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.”

Deadly Snails Invading the US!

Yesterday evening my wife and I were having supper with family friends when a young lady – very much attuned to social media of diverse sorts, as so many of the youth these days – mentioned that she had been “bombarded” with alerts about dangerous snails in North Carolina.  This was completely out of the blue.  She’s not a biologist – does not follow technical news feeds – just a regular citizen of the Charleston area in her mid-20s.

NCWRC

I, very much the opposite, confessed complete ignorance of the situation.  So, our young friend whipped out her smart phone, deftly touched off three key strokes and a swipe, and there was the news.  Invasive Pomacea of the maculata/canaliculata sort have been reported in the Lumber River at Lumberton, NC.  But my goodness, the hysteria!

The media frenzy seems to have been kicked off by a perfectly responsible press release from the North Carolina Wildlife Resources Commission on Monday 2Oct23 [1].  Initially alerted by a concerned citizen, the NCWRC conducted a survey that did indeed confirm an invasive Pomacea population extending from the I-95 bridge just above Lumberton [2] to a boat ramp about 6 km downstream.  In measured tones, the press release cautioned:

“Apple Snail grazing habits can damage plants used by many native aquatic species and they have even been observed feeding on amphibian eggs. Additionally, Apple Snails can present human health risks. They may carry rat lungworm, which can cause a potentially fatal disease in humans if the snails are eaten raw or undercooked.”

From that relatively innocuous paragraph came the New York Post headline of 4Oct23, “Deadly Apple Snails found along North Carolina River,” and from CBS News, “Invasive snails that can be deadly to humans found in North Carolina.”  But my favourite headline came from the UK Daily Mail, “Invasive Snails Deadly to Humans are Invading the US!” [html] [pdf] 

The Lumber River continues into South Carolina to unite with the PeeDee River about 50 km downstream from Lumberton.  Another 80 km downstream by kayak through impenetrable swamp would bring us to the mouth of the Waccamaw River, from whence it is but 10 – 15 km back upstream to Socastee, SC, where invasive Pomacea were first reported in 2008 [3].  Whether the North Carolina population represents a new introduction, or simply a 150 km expansion of the South Carolina population, remains to be determined.

We saw a similar wave of concern spread through the Myrtle Beach area of South Carolina when the snails first arrived here 15 years ago, although much lower in amplitude and local in extent.  The local newspapers here described apple snails as merely “harmful” or “worrisome,” not “deadly.”

In retrospect, the NCWRC might have added significantly more context to their press release.  South Carolina researchers have found no evidence of Angiostrongylus parasitism in samples of Pomacea taken here in The Palmetto State [4].  Indeed, the extensive 2013 survey conducted by Teem and colleagues across Louisiana, Texas, Mississippi, and Florida yielded only 8 cases of Angiostrongylus parasitism in 296 Pomacea tested, all from the New Orleans area [5].  And as for cases of actual rat lungworm disease in humans, the CDC was only able to confirm 12 cases in the continental USA 2011 - 2017, the majority of which were linked to eating raw vegetables, not snails [6].

So when invasive Pomacea arrive in Virginia, here’s a suggestion for that press release.  Bold the clause, “if the snails are eaten.”  And suggest that the readership resist the temptation to pop one in their mouths.  Everything will be OK.

Notes

[1] Invasive Apple Snails Now Confirmed in North Carolina.  North Carolina Wildlife Resources Commission, 2October23. [html] [pdf]

[2] In my blog post of 13June18, I advocated legislation to build “a big, beautiful wall on the North Carolina line from Cape Hatteras to Tennessee, 50 feet tall by back-of-the-envelope calculation, Pedro himself manning the I-95 guardhouse just two mucus trails and one gigantic traffic jam North of the Border” to intercept just such a Pomacea invasion as North Carolina is now experiencing here in 2023.  See, I told you so.

[3] More about Pomacea in South Carolina:

• Pomacea spreads to South Carolina [15May08]
• Two dispatches from the Pomacea front [14Aug08]
• Pomacea News [25July13]
• Invasive species updates [13June18]

[4] Underwood, E.B., M.J. Walker, T.L. Darden & P.R. Kingsley-Smith (2019) Frequency of occurrence of the rat lungworm parasite in the invasive island apple snail in South Carolina, USA.  Journal of Aquatic Animal Health 31(2): 168 – 172.

[5] Teem, J.L., Y. Qvarnstrom, H.S. Bishop, A.J. DaSilva, J. Carter, J. White-Mclean, and T. Smith (2013)  The occurrence of the rat lungworm, Angiostrongylus cantonensis, in nonindigenous snails in the Gulf of Mexico region of the United States.  Hawaii J. Med. Publ. Health 72: 11 – 14.

[6] Liu EW, Schwartz BS, Hysmith ND, et al. (2018) Rat Lungworm Infection Associated with Central Nervous System Disease — Eight U.S. States, January 2011–January 2017. Morb Mortal Wkly Rep 67:825–828.

Potamopyrgus in New Jersey

Last Wednesday I received an email from Mike Cole of Cole Ecological, forwarded by our good buddy Tim Pearce.  Attached to Mike’s message was the JPEG below, from samples taken in the Musconetcong River of northwestern New Jersey, a tributary of the Delaware.  This is the third introduction of the New Zealand Mud Snail, Potamopyrgus antipodarum, confirmed for US Atlantic drainages.

From Mike Cole 4July18

We reported Potamopyrgus in Spring Creek, a tributary of the Susquehanna in central Pennsylvania, back in 2013, and the 2017 discovery in Maryland’s Gunpowder River just last month [1].  The Musconetcong, much like the Gunpowder and Spring Creek, is a lovely body of water, tumbling cold, clear and rich about 73 km through a surprisingly unspoiled valley.  Mike reports Potamopyrgus at five sites scattered along the lower 10 km of the river.

