Warning. The essay below is the fifth in a five-part series on species relationships in the
enigmatic North American “stagnicoline” lymnaeids. I will assume that you have read all four of
my previous posts. In addition, I also
make explicit references to two essays in my 2012 series on this same subject,
[10May12] and [4June12]. In fact, it
would probably help if you started with my [20Nov06] essay on F. C. Baker, and
read my [28Dec06] essay on the classification of the Lymnaeidae as well. Stand back, I’m going to try Science! [1]
Subsequently published as Dillon, R.T., Jr. (2019b) The lost thesis of Samantha Flowers. Pp 95-106 in Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates. FWGNA Press, Charleston.
Insofar as I was able to determine from my vantage point
820.64 miles south of Ann Arbor, Samantha made good progress on her thesis
research through the 2012-13 academic year, and into the field season
following. I was pleased to see the
abstract of a talk she gave at the AMS meeting in The Azores in July of 2013,
although I myself was unable to attend.
And on 15Aug13 I received a very upbeat email from her, my first in over
a year. She reported that she was
“currently on the last leg of my Master's journey, prepping a manuscript to
recount my arduous tale of stagnicoline systematics that should be wrapped up
within the next month or two.” She also
promised to keep me posted “for when the sequences are thrown up on GenBank
[2].” And that was to be the last I ever
heard from her.
As that “month or two” stretched into 2014, with no reply to
my repeated emails, I began to worry that something might be amiss with our
promising young malacologist. Googling
around on the University of Michigan website, I was able to confirm that
Samantha did indeed defend her thesis, “Inferences into species delimitation of
Nearctic Stagnicola using geometric morphometric and phylogenetic methods,” on
November 15, 2013. The outcome I was
unable to determine. But surely, I thought,
if her thesis were successfully defended and signed, it must ultimately appear
for download (or purchase?) through some public outlet somewhere, yes?
No. After more than a
year of watchful waiting, in January of 2015 I finally emailed her major advisor,
Tom Duda, to inquire about the fate of Samantha and her thesis. Tom confirmed that Samantha’s 2013 defense
was indeed successful, and that he himself was surprised not to find her thesis
uploaded to the University of Michigan’s “Deep Blue” server. Apparently The University does not have firm
rules regarding the deposition of MS theses.
And Tom further confessed, “In the past we have requested that theses
and dissertations be deposited in our Mollusk Division library, but regret that
it was my oversight (in combination with her rapid departure and her not
responding to emails after she left) that got in the way of this happening with
Samantha.”
And in fact, as our conversation developed, it materialized
that Tom did not have a clean, final copy of Samantha’s thesis himself. He had apparently returned his only copy to
her with written comments, and she disappeared.
I suggested that he might check with some of the other members of her
committee, and he was able to locate a “near final form” version which he
shared with me in April. But Tom has
asked me not to distribute the document any further, since the version from
which I am working still has some errors.
Samantha’s Thesis [3] is a blockbuster. Her results
simultaneously reinforce a large and growing body of research confirming the
dramatic ecophenotypic effects of habitat on freshwater gastropod shell
morphology, and shatter 200 years of set notions about systematic relationships
in the North American stagnicolines.
Let’s digest her work in five steps.
First, Samantha’s CO1 sequence data suggest two biological
species. Perhaps some of you will recall
the review of interpopulation sequence divergence in L. stagnalis I posted on
the FWGNA blog in [4June12]. There I
argued that the general rule-of-thumb estimate of 5% CO1 divergence often
observed among biological species of pulmonate snails seems applicable to
within-continent comparisons of lymnaeid populations worldwide. This is not a law, it is a very broad-brush
guideline [4].
So below I’ve reproduced a (rather heavily-edited) version
of Samantha’s “Collapsed Bayesian-inferred CO1 tree,” with state and province
abbreviations marking samples from ME = Maine, MI = Michigan, MB = Manitoba,
and so forth. Setting aside the single
Lymnaea arctica sequence that Samantha mined from the Barcode of Life Database
[5], the ABGD prior max distance bars at right seem to suggest the two clusters
of stagnicolines I have labelled “V1” and “V2”.
Although the pairwise sequence differences between these two groups
apparently do not consistently reach 5%, an eyeball estimate from Samantha’s
scale bar, together with the plot of pairwise genetic distances Samantha
offered elsewhere in her thesis, suggests to me that they probably often do.
Note that I have modified the noun “species” with the
adjective “biological” here. This is
because populations of the two putative species seem to occur sympatically, at
least in some cases. More under my
fourth point, below.
Second, the distinction between these two putative
biological species does not coincide with taxonomic tradition, as historically
based on shell morphology. Samantha
classified each of the individual snails she sampled for her C01 analysis using
geometric morphometrics, digitizing their shell outlines with the large set of
sliding landmarks [6] shown in the colorful figure I have reproduced at the top
of this essay [7]. She recognized four
nomina by shell fatness – identifying the green and red categories as emarginata,
the gold as elodes and the blue as exilis.
The gold was an unfortunate color choice – nearly invisible between the
red and blue in her figure.
