Editor’s Note – This essay was subsequently published as:
Dillon, R.T., Jr. (2019d) To Only Know Invasives. Pp 63 - 71 in The Freshwater Gastropods of North America Volume 4,
Essays on Ecology and Biogeography.
FWGNA Press, Charleston.
I’ve never compiled any statistics, but it is my impression that the most common category of inquiry I find delivered to my email inbox on any long-term basis probably bears the modifier “invasive” on the subject line. Such messages are usually requests for identification of putatively exotic freshwater gastropods, with jpeg images attached. Sometimes these inquiries come from professional biologists working with agencies, and other times just from ordinary concerned citizens. I always try to help.
So a couple years ago I receive a variant of the typical
email described above, from a NOAA biologist in the Great Lakes area. The attached jpeg image depicted a couple beach worn
shells of the common pleurocerid Pleurocera semicarinata livescens. And in the body of his message the biologist
confessed, amidst other routine background matter, “I really only know the
invasives.” He was quite certain that
these freshwater gastropods were not Bithynia, or Bellamya, or
Potamopyrgus. But he didn’t know what
they were.
And this struck me as a sad way of looking at the
world. One is reminded of a clinical
practitioner who never leaves the hospital – only knowing the sick, never
meeting the well.
But in a larger sense, the existence of a biologist, or an
agency, or indeed multiple governmental agencies, who “only know invasives” has
become an article of public policy in recent years. The Nonindigenous Aquatic Nuisance Prevention
and Control Act of 1990 established a body called the “Aquatic Nuisance Species
Task Force,” with representation from 13 federal agencies, which in turn established
a National Nonindigenous Aquatic Species Information Center “to collect,
analyze, and disseminate information about the presence and distribution of nonindigenous aquatic species and their effects.” That Center is currently located at the US
Geological Survey’s Southeast Ecological Science Center in Gainesville, Florida [1].
The most visible product of the USGS Information Center has
been the development of the Nonindigenous Aquatic Species database, which can,
under some circumstances, be a useful resource, if you understand its
limitations, which are extensive. Do
open this site in a new window and check it out, if you’re not familiar with it
already:
The USGS-NAS tracks a staggering 44 species (or
categories of species) of freshwater gastropods: 11 ampullariids, 8 viviparids,
3 thiarids, 7 miscellaneous prosobranchs, 7 planorbids, 5 physids, and 3
lymnaeids. Of this extensive list only
32 are exotic; 12 species or categories are native to North America, but have
expanded their ranges so recently or so dramatically, possibly by human agency,
as to attract the attention of The Feds.
Current distributions of these 44 species are displayed as good quality
dot maps which are zoomable and clickable, to see the underlying records. Public users are allowed to add records to
the database through a process that is not terribly difficult or onerous, which
certainly has a downside, but which (on the whole) I think is good.
The site is, however, a taxonomic mess. It reminds me of my son’s bedroom, when he
was nine years old. Stuff is thrown
around everywhere, and it would be very difficult to find anything you might
want in there, if (hypothetically) you wanted to find it, and most of the stuff
you can see when you pass by the open doorway is just junk, and should be thrown
out.
 |
| USGS-NAS chinensis |
For example, the USGS-NAS collects data on four separate
categories of the Asian Mystery Snail: Cipangopaludina japonica, Cipangopaludina chinensis, Cipangopaludina chinensis malleata, and "Cipangopaludina species.” The most
current science suggests, however, that just two species of mystery snail have been
introduced to North America, best identified as Bellamya japonica and Bellamya chinensis. It is very difficult for
laymen, or indeed most field biologists, to distinguish japonica from
chinensis. In fact, both of the
thumbnail photos of shells adorning the clickboxes for chinensis and japonica
on the USGS-NAS website depict chinensis [2].