So NZMS were completely unknown in US Atlantic drainages until one day they popped up in the middle of Pennsylvania, and the next day they popped up in Maryland 50 km south, and the next day they popped up in New Jersey 200 km east.  What is going on here?

What the Musconetcong shares with Spring Creek and the Gunpowder River, in addition to NZMS introductions, is trout.  New Jersey Monthly counted the “mighty Musky” as “among the state’s most revered waterways, thanks to its ever-changing landscape and world-class fly-fishing [2].”  The river is stocked by NJ Fish & Wildlife biologists weekly in April and May, pretty much down its entire length.

My father and I used to enjoy a fair amount of trout fishing ourselves, when I was young.  Early in the spring… doggone that water was cold… we always wore hip boots.  And in fact, I’ve got at least two or three pair of boots hanging upside down in my storage room this morning.

My right boot toe

Almost all the hip boots and waders I’ve ever worn in my life had deep, heavy tread on the soles, into which mud was always caked.  But I never cleaned the soles of my boots… not in 50 years… never really thought about it.

We fishermen (and biologists) need to start thinking about it.  That mud in the soles of our boots has the potential to track a lot of hitch-hikers from one stream to the next.  And remember – Potamopyrgus are parthenogenic brooders.  There’s no such thing as “one.”  And there’s nothing to stop those critters from washing down into the Delaware River.  Darn it.

Notes

[1] Previous posts on Potamopyrgus:

• Invaders Great and Small [19Sept08]
• Potamopyrgus in US Atlantic drainages [19Nov13]
• Invasive Species Updates [13June18]

[2] NJ Monthly, 19Mar14:

Gone Fishing: Musconetcong River [html]

Invasive Species Updates

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Invasive Species Updates.  Pp 15 – 22 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays.  FWGNA Project, Charleston, SC.

Potamopyrgus in Maryland.  Early last September our good buddy Matt Ashton of the Maryland DNR sent me an email with a couple jpegs attached, including the photo below.  Matt was forwarding the report of a concerned citizen with very good reason to think he had discovered New Zealand Mud Snails in the Gunpowder River at the Prettyboy Dam tailwaters, about 30 km north of Baltimore.  See the 21Sept17 article in The Baltimore Sun from note [2] below for more details.

From Matt Ashton, MD-DNR

The Gunpowder is a lovely little river through most of its course, passing through four state parks on its 90 km journey from headwaters in southern Pennsylvania to mouth at the Chesapeake Bay.  In the late 1980s the Prettyboy Dam tailwaters were stocked with brown and rainbow trout, eggs and fingerlings as well as adults, and both populations have now become self-sustaining [3].  Attention from an avid community of fishermen has quite predictably followed.

This is the second population of Potamopyrgus antipodarium reported from a US Atlantic drainage.  My readership will probably remember the first report I posted back in November of 2013, from a small tributary of the Susquehanna River in central Pennsylvania, also heavily fished by trout anglers [4].  Spring Creek is perhaps 50 km north of the Gunpowder headwaters.  The implication would be that the little snails were transported from Pennsylvania to Maryland via muddy waders or bait buckets.

From the mouth of the Gunpowder to the heart of our nation’s capital is but another 80 km.  I understand that legislation has been introduced in Congress to build a big, beautiful six-inch wall from the Delaware River to the West Virginia line, with a three-inch guardhouse on the Baltimore/Washington parkway.

And (less surprisingly) in Syracuse.  The Great Lakes populations of Potamopyrgus are much older than the Atlantic populations; New Zealand mud snails were first reported from Lake Ontario as early as 1991 and Lake Erie in 2005.  So, surfing around in the USGS-NAS Nonindigenous Aquatic Species Database [5], I picked up a couple 2016 reports of Potamopyrgus in tributaries of Onondaga Lake, near Syracuse.

The Erie Canal runs from the eastern end of Lake Erie across the width of New York, intersecting Oneida Lake and the tailwaters of several of the Finger Lakes, as well as Onondaga Lake, to meet the Hudson River just north of Albany.  How long will it be until tired, poor Potamopyrgus are tempest-tossed down the Hudson into New York Harbor, to enter Ellis Island through the back door?  Is that one golden, too?

Pomacea progress.  As threatening as an invasion of 5 mm snails down from the north most certainly must be to homeland security, the threat posed by an army of 5 cm snails advancing up from the south is two orders of magnitude worse.  On the plus side, however, we are getting some very good science out of it.

In late 2015 our colleague Ken Hayes and 32 coauthors published a comprehensive review of the biology of the entire family Ampullariidae [6], broader than their systematic paper of 2012 [7] and better-integrated than the big 2006 book edited by Joshi & Sebastian [8].  More than just the economically-important pests and pets, the authors reviewed the entire scientific literature published on all 117 ampullariid species (by their estimate) in all 7 genera worldwide.

But though Ken and his colleagues cast their nets as broadly as possible, what they drew back was almost entirely Pomacea canaliculata.  They concluded with a call for additional research of a comparative nature, including behavioral studies that might reveal patterns that may have played a role in the evolution of the other 116 species.  We most certainly agree.

PhilRice II.  So speaking of Joshi & Sebastian.  Last year the Philippine Rice Institute published a second big collection of research focused on the Pomacea invasion, this volume with our good friend Rob Cowie among the editors [9].  A free pdf download is available from the link below.  

Fig. 7 of Rama Rao et al. [14]

Maculata/canaliculata hybridization.  Since the first appearance of pest Pomacea in North America, I have continued to be especially curious about reproductive isolation between populations of P. canaliculata and what is now called P. maculata.  So perhaps the most interesting Pomacea news in recent years, from my perspective, was the 2008 discovery that P. maculata has been introduced together with P. canaliculata throughout east and southeast Asia [10].

