist 19: 373-390.
Dillon, R.T., Jr. (2006)  Freshwater Gastropoda. pp 251 - 259  In The Mollusks, A Guide to their Study, Collection, and Preservation. Sturm, Pearce, & Valdes (eds.) American Malacological Society, Los Angeles & Pittsburgh. 
Evans, R. & Ray, S. (2008)  Checklist of the freshwater snails (Mollusca: Gastropoda) of Pennyslvania, USA.  Journal of the Pennsylvania Academy of Science 82: 92-97.
Evans, R. & Ray, S. (2010)  Distribution and environmental influences on freshwater gastropods from lotic systems and spring environments in Pennsylvania, USA, with conservation recommendations.  American Malacological Bulletin 28: 135 - 150.
Pearce, T. A. & Evans, R. (2008)  Freshwater Mollusca of Plummers Island, Maryland.  Bull. Biol. Soc. Washington 15: 20 - 30.
Richards, H. G. (1934) A list of the mollusks of the District of Columbia and vicinity.  Amer. Midl. Natur. 15: 85 - 88.
PA-DEP.  Pennsylvania’s Surface Water Quality Monitoring Network (WQN):
Friedman, E. S. (2009)  Benthic macroinvertebrate communities at Maryland’s Core/Trend monitoring stations: Water quality status and trends.  Maryland Department of Natural Resources Publication MN-09-01. 95 pp.
Stranko, S, D. Boward, J. Kilian, A. Becker, M. Ashton, A. Schenk, R. Gauza, A. Roseberry-Lincoln & P. Kazyak (2010)  Maryland Biological Stream Survey, Round Three Field Sampling Manual.  Maryland Department of Natural Resources Publication EA-07-01.  66 pp.