But in any case, the correct way to define any of these
nominal taxa would have been by reference to populations sampled from their
type localities. God Knows I Tried to
help Samantha with this critical component of her thesis, but for a variety of
reasons, it just didn’t work out. So I
have deleted Samantha’s specific names from the CO1 tree above [8] and
substituted simple color coding according to her morphometric analysis
[10]. And it will be obvious that the
four color categories do not correspond to the two putative biological
species. Cluster V2 shows all four
colors, and cluster V1 shows three of the four.
Third, the putative biological species do coincide with
Brady & Turner’s V1/V2 taxonomy.
Here I’ll ask you to open my essay of [10May12] in a separate window,
and refresh your memory regarding Brady & Turner’s “cryptic stagnicoline”
populations from NW Pennsylvania.
Although all four of the B&T populations inhabited fishless marshes,
and all four bore dark, skinny shells typical of elodes, their “Hartstown
Marsh” population demonstrated a consistently larger (and perhaps more
“flat-sided”) body whorl than their Conley, Osgood, and Killbuck
populations. Kip Brady’s common garden
experiments suggested that this body whorl difference seemed to be heritable.
Brady & Turner [11] considered that their Conley,
Osgood, and Killbuck populations demonstrated “typical” L. elodes shell morphology,
and called them V1. They called their
Hartstown Marsh population V2. See note
[12] for an interesting story about the example specimens figured at right below.
I forwarded samples of all four B&T populations to
Samantha in July of 2012 [13]. And sure
enough, samples from the Hartstown population (marked as PA-h on my version of
Samantha’s CO1 tree) appeared genetically distinct from the Conley, Osgood, and
Killbuck samples (marked PA-c,o and PA-k).
So although subtle, there does appear to be a shell morphological
correlate to C01 sequence divergence between the two putative species. The key character does not seem to be the
traditional fat/skinny dichotomy, but rather the relative size of the body
whorl [14].
Fourth, evidence suggests that the traditional taxonomy of
North American stagnicolines may have been based on shell characters largely
ecophenotypic in their origin. The best
example, ironically, comes from Douglas Lake, the home of the University of Michigan
Biological Field Station. Samantha
sampled 4 individuals from the waters of Douglas Lake itself, all of these
being classified as “emarginata-ovate” by her morphometric criteria, which I
have marked with green letters “d” in the C01 tree above. This small sample included three individuals
belonging to putative biological species V2, and a single individual belonging
to putative species V1. Samantha also
sampled 8 individuals from “Douglas Lake Pools,” presumably marginal ponds not
directly connected to the lake itself.
All of these individuals were classified as exilis by Samantha’s
morphometrics, and are marked with blue letters “d” above. This included 6 individuals classified as
putative species V2, and 2 classified as species V1.
Thus Samantha’s data suggest that two biological species of
stagnicoline lymnaeids seem to co-occur sympatrically in Douglas Lake, both
bearing fat shells of emarginata morphology in the main lake, and both bearing
skinny shells of exilis morphology in marginal pools. We search the world over, and sometimes the
answers we seek are right on our own doorsteps.
And fifth, we do not actually know the correct names for
either of the putative biological species.
Here I must pause, and wipe a tear from my eye. For some reason known but to God, Samantha did
not sequence that sample of topotypic L. catascopium I gave her in June of
2012. Was this tragic oversight related
to some sort of funky decision-making late in her research, regarding the
taxonomy to be employed in her thesis as a whole? See note [8] below for more.
In any case, as I have repeatedly emphasized (to Samantha,
and to you all as well!), catascopium (Say 1817) is the oldest name available
for any North American stagnicoline population.
One of Samantha’s two putative biological species almost certainly must
be catascopium by definition, and the correct name of the other species
depends.
So what to do? Almost
all of Samantha’s pale/fat snails, which might conventionally be identified as
catascopium, were classified as V2. This
set included the sample I sent her from Maine, graphed as a big green triangle
at the top of her C01 tree. And almost
all of Samantha’s V1 individuals demonstrated the dark/skinny shell morphology
conventionally associated with elodes.
So let us provisionally call the V2 species Lymnaea catascopium, leaving
the name Lymnaea elodes for putative species V1. This is admittedly a judgement call, but
seems most consistent with the taxonomy currently employed by workers in the
field.
Have I beat this horse long enough? Let me conclude with two recommendations for
further study. First, the hypotheses
advanced here can be tested with a good genetic survey of the stagnicoline
populations inhabiting the Douglas Lake area.
Somebody needs to use microsatellites, or old-fashioned allozymes, or
even older-fashioned breeding studies, to test the hypothesis that two
reproductively isolated stagnicoline species are sympatric in that lake, not
corresponding to the traditional fat catascopium / skinny elodes dichotomy, but
rather corresponding to the new V1 elodes / V2 catascopium dichotomy. And second, somebody needs to go back up the
Delaware River and fetch us some more topotypic catascopium. And find us some topotypic elodes at Lake
Canandaigua, while shopping around in Yankeeland for lymnaeids anyway. Not it.