Apparently published a bit too late to be collected in the nets of Hayes and his 32 colleagues was an excellent 2013 study by Matsukura et al. documenting extensive hybridization between P. canaliculata and P. maculata throughout Japan, Korea, Vietnam and The Philippines [11].  Of the 16 populations sampled, 7 (all Japanese) were apparently pure canaliculata.  The remaining 9 were mixtures, including one population that was primarily maculata with a few hybrids, and one that was primarily hybrid, with a few pure maculata and canaliculata.

Even more interestingly, Matsukura performed a set of no-choice mating experiments between canaliculata and maculata, suggesting that F1 hybrid clutches suffer significant reduction in their hatching rates, from around 80% down to maybe 20% [12].

Pomacea release sex pheromones and demonstrate elaborate courtship behaviors, featuring nuptial gifts [13].  It would be tempting to hypothesize that some sort of prezygotic reproductive isolating mechanisms may have evolved in their native ranges, which have now broken down in Asian rice fields, as lions and tigers mate in zoos.  But in a surprise finish, Matsukura and colleagues also obtained a sample of 17 individual Pomacea from 5 sites in Argentina and discovered evidence of hybridization in ten of them!  Even in the home range.  Fascinating.

If maculata and canaliculata cannot tell each other apart, how can we?  Hayes et al. [7] observed that typical egg masses of P. maculata contain significantly more eggs of a significantly smaller size than P. canaliculata, a result that Matsukura and colleagues confirmed in Japan.  But that’s not terribly helpful with an adult in hand. 

So Hayes et al [7] also suggested a variety of distinguishing shell traits [14], including adult size, ratio of spire height to shell length, shell thickness, umbilicus, shouldering, and lip pigmentation.  But just last month Rama Rao and colleagues [15] published an interesting study of 130 Pomacea sampled from 8 populations in peninsular Malaysia, 5 of which turned out to be canaliculata/maculata mixtures, judging from CO1 sequence. 

Rama Rao and colleagues did not employ nuclear markers, and hence could not identify hybrids [16].  But judging from mtDNA haplotype, they selected 26 canaliculata and 26 maculata and performed an extensive series of shell morphological and morphometric analyses, including simple linear measures and ratios as well as the geometric analysis shown in the figure above.  They did discover a statistically-significant difference between the two groups in ratios of shell height to shell width and aperture height, but of such a fine nature as to be practically useless.  The bottom line is that the Pomacea of Malaysia seem to comprise one single heap of big slimy snails, darn near indivisible.

And meanwhile, in South Carolina.  Our colleague Elizabeth Gooding and six coauthors (including yours truly) have just published a statewide survey [17] for Pomacea at the highest latitudes of its current US range [18].  The Socastee/Myrtle Beach area [19] is on the northern tip of USDA cold-hardiness zone 8b, with average annual extreme minimums around 15 to 20 degrees F, or -9 to -6 degrees C.  We report both copulation and egg laying year-round, even in coldest months of winter [20].

I understand that legislation has been introduced in Washington to build a big, beautiful wall on the North Carolina line from Cape Hatteras to Tennessee, 50 feet tall by back-of-the-envelope calculation [21], Pedro himself manning the I-95 guardhouse just two mucus trails and one gigantic traffic jam North of the Border.

Notes

[1] My earlier Pomacea reviews include:

• Pomacea News [25July13]
• Global Advances in Apple Snails [24May07]

[2] Invasive New Zealand mud snails found in Gunpowder River. The Baltimore Sun 21Sept17.

[3] The situation is actually a bit more complicated.  The first 7.2 miles below the dam are managed as a wild trout stream, catch-and release.  Anglers in the next 4.2 miles can keep two trout/day.  And the next 6.1 miles are stocked in the spring and fall with ordinary hatchery-reared trout, creel limit five trout/day.  See MD-DNR-fisheries.

[4] For more about the discovery of New Zealand Mud Snails in central Pennsylvania:

• Potamopyrgus in US Atlantic Drainages [19Nov13]

[5] For my reviews of state and national invasive species online databases, see:

• To Only Know Invasives [16Oct15]
• To Only Know Invasives in My General Vicinity [9Nov15]

[6] Hayes KA, Burks RL, Castro-Vazquez A, Darby PC, Heras H, Martín PR, et al. (2015) Insights from an integrated view of the biology of apple snails (Caenogastropoda: Ampullariidae). Malacologia 58(1–2):245–302.

[7] Hayes KA, Cowie RH, Thiengo SC, Strong EE (2012) Comparing apples with apples: clarifying the identities of two highly invasive Neotropical Ampullariidae (Caenogastropoda). Zool J Linn Soc. 166(4): 723–753.

[8] Joshi RC, Sebastian LS. (2006) Global advances in ecology and management of golden apple snails: Philippine Rice Research Institute (PhilRice), 600 pp.

[9] Joshi, RC, Cowie RH, Sebastian LS. (2017)  Biology and management of invasive apple snails.  Philippine Rice Research Institute (PhilRice) 406 pp.  [PDF]

[10] Hayes, K. A. et al. (2008) Out of South America: multiple origins of non-native apple snails in Asia.  Diversity and Distributions 14: 701-712.

Matsukura, K. et al. (2008) Genetic divergence of the genus Pomacea (Gastropoda: Ampullariidae) distributed in Japan, and a simple molecular method to distinguish between P. canaliculata and P. maculata.  Appl. Entomol. Zool. 43:535-540.

[11] Matsukura, K et al.  (2013) Genetic exchange between two freshwater apple snails, Pomacea canaliculata and Pomacea maculata invading East and Southeast Asia.  Biol. Invasions 15: 2039-2048.

