Potamopyrgus, water hardness, and the Gatorade hypothesis

Volume 28(1) of Freshwater Mollusk Biology and Conservation hit the newsstands a couple weeks ago with the unwelcome news that populations of the invasive New Zealand mudsnail, Potamopyrgus antipodarum, have been discovered in two small tributaries of the Monongahela River in Allegheny County, Pennsylvania [1].  These are the first records of Potamopyrgus in the Ohio drainage.

For as many seasons as I have been watching Invasive Species Baseball from the grandstands, the Commonwealth of Pennsylvania has been a perennial contender for the MVA (Most Vigilant Agency) award.  The Pennsylvania Natural Heritage Program iMapInvasives web-based system is peerless, or almost so [2], anywhere in The East.  And in addition to maintaining the state’s elaborate citizen-friendly tool for online reporting, Ms. Amy Jewitt and her iMapInvasives staff post email alerts, publish a newsletter and a blog, sponsor workshops and webinars, and recently even produced a 44-minute, documentary-style film.

Rarely does a month go by that I don’t hear from Amy.  And I’ve also developed longstanding correspondence relationships with Steven Means of the PADEP and Sean Hartzell of the Pennsylvania Fish and Boat Commission (PFBC), who anchors the scientific side of the enterprise [3]. And more often than not, the subject line of all this correspondence with all these colleagues features the abbreviation “NZMS,” New Zealand mudsnail.

Peters Creek, Allegheny Co, PA

So, when one of my clerks dumped the morning mailbag onto the sorting table in my office back on June 26, 2023, I was not surprised to find a cordial message from my good buddy Sean.  And attached to his message was the usual quota of jpeg images of Potamopyrgus, including the one reproduced above.

But although the news that Sean relayed in his 26June23 message was not surprising, I don’t suppose, it was anything but routine.  Three days previous, a PFBC colleague, Mr. Mike Depew, had collected the gastropod sample depicted on those jpegs from a pair of small direct tributaries of the Monongahela River just upstream from Pittsburgh – Peters Creek and Turtle Creek.  Rats.  The first records of Potamopyrgus in an interior drainage of the Eastern United States [4].

My loyal readership will be quite familiar with Potamopyrgus from many previous posts on this blog; occasional visitors are invited to hit the “Invasive species” label at right for a review.  The first Atlantic drainage NZMS population was reported in a mid-Pennsylvania tributary of the Susquehanna River back in 2013 [19Nov13], with reports from Maryland [13June18] and New Jersey [9July18] following rapidly.

In their newly-published note [1], Sean, Mike, and three additional PFBC colleagues have done an excellent job reviewing the ten-year history of this recent Eastern U.S. range expansion, contributing a very nice map of the current distribution of Potamopyrgus across The Keystone State.  Quoting the authors, “both Peters Creek and Turtle Creek contain sections managed as stocked trout fisheries with public fishing access.”  They suggest that this most recent range expansion likely comes “via angling gear from previously invaded sites.”  We concur.

But returning to June of 2023.  In his original message to me, Sean called my attention to the pitting on the shells of the little snails in his jpegs (clearly visible in the figure at the top of this essay), which he “hadn’t seen in Potamopyrgus antipodarum before.”  I agreed that such pitting is unusual, and we swapped a couple additional emails on the topic.  Quoting myself:

The short answer to your question about shell pitting would be, “soft water.”  Presumably Peter’s Creek and Turtle Creek have lower concentrations of calcium, right?  So anywhere the outer proteinaceous periostracum of the shell gets nicked a little bit, the calcium carbonate core of the shell is exposed to dissolution, forming a pit.

That said.  If you’d like to re-write the paragraph above, and scratch out “soft water,” and substitute “Low pH” or “low alkalinity” or “low carbonate” or “Low buffering capacity” or maybe even overall “low conductivity,” it would all be just as true [5].

My 26June23 hypothesis, however, turned out to be unsupportable.  Even as I was offering it, Sean and his PADEP colleague Matthew Shank were embarking on an extensive, statewide study on the relationship between water chemistry and Potamopyrgus invasion that would find the hardness of Peters and Turtle Creeks perfectly suitable.

