Dr. Rob Dillon, Coordinator





Tuesday, January 14, 2025

Taxonomy of the Pleurocera laqueata/troostiana complex. Part II, Ly - Z. Or, Just 974 Starfish Left to Go.

Editors note – This is the fifth (and final) installment of a series I started way back in September of 2024, starring Pleurocera laqueata with P. troostiana and P. simplex in supporting roles.  If you have a serious interest in the evolution and systematics of the North American Pleuroceridae, you might want to go down to footnote [1] and refresh your memory of previous episodes before proceeding.  If you are not interested in pleurocerid snails, on the other hand, I cannot imagine how the essay that follows could be much more than an irritation.

We opened last month’s essay with a tally of the pleurocerid nomina that Calvin Goodrich [2] recognized as valid to describe representatives of his Groups of Goniobasis catenaria and Goniobasis laqueata in the Ohio, Cumberland, and Tennessee River systems.  We observed that there  are 25 such names and reviewed the first 12 of them (alphabetically), promising to finish the job this month.

I did not mention it at the time, because it is a bit embarrassing, but I have found one Latin nomen useful for certain pleurocerid populations of the greater Ohio River basin that Goodrich synonymized under something else, and hence was not listed by him in 1940.  So, the total is actually 26 names, and we have 14 to review today.  Sorry – I know that’s going in the wrong direction, and I apologize.

Lyonii.  Isaac Lea [3] described Goniobasis lyonii in brief Latinate form from “Grayson County, Kentucky” in 1862.  Goodrich [2, 4] synonymized lyonii under Goniobasis laqueata and the nomen was carried passively (with a long list of other junior synonyms) into his Group of Goniobasis laqueata.  From there it disappeared, not mentioned at all by Burch [5], forgotten and consigned to the boneyard.

We consider the nomen lyonii valid and useful at the subspecific level [6], Pleurocera troostiana lyonii (Lea 1862), identifying laqueata/troostiana hybrids at the western and northern limits of the phenomenon. For our rationale, together with a copy of Lea’s [7] original figure and a modern topotype, see Dillon [8] pp 81 – 88 or my essay of [6July20].  An image of the holotype (USNM119147) is collected below.

USNM119147 (23.1 mm), MCZ53965 [27],
USNM118923 (13.6 mm),USNM118429 (20.1 mm)

Nassula. Timothy Abbot Conrad [9] described Melania nassula in 1834 from “the limestone spring at Tuscumbia, Alabama.”  Goodrich [2, 4] considered that the taxon named a distinct and valid species in his Group of Goniobasis catenaria; Burch followed suit in his Elimia catenaria Group.

No original type material seems to have survived, according to Graf [10], although the MCZ holds the “possible syntype” figured above (MCZ 53965).  And a pleurocerid population matching Conrad’s original 1834 description and figure (#9 below) quite well still inhabits the Tuscumbia Big Spring to the present day.  It does, indeed, look very much like an Atlantic drainage (or Floridian) population of Pleurocera catenaria has been airlifted 300 miles west and dropped into North Alabama.  This is a distinct and valid biological species, Pleurocera nassula (Conrad 1834).

Paupercula.  Isaac Lea described Goniobasis paupercula in brief Latinate form in 1862 [3], giving the type locality as “North Alabama, Prof. Tuomey,” with a more complete English description and figure following in 1863 [7].  Goodrich [2,4] recognized it as a valid species in his “Group of Goniobasis laqueata,” as did Burch [5] in his “Elimia laqueata group.”

We consider the nomen a junior synonym of Pleurocera troostiana perstriata (Lea 1853) [11], identifying laqueata/troostiana hybrids with decollate shells in North Alabama.  For our rationale, together with a copy of Lea’s [7] original figure and images of two topotypes (an adult and a juvenile, both R), see Dillon [8] pp 61 – 71 or my essay of [10May20].   An image of the holotype (USNM 118923) is collected above.

From Conrad [9], Lea [16], Lea [7]

Perstriata. Isaac Lea [11] described Melania perstriata from “Coosa River, Alabama, Prof. Brumby, Huntsville, Tenn., Mr. J. Clark” in 1853.  Goodrich [2, 4] recognized it as a valid species in his “Group of Goniobasis laqueata,” restricting its type locality to the Big Spring at Huntsville, Alabama.  Burch [5] concurred on the specific value of the nomen but transferred it to his “Elimia catenaria group.”  We consider the nomen valid at the subspecific level, Pleurocera troostiana perstriata, identifying laqueata/troostiana hybrids with little or no costation on the body whorl.

