Dr. Rob Dillon, Coordinator

Friday, January 23, 2009

The Freshwater Gastropods of Indiana

Kudos go to our colleague Mark Pyron and his students Jayson Beugly, Erika Martin and Matthew Spielman for their thorough survey of the freshwater gastropods of Indiana, appearing in the most recent American Malacological Bulletin (1). No big surprises here, but a job well done!

Physiographically, Indiana can be divided into three regions from north to south: the Great Lakes plains, the till plains, and the southern low plateau (which glaciation did not reach.) Rivers in a northern sliver of the state flow north to the Great Lakes, but the great majority of the state drains south through the Kankakee/Illinois or the Wabash/Ohio systems. Prehistorically the entire state would have been covered with mixed woodland, grassland, and wetland, but today 98% of Indiana has been converted to agriculture. Industrial regions have sprung up in the north and central.

Goodrich & van der Schalie (2) published a nice survey of the entire molluscan fauna of Indiana in 1944, and both the University of Michigan Museum of Zoology and the Ohio State Museum have substantial Indiana holdings today. Pyron's initial review of these historical records led him to expect a freshwater gastropod fauna of 39 species, nothing (to my eye) especially rare or surprising. He and his students then designed a survey of 123 ponds, lakes, rivers and streams around Indiana, covering all drainages and physiographic provinces, revisiting 86 sites for which historical data were available. They documented 36 freshwater gastropod species (3) – missing several that might have been expected from historical records, but discovering a few species not previously listed. Pyron concluded that "the majority of the freshwater gastropod taxa in Indiana are of local conservation concern."

This is the third squarish Midwestern state to be surveyed in recent years, and one is tempted to begin looking for patterns. Shi-Kuei Wu and colleagues (4) published a big monograph of the Missouri freshwater gastropods in 1997 which, boiling off 17 synonyms, listed 39 valid species. Our colleague Tim Stewart (5) had a nice 2006 paper in the AMB reporting 46 species of freshwater gastropods in Iowa (6). Both the Missouri and the Iowa inventories relied on historical records, and in fact, Tim developed his Iowa list without fresh field surveys of any sort. Thus the fair comparison would be with the total fauna of Indiana developed by Pyron - historic as well as modern - a list of 40 species (7).

The figure at left above is a Venn diagram of Midwestern States, using rectangles rather than circles, each state oriented geographically and scaled in proportion to its surface area (8). The most striking feature of this diagram to my eye is an effect of latitude (or perhaps physiography), making the Indiana/Iowa pair far more similar than either state is to Missouri.

Indiana has four unique species - almost entirely pleurocerids (including Leptoxis praerosa, Pleurocera canaliculata, and Pleurocera semicarinata "obovata"). Iowa has five unique species - almost entirely northern pulmonates (including Lymnaea catascopium "emarginata", L. megasoma, L. haldemani, and Physa skinneri). The two states share an impressive 35 freshwater gastropod species – 20 that also occur in Missouri and 15 that do not.

The Missouri fauna is characterized by 12 unique species of freshwater gastropods, including three pleurocerids and seven endemic hydrobiids of the Ozark Plateau. But as its diversity rises by extension into a new physiographic province, Missouri's fauna also declines by the subtraction of quite a few northern elements common in Iowa and Indiana, for example Lymnaea stagnalis, Helisoma campanulata, Aplexa and several Valvata. Thus although the largest of the three states, in net effect Missouri finishes with the shortest faunal list.

Meanwhile, back in Muncie, I'm pleased to report that our colleague Mark is busily developing an FWGNA-style website to disseminate the results of his Indiana survey more fully. And he’s thinking about collaborating and expanding this effort into the surrounding states of the upper Mississippi watershed. Exciting developments, to which we can all look forward!


(1) Pyron, M., J. Beugly, E. Martin, and M. Spielman (2008) Conservation of the freshwater gastropods of Indiana: Historic and current distributions. Am. Malac. Bull. 26: 137-151. [download PDF]

(2) Goodrich, C. and H. van der Schalie (1944) A revision of the Mollusca of Indiana. Am. Midl. Natur. 32: 257-326.

(3) Pyron's estimate of 36 extant species included four species reported by E. H. Jokinen (2005) Pond mollusks of Indiana Dunes National Lakeshore: Then and now. Am. Malac. Bull. 20: 1 - 9.

(4) Wu, S-K, R. D. Oesch and M. Gordon (1997) Missouri Aquatic Snails. Missouri Department of Conservation, Jefferson City. Natural History Series 5: 1 - 97.

(5) Stewart, T. W. (2006) The freshwater gastropods of Iowa (1821-1998): Species composition, geographic distributions, and conservation concerns. Am. Malac. Bull. 21: 59-75.

(6) Stewart listed 49 species, but here I combine Lymnaea exilis under L. palustris, Helisoma truncata under H. trivolvis, and Laevapex diaphanus under L. fuscus.

(7) Pyron's complete Indiana list included 41 species, but again I have combined Lymnaea exilis under L. palustris.

(8) Indiana 36,000 mi2, Iowa 56,000 mi2, Missouri 70,000 mi2.