A New Invasive Gastropod in the Great Lakes?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) A new invasive gastropod in the Great Lakes?  Pp 223 - 228 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

“I have absolutely no idea what this is.  I got nuthin.  Where did you say this sample came from, again?”

It was day four of the Society for Freshwater Science meeting in Milwaukee this past spring, and I had spent the afternoon enthroned under the “Gastropoda” sign at the Taxonomy Fair [1], identifying freshwater gastropod samples from all corners of the continent, from all baskets of molluscan biodiversity.  Well, juvenile Physa, mostly.

So up walked Adam Frankiewicz and Jerry Shepard from the EPA lab in Duluth, bearing several small vials of small hydrobiid snails from benthic grab samples taken on the Canadian side of Lake St. Clair, maybe 25 miles east of Detroit.  Some of their samples were collected from sand bottoms as little as 0.9 m deep, others from muck/clay as deep as 5.6 m.

Of course, I was expecting Potamopyrgus.  The invasive “New Zealand Mud Snail” was first reported from Lake Ontario in 1991, and has since popped up in four of the five Great Lakes [2].  But the snail sample that spilled out under my scope on that May afternoon immediately struck me as too small in adult size, even by hydrobiid standards.  Although apparently mature, I saw no specimen in excess of 3.0 mm standard shell length, and most were closer to 2.0 mm.  And the shell morphology simply did not match Potamopyrgus.  The example specimens shown in the growth series above (from Ron Griffiths) range from 2.0 mm to 2.8 mm only.

Moreover, there really weren’t a whole lot of other candidate hydrobioid species in the Great Lakes section of my mental database.  Marstonia lustrica is vaguely similar, but bigger and fatter.  None of the other familiar hydrobioids are even close - Cincinnatia, Amnicola, Lyogyrus, Bithynia?  No way.  I have never, in eight years of service to the SFS/NABS Taxonomy Fair, been so completely stumped as I was that May afternoon in Milwaukee.

But strange and unexpected as this episode was, it became stranger.  Our good friend Ron Griffiths, who works as a consultant in Ontario, happened to be passing by the Gastropoda table at that very moment.  And Ron said, “Oh, I’ve seen those little snails too.”  Ron had also been working on benthic samples from the Canadian side of Lake St. Clair, in the vicinity of the Thames River mouth, and had been similarly puzzled by the tiny hydrobiid snails they contained.  He did not have a sample with him in Milwaukee, however.

“Gentlemen,” said I, “We are not going to solve this mystery today.”  I suggested to the working group assembled that if Adam or Ron could take some good jpeg images of our little critters, I would forward the images to my buddy Bob Hershler at the USNM.  And I further suggested that if, as seemed likely, Bob wanted to see some actual specimens, our little group should stand ready to send samples to Washington directly.  And I closed with a request that I be kept on the CC line.  I was fascinated by my own ignorance.

Lake St. Clair ain’t the Peruvian Amazon!  It is one of the most heavily-traveled commercial waterways in the world.  Cradled in the greater Detroit/Windsor metropolitan area, the gentle waters of Lake St. Clair lap upon shores of great wealth, great power, and great learning.  Could we North American biologists be utterly ignorant of our own commonplace biota?

So Adam and Ron did indeed follow up with several jpeg images within the week, Adam’s comprising the top half of the figure below.  I myself photographed the juvenile Potamopyrgus specimens shown in the bottom half of the figure at the same scale, pasted the two images together for comparative purposes, and forwarded the montage onward to Bob Hershler in early June, with a cover message.  Sure enough, Bob wanted to examine some actual specimens, which Ron promptly posted to Washington.

And this is what our buddy Bob said (6/10): “These are not the New Zealand mudsnail (Potamopyrgus antipodarum), nor are they (juvenile) Elimia [3].  They are phallate and oviparous, and probably are ‘hydrobioids’ but I will need to examine additional material to get a better handle on their identity.  If your colleague is sequencing specimens then the resulting data could also be used toward this end.”    

So yes, quite fortunately, Jerry did indeed have connections with the Molecular Ecology Research Branch of the EPA lab in Cincinnati.  And even as Bob, Ron, Adam and I had been fumbling around with the old fashioned morphology, Jerry was packing a sample of our Lake St. Clair unknowns off to Dr. Erik Pilgrim in Cincinnati for sequencing.

And by late August Erik, the newest member of our working group, had been able to amplify “nice, clean CO1 and 16S sequences for two specimens.”  He reported (8/26): 

“Searching the COI barcode sequences places these snails in Marstonia, and they appear most closely related to, but not the same species as, M. pachyta (95% match), M. lustrica (95%), M. comalensis (95%), M. hershleri (94%), M. agarhecta (94%), M. halcyon (93%), and M. castor (93%). The 16S sequences also place them as close relative of M. agarhecta, M. hershleri, and M. halcyon (apparently not as many 16S sequences are in the database). So, bottom line, those snails are some species of Marstonia.”

 I will confess that Erik’s conclusion seemed highly improbable to me.  Marstonia is a North American genus, with approximately 10-15 species, most of which are endemic to the southeastern United States.  And although I may not have expressed my opinion openly prior to August, it had been my strong impression, from the very moment I laid eyes on these little critters in Milwaukee, that the Lake St. Clair unknowns must represent a heretofore unrecognized alien invasion. 

I said this to the group (8/27): “Geeze, there is NOTHING about those little hydrobiids that speaks Marstonia to me.  I’m not the expert, but they don’t look like Marstonia or smell like Marstonia – they’re too small, they’re in the wrong habitat, in the wrong part of the world.”  I then fussed and fumed for a couple paragraphs, advancing my invasive species hypothesis to no effect, and concluded, “When I first looked at those snails way back in May, I told you I got nuthin.  And nuthin is what I still got.”  To which Bob Hershler replied (8/28):

“As I said previously, I would have to examine a well preserved anatomical series to confidently identify this snail.  The single male that I dissected had a bifurcate penis consistent with Marstonia, which is widely distributed in the Great Lakes region.  Although Ron's shells are more elongate than what I am used to seeing in Marstonia, I would not dismiss the mtCOI results out of hand given that (in my experience) this bar code is almost always spot on in pinpointing identities of hydrobioid snails.”

So that’s where the question sat, when our little Lake St. Clair hydrobiid working group adjourned sine die on the morning of 28Aug15.  In retrospect, I understood Bob to be referring to Marstonia lustrica, which is indeed “widely distributed in the Great Lakes region.”  But I myself had ruled out Marstonia lustrica immediately, way back in May, because (as Bob observed) the shell morphology of our unknowns is much “more elongate.”  And lustrica has a larger adult size as well.  The Lake St. Clair unknowns don’t look any more like Marstonia lustrica than they look like Potamopyrgus.

Ah, but there is a second Great Lakes species of Marstonia for which the descriptor “widely distributed” does not come to mind, but which fits the modifier “obscure” as well as any creature on Noah’s Ark.  What diminutive denizen of the benthos might lurk forgotten in the inky darkness of Lake St. Clair?  Tune in next time!

Notes

[1] See You In Milwaukee? [30Mar15]

[2]  Potamopyrgus has not been reported from Lake St. Clair as of this writing, however.  See the USGS Nonindigenous Aquatic Species Database.  [USGS-NAS]

[3] I am sure that our good buddy Bob meant to write Pleurocera here, rather than the obsolete nomen “Elimia.”  And surely Bob’s observations in this regard must have been addressed to our EPA colleagues, not to yours truly, who has dedicated much of his career to the Pleuroceridae, and has 19 papers published in the peer-reviewed literature on the subject, and would know a juvenile Pleurocera if he saw one.  Surely.