Notes
[1] This catch phrase comes from the online comic,
xkcd.com. And although the xkcd logo
shows a stick-figure scientist flamboyantly flourishing a beaker and a calculator,
real science is at least as much theoretical as applied. To be quite precise, science is the
construction of testable models about the natural world. This essay is real science. Stand back.
[2] Not only did Samantha ultimately fail to make her MS
thesis available from any public outlet, she also failed to “throw up” any of
her sequence data on GenBank. Alas.
[3] Flowers, S. L. (2013)
Inferences into species delimitation of Nearctic Stagnicola (Gastropoda:
Lymnaeidae) using geometric morphometric and phylogenetic methods. M.Sc. Thesis, University of Michigan, Ann
Arbor.
[4] Two disclaimers.
First, gene trees are NOT species trees!
They are weak, null models of population relationship. And second, there is no cut point for
species-level sequence divergence that isn’t more exception than rule. See, for example:
- Phylogenetic Sporting and the genus Laevapex [20July07]
- Gene Trees and Species Trees [15July08]
- The Snails The Dinosaurs Saw [16Mar09]
- What is a Species Tree? [12July11]
[6] Samantha really should have digitized more than the
shell outlines. In particular, the
relative sizes of the shell whorls, especially the body whorl, seem to contain
a great deal of heritable information in freshwater gastropod populations [14],
which may be difficult to recover without landmarks on the suture lines or
aperture. See for example:
- Dunithan A, Jacquemin SJ, Pyron M (2012) Morphology of Elimia livescens (Mollusca: Pleuroceridae) in Indiana, U.S.A. covaries with environmental variation. Am Malac Bull 30:127–133.
- Dillon, R. T., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata. Hydrobiologia 709: 117-127. [PDF]
- Dillon, R. T. & S. J. Jacquemin (2015) The heritability of shell morphometrics in the freshwater pulmonate gastropod Physa. PLoS ONE 10(4) e0121962. [PDF]
[8] Although Samantha did not explicitly cite any reference
works to support her taxonomy, it is my impression that her choices of the
category names emarginata-ovate, emarginata-narrow (“canadensis”), elodes and
exilis follow the 1992 work of Burch & Jung [9]. Whatever the origin, her
taxonomy is most unfortunate. For
unexplained (indeed unexplainable) reasons, Samantha seems to have dropped the
oldest specific nomen available for stagnicoline lymnaeids, catascopium (Say
1817), from her methods and results sections in favor of emarginata (Say
1821). But (I’m guessing here) the
decision may have come late in her project?
Because a couple samples remain identified as “catascopium” in her draft
Table 1, with purple coding in her draft Figures 10, 11, and 12. What a mess.
[9] Burch, J. B. & Jung, Y. (1992) Freshwater Snails of
the University of Michigan Biological Station Area. Walkerana 6(15): 1 – 218.
[10] It is my broad-brush impression, based on nothing more
than inspection of the figures in Samantha’s draft thesis, plus the sets of
stagnicoline shells figured by Burch and Jung, that samples our Michigan
colleagues tend to call emarginata-narrow (“canadensis”) and elodes may tend to
represent putative species V1, and the samples our colleagues call
emarginata-ovate or exilis may tend to represent V2, But since Samantha didn’t digitize the shell
aperture or suture lines, the distinction was not recovered by her
morphometrics. So let’s just focus on
Samantha’s color coding, and set her taxonomy aside to the extent possible.
[11] Brady, J. K & A. M. Turner (2010) Species-specific
effects of gastropods on leaf litter processing in pond mesocosms. Hydrobiologia 651: 93-100.
[12] For several weeks during the spring and early summer of
2012, I held cultures of the four B&T populations here in Charleston,
dissecting samples to hunt for anatomical distinctions that I ultimately did
not find. During that period both the Killbuck (V1) and Hartstown (V2)
populations laid eggs. These I hatched and
reared for quite a few months in my standard plastic aquarium boxes, at
densities that were certainly too high, largely neglecting them, changing their
water infrequently.
In any case, my results seem to confirm those of Kip
Brady. My standard culture water here in
Charleston is almost certainly much softer than that to which stagnicoline
lymnaeids are usually adapted, yielding the chalky appearances of the two
example shells figure above. Yet the V1
offspring did indeed seem to develop relatively smaller body whorls than the V2
offspring.
[13] And here’s another little confusion. Unknown to me, Kip Brady sent Jack Burch
samples from a couple of his stagnicoline populations several years prior to my
shipment to Samantha. Kip never heard
anything further. But apparently 5 of
Kip’s earlier samples were sequenced at some point along with 8 of the
individuals I sent to Samantha in 2012, and all 13 appear graphed side-by-side
in Samantha’s C01 tree, under two different labelling schemes.
[14] Body whorl differences of this subtle sort seem to be
quite heritable in freshwater pulmonates as a general rule. In fact, body whorl differences were the way
we initially distinguished Physa carolinae from Physa acuta back in 2009
[15]. For more, see:
- The Lymnaeidae 2012: A clue [9July12]
- The heritability of shell morphology in Physa h^2 = 0.819 [15Apr15]
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