[12] The hatchability experiment was not as neat as one might hope, and the results a bit ambiguous.  See the paper itself [10] for details.

[13] Burela S, Martin PR. 2009.  Sequential pathways in the mating behavior of the apple snail Pomacea canaliculata (Caenogastropoda: Ampullariidae).  Malacologia 51: 157 – 164.

Burela S, Martín PR. 2011. Evolution and functional significance of lengthy copulations in a promiscuous apple snail, Pomacea canaliculata (Caenogastropoda: Ampullariidae).  Journal of Molluscan Studies 77: 54–64.

Takeichi M, Hirai Y, Yusa Y. 2007. A water-born sex pheromone and trail following in the apple snail, Pomacea canaliculata.  Journal of Molluscan Studies 73: 275–278.

[14] There are also anatomical differences between canaliculata and maculata, most notably a difference in penial sheath.  A study of anatomical hybrids would be most salutary.

[15] Rama Rao S, Liew T-S, Yow Y-Y, Ratnayeke S (2018) Cryptic diversity: Two morphologically similar species of invasive apple snail in Peninsular Malaysia. PLoS ONE 13(5): e0196582.

[16] Back in the allozyme days, I could have solved that problem with one afternoon in the laboratory and $30 for reagents.  It does frustrate me that science often seems to lose good old technology in its restless quest for the new.

[17] This is the same survey of which I made passing mention in my essays of November and December of 2015.  Although we did not discover any Pomacea on Hilton Head Island, we do formally report invasive populations of Bellamya, Biomphalaria, Melanoides and Pyrgophorus in our 2018 paper [17].  To refresh your memory:

• The Many Invasions of Hilton Head [16Dec15]

[18] Gooding, E. L., A. E. Fowler, D. Knott, R. T. Dillon, T. Brown, M. R. Kendrick, & P. R. Kingsley-Smith (2018) Life history and phenological characteristics of the invasive island apple snail, Pomacea maculata (Perry, 1810) in stormwater retention ponds in coastal South Carolina, USA.  Journal of Shellfish Research 37: 229 - 238. [PDF]

[19] Yes, the introduction of Pomacea we first reported from the Myrtle Beach area in 2008 may even be spreading.  I understand that the initial site of discovery around that trailer park in Socastee was treated with copper sulfate, but the snails seem to have jumped a couple km SE into ditches and stormwater retention ponds around a sprawl of strip malls and big-box retailers.  For ancient history, see:

• Pomacea spreads to South Carolina [15May08]
• Two dispatches from the Pomacea front [14Aug08] 

[20] Although no juveniles seem to hatch in the South Carolina winter.  One presumes that egg masses laid in the winter will hatch in the spring.

[21] If a 6-inch wall is sufficient to stop a 5 mm gastropod coming south between Baltimore and Washington, then to stop a 5 cm gastropod advancing north from the Carolinas we need 100 x 6 inches = 600 inches = 50 feet.

Snails by Mail

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Snails by Mail.  Pp 45 – 49 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays.  FWGNA Project, Charleston, SC.

Last month we surveyed the elements of the freshwater gastropod fauna widely available to hobbyists in the Big Box retail outlets that seem so dominant on the landscape of aquarium supply today [1].  We found two categories of snails reliably offered for sale, strikingly different in their biology but ironically similar in their provenance – the “mystery snails” (Pomacea bridgesii/diffusa) and the nerites.  But, as my readership has already doubtless inferred from my essay of 9Oct17 [2], ampullariids and neritids do not the entire market comprise.  What else might be available online?

Assassin Snail - Aquatic Arts

If one simply enters “freshwater snails” on the subject line of a google search, the first 50 hits include four major retail suppliers – Amazon, eBay, aquaticarts.com, and liveaquaria.com.  Most of the stock available for purchase from these sources are (once again) nerites or mystery snails in their various color varieties.  But below I have compiled a brief review of the remainder, sorted into seven pigeonholes.  The first four taxa or groups of taxa appear to be widely available for purchase online, the next two categories seem to be occasionally available, and the last category is what I would call a “wastebasket.”

Ramshorns – These easy-to-culture snails seem to have remained a perennial favorite of aquarium hobbyists for many years, at least since I was a kid.  All the stocks with which I had any personal experience growing up were North American Helisoma trivolvis, but today it is my impression that most "ramshorns" are Floridian Helisoma scalaris duryi [3].  Ng and colleagues [4] identified Singapore ramshorns as Oriental Indoplanorbis exustus, on the other hand, and I've even seen European Planorbarius corneus implicated in what seems to be a global planorbid conspiracy.  What the heck are these snails?  Most of the offerings for sale online today are “red ramshorns,” which are actually albinos, their absence of body pigmentation allowing that red hemoglobin so characteristic of planorbids to show through.  Stocks with wild pigmentation are marketed as either “brown” or “black.”  There is also a “leopard” variant for sale that has patchy pigmentation on its mantle, and a “blue” that (I think) demonstrates some sort of mutation in shell pigmentation.  I wish I knew more about that, too.

Assassin Snails – Approximately thirty nominal species of the nassariid genus Clea (or Anentome) burrow in the soft bottoms of broad, coastal rivers from southern China and Southeast Asia into The Philippines.  What fascinating creatures!  The group is one of only two neogastropod genera to have successfully invaded fresh waters [5].  As their name implies, assassin snails are predatory – hunting other freshwater snails and sucking them out of their shells.  The little tigers widely marketed to the aquarium hobby today are universally identified as Clea helena, but the excellent recent study by Ellen Strong and colleagues [6] suggests that commercial stocks may represent as many as four species, none of which seems to match topotypic Anentome helena from Java.