The Hartzell & Shank paper, published online "early view" last fall and February in hard copy [6], correlated NZMS presence/absence at 443 sites in Pennsylvania to 57 water chemical parameters, including (of course) all those hardness-related variables I had suggested in 2023.  They plotted their 71 present observations (red) and 372 absent observations (blue) on the simplified geological map of Pennsylvania reproduced below.  The relationship between successful Potamopyrgus invasion and the presence of limestone and dolomite in the drainage is striking, am I right?

From Hartzell & Shank [6]

And indeed, the Hartzell & Shank map does show a pair of red “present” dots in the west corresponding to Peters Creek and Turtle Creek [7], indicating limestone in their drainages as well.  Quoting Sean from our more recent (14Apr25) correspondence:

“Although the area has been (mostly historically) impacted by mining, the pH and conductivity samples we had on file for those streams were not reflective of soft water. Additionally, the statewide water chemistry suitability analyses that my colleague Matthew Shank and I worked on more recently . . . suggests that the two respective HUC12s that the snails were found in contain various highly suitable chemical parameters for this species.”

So what, then, might have been the cause of the shell pitting in the original 2023 samples of Potamopyrgus from Peters and Turtle Creeks?  Again, quoting my good buddy Sean, from our 14Apr25 correspondence:

“When Mike (Depew) sampled these streams (both on the same date) and upon finding the P. antipodarum, he realized he neglected to pack any containers to bring snails back to the lab and so upon improvising, he placed them in half-empty Gatorade bottles that he had on hand. As a result, the snails were submerged in Gatorade for about 24 hours before they were transferred to distilled water. From my understanding, Gatorade is quite acidic (a quick Google search indicates a pH of 2.9 to 3.6 depending on the exact flavor purchased) and so this is likely the cause of the pitting observed in the snails.”

Flavor not being among the 57 parameters analyzed by Hartzell & Shank, however, I fear that we shall never have a conclusive answer to this particular mystery.

 

Notes

 

[1] Hartzell, S.M., M.A. Depew, D. Byington, L. Hartman, and R. Pletcher (2025) Collections of the invasive New Zealand mudsnail, Potamopyrgus antipodarum (J.E. Gray, 1843) in the Ohio River basin.  Freshwater Mollusk Biology and Conservation 28: 22 – 25.

[2] To be fair, both New York and Maine also participate in NatureServe's iMapInvasives Network.  But I don't know any of those dedicated folks way up there.  

 

[3] Recent publications from our good friend Sean:

• Hartzell, S.M. and N. Macelko. 2022. Range expansion of the invasive New Zealand Mudsnail (Potamopyrgus antipodarum) in the Susquehanna and Delaware River Basins of Pennsylvania.  Journal of the Pennsylvania Academy of Science 96: 36 - 45.
• Hartzell, S.M. and J.R. Frederick. 2023. First records of the invasive New Zealand mudsnail (Potamopyrgus antipodarum) in the Potomac River Basin.  Northeastern Naturalist 30 (1): N13 - N16.

[4] The USGS Nonindigenous Aquatic Species database does show quite a few records in Wisconsin tributaries of the Rock River, which I think of as being midwestern.

 

[5] I reviewed the subject of environmental calcium as a factor in freshwater gastropod distribution at great length in Chapter 8 of my book, pp 326 - 338:

• Dillon, R.T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.  509 pp.

[6] Hartzell, S.M. and M.K. Shank. 2025.  Chemical variables predicting colonization risk of the invasive New Zealand mudsnail (Potamopyrgus antipodarum) in Pennsylvania's flowing waters.  Hydrobiologia 852: 645 - 658. https://doi.org/10.1007/s10750-024-05711-2.

 

[7] At the earlier date of the publication of their water quality paper, Hartzell & Shank listed their observations of Potamopyrgus in drainages of The Ohio as “Hartzell et al. unpublished data.”