For our rationale, together with a copy of Lea’s [11] original figure, an image of a modern topotypic specimen, and example shells from several additional populations, see Dillon [8] pp 51 – 59 or my essay of [15Apr20].  An image of the holotype (USNM 118429) is collected above.

Plicata-striata.  Albert G. Wetherby’s [12] 1876 description of Goniobasis plicta-striata [13] is very difficult to obtain today.  But Walker [14] quotes his type locality as “Stone River and Mill Creek, Rutherford County, and Sinking Creek, Shelbyville, TN.”  Goodrich [2] assigned the nomen to his Group of Goniobasis laqueata.  Burch re-spelled the nomen without the dash and shifted it to his Elimia catenaria group.

The entire main stem of the Stone River is impounded today, as is Mill Creek, but Sinking Creek is inhabited by apparently healthy populations of both P. laqueata and P. troostiana edgariana, not especially helpful for our understanding of Wetherby's taxon today.  Fortunately, Wetherby donated N = 65 (rather ratty) syntypes “ex original lot” to Harvard’s Museum of Comparative Zoology (MCZ 149453), the least-ratty of which is selected and figured above.  Goniobasis plicata-striata (Wetherby 1876) appears to be a simple synonym of Pleurocera troostiana edgariana (Lea 1841).

MCZ149453 [27], USNM118834 (17.7 mm),
USNM119329 (19.8 mm), USNM119296 (16.9 mm)

Porrecta.  Isaac Lea [15] described Goniobasis porrecta in brief Latinate form from “Gap Creek and Spring” (Cumberland Gap, TN) in 1863, with more complete English description and figure (#47 above) in 1866 [16].  Goodrich [2] considered it a valid species in his “Group of Goniobasis catenaria,” subsuming vittatella (Lea 1863) under it, as did Burch [5] in his “Elimia catenaria group.”

We consider the nomen a junior synonym of Pleurocera troostiana troostiana (Lea 1838).  For our rationale, together with an image of a modern topotype, see Dillon [8] pp 1 – 7 or my essay of [4Aug19].  An image of the holotype (USNM 118834) is collected above.

Pybasii.  Isaac Lea [3] published a brief Latinate description of Goniobasis pybasii from “Tuscumbia, Alabama” in 1862, with English description and figure following in 1863 [7].  This is the third of Lea’s 1862/63 “eighty-two new species” of Goniobasis we have reviewed in the present essay, along with lyonii and paupercula, all synonyms of the same species.  A nineteenth-century malacological hat trick!  There will be two more.

Goodrich [2, 4] recognized pybasii as a valid species in his “Group of Goniobasis laqueata,” as did Burch [5] in his “Elimia laqueata group.”  We consider the nomen a junior synonym of Pleurocera troostiana perstriata (Lea 1853), identifying laqueata/troostiana hybrid populations in North Alabama.  For our rationale, together with a copy of Lea’s [7] original figure and an image of a modern topotypic specimen (Q), see Dillon [8] pp 61 – 71 or my essay of [10May20].   An image of the holotype (USNM 119329) is collected above.

Rubella.  This is the fourth of the “eighty-two new species” that Isaac Lea [3] described in 1862 we have reviewed this afternoon.  Lea’s original figure [7] is reproduced above (#191), as well as a fresh image of the holotype (USNM 119296).

Goodrich [2] considered rubella a valid species in his “Group of Goniobasis catenaria,” noting as he did, however, that the species was “reported originally from Cherokee County, North Carolina, and not found there since.  May be the same as porrecta.”  Burch [5] did not list rubella but did reproduce Tryon’s figure of it (#369), with the caption “E. rubella = ?E. porrecta.”

Right.  Lea’s original description, “very near to Melania (Goniobasis) teres but differs in being carinate,” together with his figure and type specimen, make it quite clear that Goniobasis rubella, like porrecta and like teres, is a simple junior synonym of Pleurocera troostiana troostiana (Lea 1838).

That said, I really think that the type locality given by Lea for his G. rubella, “Near Murphy, Cherokee County, North Carolina,” must have been in error.  The modern range of P. troostiana does not extend any further east up the Hiwassee drainage than Polk County, TN.

USNM119269 (16.3 mm), USNM118448 (14.2 mm),
USNM121603 (19.4 mm)

Spinella.  The fifth of Isaac Lea’s 1862/63 creations [3, 7] we have reviewed this month, Goniobasis spinella was described from “Sycamore, Claiborne County, Tennessee” as “very nearly of the same outline of Melania (Goniobasis) strigosa but much smaller, slimmer, and darker color.”  Goodrich [2] considered the nomen a subspecies of Goniobasis arachnoidea in his “Group of Goniobasis catenaria,” as did Burch [5] in his “Elimia catenaria group.”