Rabbit Snails – Several species of the pachychilid genus Tylomelania are not uncommonly offered for purchase online, variously marketed as “giant” or “orange” or “golden” rabbit snails.  It may be recalled from my October essay that Ng and colleagues [4] identified four Tylomelania species in the Singapore aquarium trade, all of which are apparently endemic to Lake Pozo on the Indonesian Island of Sulawesi.  Some conservation concern has been expressed, but see the follow-up essay I published on this subject in November [7].

Japanese Trapdoor Snails – Yes, our old familiar Bellamya japonica (or maybe B. chinensis?) often seems to be marketed to the indoor aquarium hobby, generally labelled as "Viviparus malleattus.”  The biology of these large oriental viviparids will be well known to my FWGNA readership, but see my species pages [japonica] and [chinensis] for a refresher.

Rabbit Snail - Aquatic Arts

Pagoda Snails – Several nominal species of the pachychilid genus Brotia bearing heavy, strikingly spiny or tuberculate shells are harvested from the rivers of Thailand and occasionally available from online retailers as “Pagoda snails.”  We touched on these back in October as well.

Chopsticks, Spikes, or Long nosed Snails – Occasionally the discriminating freshwater gastropod connoisseur will find thiarids of the genus Stenomelania offered for sale online.  Again, Ng and colleagues [4] identified four Stenomelania species marketed in the Singapore pet trade, although raising no conservation concerns.  The Discover Life website lists 36 nominal species in the genus, ranging throughout Southeast Asia, Australia, and Oceania.  The most common specific nomina mentioned in the pet trade are Stenomelania torulosa and S. plicaria, both distributed widely from India through Indonesia to China.

The Wastebasket – Although (almost) universally reviled, stocks of the “Malaysian Trumpet Snail” (Melanoides tuberculata) are available for purchase on Amazon and eBay.  This invasive thiarid, apparently native to low latitudes throughout the Old World (in various clones), has been widely introduced into the New.  See my FWGNA species page [tuberculata] for more.  And (if you can believe it) hobbyists with a thirst for the small, brown, and mundane can also purchase Physa acuta stocks from Amazon.  I get the impression that both the Physa and the Melanoides are primarily marketed as prey for Assassin snails.  The Physa listing on Amazon advertises, “great natural food for your puffer.”

What I did not find for sale online last week, thank heaven, was any ampullariid stock other than Pomacea bridgesii/diffusa. I remember in years past being able to purchase, at least occasionally through mail order or mom-and-pop aquarium stores, Pomacea insularum/maculata (“Golden Apple Snails”), Pomacea paludosa (“Florida Apple Snails”) and Marisa cornuarietis (“Giant Ramshorns.”)  But I was unable to find, at least upon superficial search, any listing for any such invasive ampullariids through the major online retail outlets today.

So to conclude.  Should we be concerned that any of the freshwater gastropod groups listed above might escape to become pests here in North America, other than the ones already introduced and spreading?  We have reviewed the criteria for invasiveness on quite a few occasions in the past [7], ultimately settling on two ecological qualities which I have called “weedy” and “different.”  So the ramshorns, trapdoors, and wastebaskets are already here.  And the rabbits, pagodas, and chopsticks are not all that ecologically different from North American pleurocerids, in many cases, nor do their life histories seem especially weedy.  That leaves the Assassin snails.

Could an introduction of Clea succeed here in North America?  Some concern has already been expressed [8].  All the range maps I have seen for the genus seem to suggest that their natural distribution is entirely tropical – apparently ranging from the equator to around 20 degrees N latitude.  So our own Key West floats in the Caribbean at latitude 24.5 degrees N, perhaps still a bit too temperate to raise concerns about the threat of gastropod assassination here in the USA.  But you all down in Mexico and Central America might best be on the lookout.

Notes

[1] Pet Shop Malacology [21Dec17]

[2] What’s Out There?  [9Oct17]

[3] Subsequent to the publication of this essay I posted a lengthy series on Helisoma scalaris duryi, starting in October of 2020 and going onward for at least five or six months.  I do suggest that you skip ahead here and read forward into 2021 if you are genuinely interested in "ramshorns":

• The flat-topped Helisoma of The Everglades [5Oct20]

[4] Ng Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130

[5] The only other neogastropod group to invade fresh waters is the marginellid genus Rivomarginella.

[6] Strong EE, Galindo LA, Kantor YI. (2017) Quid est Clea helena? Evidence for a previously unrecognized radiation of assassin snails (Gastropoda: Buccinoidea: Nassariidae) PeerJ 5:e3638 https://doi.org/10.7717/peerj.3638

[7] Loved to Death?  [6Nov17]

[8] For the biology of freshwater gastropod invasions, see:

• Invaders Great and Small [19Sept08]
• Community Consequences of Bellamya Invasion [18Dec09]
• The Most Improbable Invasion [11Oct12]
• The Many Invasions of Hilton Head [16Dec15] 

[9] Mienis HK. 2011. Will the uncontrolled sale of the snail-eating gastropod Anentome helena in aquarium shops in Israel result in another disaster for Israel’s native freshwater mollusc fauna? Ellipsaria 13(3):10-11.  Bogan AE, Hanneman EH. 2013. A carnivorous aquatic gastropod in the pet trade in North America: the next threat to freshwater gastropods. Ellipsaria 15(2):18-19

What's Out There?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  What's Out There?  Pp 23 – 28 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays.  FWGNA Project, Charleston, SC.

Last summer an interesting paper came across my desk – so interesting, in fact, that I filed it in my stack of promising subject matter for the FWGNA blog.  Where it was promptly buried.  But from which this week it has been exhumed, to stimulate my imagination afresh.