I really don’t trust the species-level identifications
upon which any of the four “Cipangopaludina” maps are based, and would advocate
combining all those data into a single map that simply shows “Bellamya species.” If the mission of the USGS-NAS database is
indeed to disseminate information about the “distribution of nonindigenous species
and their effects,” B. chinensis and B. japonica are ecologically equivalent
[3], and combining them would make sense.
 |
| USGS-NAS "japonica" (not, actually) |
The situation is much more complicated in the
Ampullariidae, I fear. It is not
entirely clear how many exotic species of Pomacea have been introduced into US
waters, but the best hypothesis at this writing is probably four: Pomacea maculata (aka insularum) which is the most common, Pomacea canaliculata (bona
fide) which is very similar but more rare, Pomacea diffusa (aka bridgesii)
which is smaller and not as voracious, and Pomacea haustorum, which is much
larger and rare. I myself am not sure I
could distinguish maculata from canaliculata (bona fide) in the field, although
their egg masses are distinctive [4].
So users of the USGS-NAS database will find these four
differently-sized pies cut into seven differently-sized pieces: maculata,
canaliculata, diffusa, bridesii, haustorum, cumingi, and “Pomacea
species.” Again, I really don’t trust
any of the identifications upon which any of the resulting maps are based, and
would advocate collapsing the categories ecologically, into “Exotics that do eat
macrophytes” (maculata, canaliculata, haustorum, and probably “species”) and
“Exotics that do not” (diffusa, bridgesii, and cumingi).
I have similar misgivings about almost all the pulmonate
groups. Lymnaea auricularia can be
identified unambiguously, but the distinction between “Planorbella duryi” and
“Helisoma species” gives me pause, and just the thought of clicking on some of
those physid links scares me to death.
What the heck is exotic “Physella species?” Some of the things in my 9-year-old son’s
bedroom did not warrant close examination.
But to be clear, I am not criticizing the hard-working
public servants who are doing their best to administer the USGS Nonindigenous Aquatic
Species database. They are trying to play
baseball in a bluegrass jam session, bless their hearts [5].
And the most important feature of the USGS-NAS database
is not the maps, but rather the ready
So what fraction of the USGS-NAS data might be useful for
the FWGNA project? And should we move to
incorporate it?
At this point I should register a scruple. The primary goal of the FWGNA project has
always been to survey the distribution and abundance of the North American
freshwater gastropod fauna, as objectively as possible. Early on we identified a phenomenon I called
“conservation-biased oversampling" [6] which is the (self-defeating) tendency
for agencies to conduct surveys directed toward putatively-endangered
species. And I have consistently tried
to avoid incorporating data which might reflect such bias in the FWGNA
database, albeit with mixed success.
The USGS-NAS database very clearly demonstrates a related
phenomenon, which I hereby dub “invasion-biased oversampling.” I suppose if the USGS ran a parallel site,
where biologists might upload data on the native species of freshwater
gastropods they found in addition to the nonindigenous species recorded at each
site, such a bias would not be a problem.
But they don’t, so it is.
With that dab of balm applied to my tender conscience,
last week I spent a few hours fishing around in the USGS-NAS database [7], looking
for good-quality freshwater gastropod records that might reasonably be added to
the FWGNA. I focused on the
Atlantic-drainage fauna of the nine-state region from Georgia to the New York
line, the region with which I am most familiar.
I disqualified the Bellamya (“Cipangopaludina”) records, the Pomacea
records, and any weirdo pulmonate records for the reasons outlined above. This left me with just the four taxa it is
harder to mess up: Viviparus, Bithynia, Potamopyrgus, and Melanoides.
 |
| USGS-NAS V. georgianus |
Well actually, Viviparus is a bit problematic. The NAS database contains 235 records in
total: 227 of Viviparus georgianus, 7 of Viviparus viviparus and 1 of Viviparus subpurpureus. The distinction between
(the putatively native) V. georgianus and (the European) V. viviparus simply is not clear [8], but lumping those two categories and sorting geographically, I
was able to extract 10 records. Three of
those records corresponded to populations already in the FWGNA database. Four referred to Viviparus populations in the
Potomac River near Washington, of which I was already aware, but which I have
been unable to confirm. The other three
records were from the 1970s, not vouchered or accompanied by sufficient data
[9].
The USGS-NAS database also contains (a very similar) 247
records of Bithynia tentaculata, of which 9 fall in our study area. All nine of these records refer to the
Potomac River Bithynia population already well-documented in the FWGNA
database.
Although the USGS-NAS database contains a whopping 1,210
records of the New Zealand Mud Snail, Potamopyrgus antipodarium, only one,
single record falls within our nine-state study area. That would be the population in Spring Creek,
Centre County (PA) with which we are already quite familiar [10].