Lea’s original figure is reproduced below (#130), and his holotype (USNM 119269) freshly imaged above.  We consider spinella another simple junior synonym of Pleurocera troostiana troostiana (Lea 1838).  For our rationale, together with an image of a modern topotype, see Dillon [8] pp 41 – 49 or my essay of [7Jan20].

Striatula.  Isaac Lea [17] described Melania “striata” in brief Latinate form from “Tennessee” in 1841, with English description and figure following in 1843 [18].  He amended the name to “striatula” in the interim [19].  Goodrich [2] considered striatula a valid species in his “Group of Goniobasis catenaria,” as did Burch [5] in his “Elimia catenaria group.”

Lea’s original [18] figure (#49) is reproduced below, and a shell catalogued into the USNM collection as the holotype (USNM 118448), that Graf [10] referred to as a “possible syntype” is imaged above.  They do not match.  Rats.

Lea’s original figure showed strong striation and no plication, looking like a synonym of typical P. troostiana troostiana, as I myself suggested in Dillon [8] pp 41 – 49 and in my essay of [7Jan20].  The nominal holotype, however, shows plicae as strong as striae [20], looking very much like P. troostiana edgariana.  And the locality information, simply “Tennessee,” is no help resolving the discrepancy.

Turning to the letter of Lea’s [17] original description as a tiebreaker, we read “shell striate” to lead off, with no mention of plication.  But in Lea’s remarks, we read “In some individuals the folds are numerous – in others the striae predominate and cover nearly all the whorls.”  Are the “folds” plicae?

In the end, I suppose it does not matter.  Melania striatula (Lea 1842) is a junior synonym of Pleurocera troostiana (Lea 1838), but whether of the purebred (typical) form or the hybrid edgariana form, I don’t think we’ll ever know.

From Lea [7], Lea [18], Lea [22]

Strigosa. Another of Isaac Lea’s 1841/43 classics [17, 18], Melania strigosa was described as “somewhat like the teres herein described” from “Tennessee, Dr. Troost, Holston River Dr. Warder.”  Goodrich [2] considered it a valid species in his “Group of Goniobasis catenaria,” as did Burch [5] in his “Elimia catenaria group.”

In my 2023 essay [8, pp 41 - 49], and in the 2020 blog post from which that essay was crafted [7Jan20], I offered four reasons to restrict the strigosa type locality to Little Flat Creek 10 miles north of Knoxville, figured a topotype, and reproduced Lea’s [18] original figure.  Lea’s holotype (USNM 121603) is imaged above.  We consider the nomen yet another simple junior synonym of Pleurocera troostiana troostiana (Lea 1838).

Teres.  And a third time.  Isaac Lea [17] described Melania teres from “Tennessee, Dr. Troost” in 1841, following with a more complete English description and figure in 1843 [18].  Again, Goodrich [2] considered it a valid species in his “Group of Goniobasis catenaria,” as did Burch [5] in his “Elimia catenaria group.”

We considered the locality data too vague to send us on a modern day teres-hunt for our blog post of [7Jan20] or the essay [8, pp 41 - 49] derived from it, but did reproduce both Lea’s [18] original figure, and figure #356 from Burch [5].  An image of Lea’s holotype (USNM 119251) is collected below.  We consider the nomen yet another simple junior synonym of Pleurocera troostiana troostiana (Lea 1838).  So, it materializes that Isaac Lea scored malacological hat tricks in both 1841 and in 1862.  Without question, Isaac Lea was the greatest of all time, of something.

Torta/tortum.  Isaac Lea’s brief, Latinate description of Melania torta from “Big Creek, Lawrence County, Tennessee” was published in 1845 [21], with more complete English description and figure following in 1848 [22].  Tryon [23] assigned the nomen to Pleurocera in 1873, changing the spelling to tortum [24].  Goodrich considered tortum a valid subspecies of Goniobasis laqueata, as did Burch, of Elimia laqueata.

Lea’s original [22] figure is reproduced above (#30), and a fresh image of the holotype (USNM 119255) collected below.  Quoting him verbatim:
“There were eight specimens of this species submitted to my examination by Mr. Clark, of Cincinnati.  […] The apices of the individuals now before me are slightly eroded … one of the specimens has small folds near the apex, with decussating striae. […] The body whorl is very long.”
Lea’s description, original figure, and designated holotype all strongly suggest that Melania torta is a laqueata/simplex hybrid, making the nomen a junior synonym of populations we designated Pleurocera laqueata castanea (Lea 1841) in our essay of [12Nov24].