The paper is entitled, “Molluscs for sale: Assessment of freshwater gastropods and bivalves in the ornamental pet trade [1].”  The senior author, Ting Hui Ng, and her six coauthors hail almost exclusively from Singapore, the primary exporter of ornamental fishes to the global marketplace.  Ting Hui and her colleagues surveyed seven local retail shops and major aquarium-stock exporters between 2008 and 2014, documenting an impressive 47 species of freshwater gastropods and 12 species of bivalves available for sale to the hobby worldwide.  Of the gastropods, the authors considered 26 to be oriental in their origin, 7 Australasian, 4 Neotropical, and 1 North American, leaving 9 classified as “cosmopolitan.” 

Figure 1 of Ng et al. (2016)

The entire malacological zoo is depicted in the author’s Figure 1 reproduced above, with their (extensive) caption copied at note [2] below.  I hate to fuss [3], but the scales are completely messed up in this figure – not just the unmarked scale bars (which are supposed to be 10 mm) but the scale bars “indicated differently.”  With regard to the actual sizes of any of these 59 creatures, you are on your own.

The single North American species was identified as “Physa sp.”  I would have bet dollars-to-donuts that this must be Physa acuta, the world’s most cosmopolitan freshwater snail [4], which is indeed widely introduced into the waters of Singapore, until I swapped emails with Ting Hui. She called my attention to her 2015 paper reporting the discovery of a single individual physid in a Singapore aquarium shop, and a second individual in Malaysia, 22% different from anything in GenBank [5].  I'm going to fight the temptation to digress here, but see note [6] below.

The main point is that Physa (and three other gastropod species) were classified as “hitchhikers” by Ting Hui and colleagues, which means that they were discovered in Singapore incidentally transported with aquatic plants, ornamental fish or other freshwater mollusks, and not literally “For Sale,” because nobody wants them.  But at least North America wasn’t skunked entirely.  USA!  USA! USA!

The four Neotropical species were all (you guessed it) ampullariids – Pomacea canaliculata, Pomacea maculata, Pomacea diffusa, and Marisa cornuarietis.  All these various South American ampullariids are also quite familiar to us here in the USA, introduced years ago into Florida and Texas, recently knocking on our doorsteps in South Carolina [9].

Among the remainder of the gastropods for sale in Singapore, I myself was most interested by the strange and exotic-looking pachychilids and thiarids.  We are certainly familiar with #48 Melanoides tuberculata here in the USA [10].  But I was surprised not to see the oriental Tarebia granifera in the gallery compiled by Ting Hui and her colleagues.  Tarebia has been introduced to several southern US states and is sporadically common in Central America and on many Caribbean islands [11].

And speaking of noteworthy absences.  Neither Bellamya japonica nor B. chinensis appears on Ting Hui's list of 59.  I’m sure their mothers must love them, but perhaps Bellamya are too plain and clunky to find a market among aquarium hobbyists?  Or has Bellamya become so common here in North America [12] that nobody buys them out of Singapore?  It has always been my impression that they are marketed here in the USA primarily by suppliers to the backyard “water garden” trade, along with koi, water lilies, and little tumbling fountains.  Not the (indoor) aquarium shops supplied by Singapore.

The research interests of Ting Hui and colleagues seem to have been motivated by two primary concerns – that exotic freshwater mollusks might spread, and that they might not.  Let’s set aside the former concern for the time being, and focus on the latter.

Eight of the 59 freshwater mollusk species for sale in Singapore were identified by Ting Hui as “narrowly-endemic.”  These included the three spiny species of Brotia, the four Tylomelania species, and the viviparid Celetaia persculpta.  Quoting directly from Ting Hui: 

“These species appeared to fetch higher prices compared to more common species (up to US$10 per individual Tylomelania sp. compared to US$5 per individual Thiaridae or Neritidae, THN pers. obs.). The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”

Pachychilid snails of the genus Brotia are ovoviviparous but sexually reproducing – males and females apparently present in equal frequency.  Brotia armata (#34) and B. binodosa (#35) are members of what Glaubrecht & Kohler called a “species flock” in the Kaek River of central Thailand [13].  Brotia pagodula (#37) is endemic to the Salween River and its tributaries on the border of Thailand and Myanmar.   Their habitat is given as “Attached to rocks in sectors with swift currents” [14]. 

The genus Tylomelania (#39 - 42) is another group of ovoviviparous pachychilids, these endemic to particular lakes, rivers, and streams on the Indonesian Island of Sulawesi.  Our friends Thomas von Rintelen and Matthias Glaubrecht recognized 62 species of Tylomelania the last time I checked, but I haven’t been home since morning [15].

So the paper by Ting Hui and colleagues opened with this sentence: “The ornamental pet trade is often considered a key culprit for conservation problems such as the introduction of invasive species (including infectious diseases) and overharvesting of rare species.”  Can a gastropod population – especially a population endemic to some narrowly-circumscribed patch of habitat – indeed be threatened by overharvest?  What is the evidence that hobbyists can drive populations of their favorite organisms to extinction with love?  Tune in next time.

Notes

[1] Ng Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130

[2] Fig 1. Freshwater molluscs in the ornamental pet trade.