Similarly, the 94 records of Melanoides tuberculata in
the USGS-NAS database also include but a single observation in our study
area. But it is a good one. Record #153847 reports a 2001 collection of
M. tuberculata in the small coastal town of Southport, NC. Zooming the USGS map inward shows the
collection site at Bonnetts Creek, near the entrance to the “Olde Southport
Subdivision.” And reference to the
online database at the North Carolina State Museum shows a lot of 82 Melanoides
tuberculata shells collected in Southport on 27Jan06 under catalog number
40541. Super – can’t beat that.
Although I have seen secondary reports of Melanoides in North
Carolina [11], this is the first well-documented, bona fide record of that
species I have ever seen north of Florida, east of Texas. So the
 |
| USGS-NAS Melanoides |
But let me conclude this month’s essay with another anecdote
from the old mail bag. A couple years
ago I got an email from a colleague in Europe asking a simple question. He wanted an estimate of the number of
freshwater gastropod species in the US Great Lakes. I was gratified to be able to answer
promptly, and I did. I said I didn’t
know [12]. So my European correspondent
wrote me back incredulous, and asked, “Do you mean to tell me that the US
Government charges 13 Federal Agencies to record and monitor the latitude,
longitude, date and sex of every pallid 5 mm European gastropod that sets foot
onto American shores, but spends not one nickel and gives not one rip about any
of its own native species whatsoever?”
Yes, I told him that was indeed the case, and suggested
that we might either swap faunas, or swap governments, to solve the problem. That former solution seems to be in the
works.
Notes
[1] The actual legislative and regulatory history is much
more complicated than this. The 1990 act
was amended and expanded by the 1996 “National Invasive Species Act,” and a
body called the “National Invasive Species Council” was also created by
Executive Order in 1999. Exactly why the
Nonindigenous Aquatic Species database landed in the US Geological Survey, as
opposed to the Fish & Wildlife Service, or the USDA Department of Pests and
Weeds, or the Coast Guard, or the Corps of Engineers (both of which were
explicitly charged in the 1990 act) I have no idea.
[2] I do feel considerable sympathy for our
long-suffering civil servants here. It
amazes me how often freshwater mollusk invasions are not single-species
efforts, but rather seem to result from conspiracies of multiple species nearly
indistinguishable from each other. The
zebra mussel / quagga mussel invasion is a prominent example, of course, as is the
situation with the purple-Corbicula and the white-Corbicula.
[4] Although the distribution data are vintage 2006, there are excellent illustrations of the various Pomacea species and their egg masses in
a colorful PDF flyer available from the Florida Fish & Wildlife Commission:
- Non-Native Applesnails in Florida [pdf]
[5] Science and Public Policy are not compatible, but
they’re not incompatible either. They’re
two entirely different things. See any
of my essays under the menu item “Science and Public Policy” at right above for more.
[7] I simply blocked and copied from the “Collection
Info” page(s) for each species of interest, then pasted into an excel
spreadsheet. I could have gotten a lot
more data if I had requested a “custom query” from the NAS staff, but screw it.
[8] The original footnote I posted here in October of 2015 remarked that it seemed possible to me "that all the Viviparus
populations that have spread throughout the American North and Midwest in
recent decades may be a cryptic invasion of the European V. viviparus." But I cautioned that "no
actual data have been brought to bear on the question as yet." Some actual data were ultimately published five years later, however, and the cryptic invasion hypothesis was not supported. See:
- A Gene Tree for the Worldwide Viviparidae [9Mar21]
[9] These three records from the 1970s got the
sticky-note treatment. I marked the
creeks involved with sticky-notes on my Maryland map book. Next time I’m in the area…
[11] Anderson, T.
K. (2004) A review of the United States distribution of Melanoides tubeculatus
(Muller, 1774), an exotic freshwater snail.
Ellipsaria 6: 15-18.
[12] This quote comes from Mark Twain’s (1875) Old Times
on the Mississippi. The dialogue
between The Captain and the author continues:
“You—you—don’t know?" The captain mimicked my drawling manner of speech.
“What do you know?”
“I—I—nothing, for certain.”
“I—I—nothing, for certain.”
“By the great Caesar’s ghost, you don’t know enough to
pilot a cow down a lane.”
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