Lawrence County, Tennessee, is on the Alabama line just north of Florence in the Shoal Creek subdrainage.  I cannot find a “Big Creek” anywhere on modern maps, but Pleurocera laqueata populations bearing shells of typical morphology are widespread in that county.  And the FWGNA database contains five records of P. laqueata castanea in Lauderdale County, AL, just south.

USNM119251 (21.6 mm), USNM119255 (18.0 mm),
USNM119256 (25.3 mm)

Troostiana.  Isaac Lea [25] described Melania troostiana from “Mossy Creek, Jefferson County, Ten” in 1838, quite early in his career.  Goodrich [2] considered it a valid species in his Group of Goniobasis catenaria, as did Burch [5] in his Elimia catenaria group.

Pleurocera troostiana is the oldest name for a distinct, valid, biological species of pleurocerid snail widespread in small streams of the greater Ohio drainage from SW Virginia through most of Tennessee, North Alabama and Southern Kentucky.  For a complete review, illustrated with a copy of Lea’s [25] original figure and images of modern topotypes, see Dillon [8] pp 35 – 40 or my essay of [9Dec19].  An image of the holotype (USNM 119256) is collected above.

I provided a photo of a living P. troostiana individual in my follow-up essay of [7Jan20], published in Dillon [8] pp 41 – 40.  I then developed the argument that a great variety of pleurocerid nomina in East Tennessee might be junior synonyms, including arachnoidea, porrecta, spinella, strigosa, striatula, and teres, as reviewed above.

Then in a series of four essays posted on this blog between April and July of 2020, and published in 2023 by Dillon [8] pp 51 – 88, I recognized three subspecies [6] of P. troostiana inhabiting the waters of North Alabama, Middle Tennessee, and Kentucky: perstriata (Lea 1853), edgariana (Lea 1841), and lyonii (Lea 1862), synonymizing a large number of additional nomina underneath them.  I also published a separate circular [26] reviewing the entire four-subspecies system, including the typical (s.s.) form.

So, we closed last month’s essay with a reference to the sanctimonious story usually entitled “Starfish on the Beach,” which seems to have evolved from a 1969 essay by Loren Eiseley.  Including the 12 starfish we dispatched last time, our two-month total is 26 starfish on the beach, 7 of which we tossed back into the sea.  At the species level we recognize laqueata, troostiana, and nassula.  At the subspecies level, all of hybrid origin, we recognize perstriata, edgariana, and lyonii under troostiana and castanea under laqueata.  The other 19 starfish we have now bagged for the dumpster.  Just 974 starfish left to go.

Notes:

[1] Here are the four essays on the Pleurocera laqueata/troostiana complex that preceded the present:

  • The type locality of Melania laqueata [18Sept24]
  • Widespread hybridization between Pleurocera laqueata and P. troostiana in streams of the Tennessee/Cumberland [15Oct24]
  • Reticulate evolution in the North American Pleuroceridae [12Nov24]
  • The taxonomy of the Pleurocera laqueata/complex.  Part I, A – La. [10Dec24]

[2] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occasional Papers of the Museum of Zoology, University of Michigan  417: 1-21.

[3] Lea, Isaac (1862) Description of a new genus (Goniobasis) of the Family Melanidae and eighty-two new species. Proceedings of the Academy of Natural Science of Philadelphia 19: 262 – 272.

[4] Goodrich, C. (1930)  Goniobases of the vicinity of Muscle Shoals.  Occasional Papers of the Museum of Zoology, University of Michigan 209: 1 – 25.

[5] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[6] Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits.  For more, see:

  • What is a subspecies? [4Feb14]
  • What subspecies are not [5Mar14]

[7] Lea, Isaac (1863) New Melanidae of the United States.  Journal of the Academy of Natural Sciences of Philadelphia (New Series) 5: 217 – 356.

[8] Dillon, R.T., Jr. (2023b) The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC. [publications]

[9] Conrad, T. A. (1834) New Fresh Water Shells of the United States, with coloured illustrations, and a monograph of the genus Anculotus of Say; also A synopsis of the American naiades.  Philadelphia, Judah Dobson.  76 pp, 8 plates.

[10] Graf, D. L. (2001) The cleansing of the Augean stables.  Walkerana 12(27): 1 - 124.