Unless indicated differently, scale bars = 10mm. 1. Batissa similis; 2. Batissa violacea; 3. Corbicula fluminea; 4. Corbicula moltkiana; 5. Hyriopsis bialata; 6. Hyriopsis desowitzi; 7. Parreysia burmana; 8. Parreysia tavoyensis; 9. Pilsbryoconcha exilis; 10. Scabies crispata; 11. Sinanodonta woodiana; 12. Unionetta fabagina; 13. Marisa cornuarietis; 14. Pomacea canaliculata; 15. Pomacea diffusa; 16. Pomacea maculata (photograph by K.A. Hayes); 17. Bithynia sp.; 18. Clea bockii; 19. Clea helena; 20. Radix rubiginosa; 21. Clithon corona; 22. Clithon diadema; 23. Clithon lentiginosum; 24. Clithon mertoniana; 25. Neripteron auriculata; 26. Neritina iris; 27. Neritina juttingae; 28. Neritina violacea; 29. Neritodryas cornea; 30. Septaria porcellana; 31. Vittina coromandeliana; 32. Vittina turrita; 33. Vittina waigiensis; 34. Brotia armata; 35. Brotia binodosa; 36. Brotia herculea; 37. Brotia pagodula; 38. Sulcospira tonkiniana; 39. Tylomelania towutica; 40. Tylomelania sp.; 41. Tylomelania sp.; 42. Tylomelania sp.; 43. Physa sp.; 44. Amerianna carinata; 45. Indoplanorbis exustus; 46. Gyraulus convexiusculus; 47. Semisulcospira sp.; 48. Melanoides tuberculata; 49. Stenomelania offachiensis; 50. Stenomelania plicaria; 51. Stenomelania cf. plicaria; 52. Stenomelania sp.; 53. Thiara cancellata; 54. Celetaia persculpta; 55. Filopaludina cambodjensis; 56. Filopaludina peninsularis; 57. Filopaludina polygramma; 58. Sinotaia guangdungensis; 59. Taia pseudoshanensis.

https://doi.org/10.1371/journal.pone.0161130.g001

[3] No, I don’t.

[4] Dillon, R. T., A. R. Wethington, J. M. Rhett and T. P. Smith.  (2002)  Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra.  Invertebrate Biology 121: 226-234.

[5] Ng, TH, SK Tan, & DCJ Yeo (2015) Clarifying the identity of the long-established, globally-invasive Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in Singapore.  BioInvasions Records 4: 189 - 194.
http://www.reabic.net/journals/bir/2015/Issue3.aspx

[6]  This is fascinating.  It is certainly possible that the 22% CO1 sequence difference reported in Singapore is another case of mitochondrial superheterogeneity, which (we do know) occurs in North American populations of Physa acuta, almost certainly a signature of cytoplasmic male sterility [7].  It is also possible that the "Physa sp" reported by Ng and colleagues in Singapore might indeed be a distinct biological species.  Over and over again, all over the world, we have seen invasions by species mixtures, which have been sorted out only at the site of introduction, and then subsequently corrected in their places of origin.  I do think it is possible that the American Pacific northwest may be inhabited by a species perhaps best identified as Physa concolor, but called Physa natricina in the Snake River, morphologically very similar to Physa acuta, but reproductively isolated from it [8].  Is this Physa concolor in Singapore?  Wild speculation here, but who knows?  Science is lots of fun, isn't it?

[7] For more on mtSH in Physa, see:

• Cytoplasmic Male Sterility in Physa! [9June22]

[8] For more on what I called "Hypothesis #2 (of 3) See:

• The Mystery of the SRALP: A bidding [5Feb13]

[9] I’ve published quite a few essays on the North American Pomacea invasion.  See:

• Pomacea spreads to South Carolina [15May08]
• Two dispatches from the Pomacea front [14Aug08]
• Pomacea news [25July13]

[10] For more on the North American populations of Melanoides, see:

• To only know invasives [16Oct15]
• The many invasions of Hilton Head [16Dec15]

[11] Pointier, J-P., R. Incani, C. Balzan, P. Chrosciechowski, and S. Prypchan (1994)  Invasion of the rivers of the littoral control region of Venezuela by Thiara granifera and Melanoides tuberculata and the absence of Biomphalaria glabrata, snail host of Schistosoma mansoni.  Nautilus 107: 124 – 128.

[12] For more on Bellamya in North America, see:

• Bellamya News [6Oct05]
• Community consequences of Bellamya invasion [18Dec09]
• Just before the bust [5Aug14]

[13] Ng and colleagues recorded only 0.5% uncorrected pairwise 16S sequence difference between their individual B. armata and B. binodosa analyzed.  Hmm…

[14] Kohler, F, and M. Glaubrecht (2006)  A systematic revision of the southeast Asian freshwater gastropod Brotia (Cerithioidea: Pachychilidae)  Malacologia 48: 159 – 251.

[15] von Rintelen T., B. Stelbrink, R. M. Marwoto, & M. Glaubrecht (2014) "A Snail Perspective on the Biogeography of Sulawesi, Indonesia: Origin and Intra-Island Dispersal of the Viviparous Freshwater Gastropod Tylomelania". PLoS ONE 9(6): e98917.

https://doi.org/10.1371/journal.pone.0098917

Marstonia letsoni, Quite Literally Obscure

Editor’s Notes - This is the second essay of a two-part series on an enigmatic population of hydrobiids in Lake St. Clair, on the US/Canada border east of Detroit.  It will only make sense if you’ve read my post of [19Jan16] first.

This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Marstonia letsoni, quite literally obscure.  Pp 229 - 234 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

In late September of last year I began to mentally outline a short series of essays to catch us up on invasive species news, such as have become a regular feature of this blog.  And I thought to myself, what better way to reintroduce the subject of introduced species than to tell the story of an unidentified hydrobiid snail from Lake St. Clair, which must certainly represent an invasion not heretofore recognized by anybody but yours truly, Rob Dillon?

I don’t know why I picked up Bob Hershler’s (1994) Pyrgulopsis monograph [1] and began leafing through it on Friday morning, 9/25.  I knew that Bob had reviewed 11 Eastern species as well as 54 Western species of Pyrgulopsis in that most thorough and useful work.  I also knew that Bob had teamed up with Fred Thompson in 2002 to “resurrect” Baker’s generic nomen “Marstonia,” transferring those 11 Eastern Pyrgulopsis species into it [2].  In the last 20 years I have continued to find Bob’s 1994 Pyrgulopsis monograph very helpful as I have stomped around Georgia and Tennessee, paying respects to endemic or narrowly-restricted populations of what are now called Marstonia agarhecta, Marstonia arga, Marstonia halcyon, Marstonia ogmorphaphe, and others.

I thought I knew the work pretty well.  But on the morning of 9/25, Bob’s image of a 2.8 mm “Pyrgulopsis” letsoni from a small lake west of Pontiac, Michigan jumped out and bit me like a snake.  There are two species of Marstonia inhabiting Great Lakes drainages, not just the one!  And everything about M. letsoni fit our Lake St. Clair unknowns: the shell morphology, the diminutive adult size, the range, indeed the very obscurity of it.

“Amnicola” letsoni was described by Bryant Walker [3] in 1901 from Pleistocene deposits on the banks of the Niagara River in upstate New York [4].  The first living specimens were not collected until 1943, however, when E. G. Berry dug the silt and mud out of the “honeycombed cavities” in rocks from the bed of a temporarily-dewatered stretch of the Huron River above Ann Arbor, and washed the goop through a sieve [5].  The species first known as Amnicola letsoni, and then later as Pyrgulopsis letsoni, and now as Marstonia letsoni is, quite literally, obscure [6].

Amnicola/Pyrgulopsis/Marstonia letsoni is so obscure that all the index entries under “letsoni” in Burch’s [6] North American Freshwater Snails direct the user to incorrect pages.  (Burch’s little line drawing of a Pyrgulopsis letsoni shell is hidden on the bottom of page 238.)  Arthur Clarke [7] did mention (but did not figure) Pyrgulopsis letsoni under Marstonia decepta (page 60), only venturing so far as to suggest that letsoni “may live in Canada.”

But returning to the morning of 9/25, especially compelling to me was the penial morphology.  In his message of 8/28, Bob had described the penis of the Lake St. Clair unknowns as “bifurcate.”  But I have never personally seen a Marstonia penis I would describe in that fashion.  Nymphophiline hydrobiids, such as Marstonia, bear a penis with a single duct plus a glandular lobe.  Typically the glandular lobe is larger than the duct that transmits the sperm, so that the overall nymphophiline penial morphology is bladelike, with a little “penial filament” hanging out along the margin.  E. G. Berry’s figure of the Marstonia lustrica penis at left below is typical.  The adjective “bifurcate” is not called to mind.

But Berry’s [5] figure of the penis of Marstonia letsoni illustrated a glandular lobe much smaller than any I have ever seen in a Marstonia – smaller than the duct that bears the sperm.  The overall morphology of the letsoni penis shown at right above might indeed be described as “bifurcate.”

So on 9/26 I swapped a couple fresh emails with Bob Hershler, asking for clarification on the penial morphology of the Lake St. Clair unknowns, and suggesting Marstonia letsoni as a possible identification.  And here is what Bob said: “I did think of letsoni in this context, but, again, I will need to see more specimens of the mystery snail to nail it down.”  A cautious worker is our good buddy Bob.

Well, those of you who know me, know that I think of evolutionary biology as a science – the construction of testable hypotheses about the natural world.  And the names I assign to populations of freshwater gastropods are hypotheses.  Sometimes they are weak hypotheses, and sometimes they are strong hypotheses, but Rob Dillon’s identifications are never “nailed down.”

So I composed the present two-part essay series in early October, firmly and boldly hypothesizing that the Lake St. Clair unknowns are Marstonia letsoni (Walker 1901), and prepared to go to press.  But there is yet one final twist in this story.  Our good buddy Ron Griffith asked me to delay release of the news as he was (even at that moment, in October) writing up a note for formal publication.

Ron reported that he had access to historical data from Lake St. Clair benthic samples, suggesting that the rise and spread of our M. letsoni population seems to be a fairly recent phenomenon.  In other words, M. letsoni may be a native invasive.  Might its range currently be demonstrating a significant expansion?  Ron also wanted time to send additional samples to Bob H., to try again for that "nailing down" thing.

So as most of you will have subsequently noticed, I wrote essays on three other invasive species topics for the months of October, November, and December.  And in the interim, Ron G. and Bob H. have been busy with the malacological hammer, confirming our identification even to Bob’s exacting standards.  And now at long last, nine months after that first little sample of tiny snails spilled under my dissecting scope in Milwaukee, our six-member team has reached consensus on an identification of M. letsoni.  Cradled in the metropolitan Detroit/Windsor area, bathed in one of the world’s busiest waterways, and yet remaining quite surprisingly, quite literally, obscure.

Notes

[1]  Hershler, R. (1994)  A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae).  Smithsonian Contributions to Zoology 554: 1-115.

[2] Thompson, F. G. & R. Hershler (2002)  Two genera of North American freshwater snails: Marstonia Baker, 1926, resurrected to generic status, and Floridobia, new genus (Prosobranchia: Hydrobiidae: Nymphophilinae).  The Veliger 45: 269 - 271.

[3] For my tribute to Bryant Walker, see:

• Bryant Walker’s Sense of Fairness [9Nov12]

[4]  Walker, B. (1901)  A new Amnicola.  Nautilus 14: 113-114.

[5] Berry, E. G. (1943)  The Amnicolidae of Michigan: Distribution, ecology, and taxonomy.  Misc. Publ. Mus. Zool. Univ. Mich. 57: 1 – 68.

[6] Burch, J. B. (1989)  North American Freshwater Snails.  Malacological Publications, Hamburg, MI.  365 pp.

[7] Clarke, A. H. (1981)  The Freshwater Molluscs of Canada.  National Museum of Natural Sciences, Ottawa.  446 pp.