[11] Lea, Isaac (1853)  Description of a new genus (Basistoma) of the Family Melaniana, together with some new species of American Melaniae.  Transactions of the American Philosophical Society (new series) 10: 295 – 302.

[12] We first met Albert G. Wetherby (1833 – 1902), author of the baffling taxon Helisoma duryi, back in 2020:

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

[13] Wetherby, A.G. (1876) Remarks on the variation in form of the family Strepomatidae, with descriptions of news species  Proceedings of the Cincinnati Society of Natural History 1:10.

[14] Walker, B. (1918)  A synopsis of the classification of the freshwater Mollusca of North America, North of Mexico, and a catalogue of the more recently described species, with notes.  Univ. Mich. Mus. Zool. Misc. Publ. 6: 1 - 213.

[15] Lea, Isaac (1863) Descriptions of fourteen new species of Melanidae and one Paludina.  Proceedings of the Academy of Natural Sciences of Philadelphia 15: 154 – 156.

[16] Lea, Isaac (1866) New Unionidae, Melanidae, etc. chiefly of the United States.  Journal of the Academy of Natural Sciences of Philadelphia (New Series) 6: 113 – 187.

[17] Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings of the American Philosophical Society 2: 11 – 15.

[18] Lea, Isaac (1843) Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society (New Series)  8: 163 – 250.

[19] Lea, Isaac (1842) Minutes of the Stated Meeting of December 2.  Proceedings of the American Philosophical Society 2: 237.

[20] If you are confused about striation and plication (costation), see my 2020 essay for a diagram:

[21] Lea, Isaac (1845) Descriptions of new fresh water and land shells.  Proceedings of the American Philosophical Society 4: 162 – 168.

[22] Lea, Isaac (1848)  Description of new fresh water and land shells.  Transactions of the American Philosophical Society 10: 67 – 101.

[23] Tryon, G. W. (1873) Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

[24] At the risk of confusing the situation further.  Isaac Lea also described a Trypanostoma tortum in 1862 from the Uchee River (Creek), a tributary of the Chattahoochee on the GA/AL border.  That one was renamed Pleurocera parkerii by Tryon.

[25] Lea, Isaac (1838-39) Description of New Freshwater and Land Shells.  Transactions of the American Philosophical Society (New Series) 6: 1 – 154.

[26] Dillon, R.T., Jr.  (2020) The four subspecies of Pleurocera troostiana (Lea 1838), with synonymy.  FWGNA Circular 2: 1 - 5. [pdf]

[27] I first reached out to our buddy at the MCZ Gonzolo Giribet about taking some photos of arachnoidea, plicata-striata, and nassula on October 10, and have since exchanged eight rounds of email with him and with his collection manager, Ms. Jennifer Trimble.  And yesterday Ms. Trimble sent me a pdf form to fill out.  So, by the standards of snail science, I am pleased to report that progress is being made.

Tuesday, December 10, 2024

Taxonomy of the Pleurocera laqueata/troostiana complex. Part I, A - La.

Editor’s Note – This the fourth installment of a five-part series on Pleurocera laqueata, P. troostiana, and hybridization between them in small streams of Kentucky, Tennessee, and North Alabama.  If your interest in the evolution of the North American Pleuroceridae is serious enough to have dropped you this deep into so dismal a swamp, and you have not previously read my posts of [18Sept24], [15Oct24] and [12Nov24], go back and do so now.  We will wait for you.

Calvin Goodrich [1] divided the Goniobasis species of the Tennessee, Cumberland, and Ohio River systems into six groups [2]. Prominent among those was a “Group of Goniobasis laqueata,” with ten species and six subspecies, and a “Group of Goniobasis catenaria” with eight species and one subspecies.  Unsurprisingly, most of the nomina in that former group are synonyms of Pleurocera laqueata (Say 1829), and most of the latter group synonyms of Pleurocera troostiana (Lea 1838).

But because P. laqueata and P. troostiana hybridize, the distinction between Goodrich’s two groups has never been clear.  Burch [3] moved three species with two subspecies from Goodrich’s Group of Goniobasis laqueata to his understanding of the “Elimia catenaria Group” and separated one species/subspecies pair from Goodrich’s laqueata group (the “Elimia acuta Group”) as entirely distinct.

So, working alphabetically, this month we will review the first twelve of the 10 + 6 + 8 + 1 = 25 pleurocerid nomina from the Ohio, Cumberland and Tennessee allocated by Calvin Goodrich to his Groups of Goniobasis catenaria and Goniobasis laqueata combined.

USNM119088 (14.9 mm), MCZ50236 (21.1 mm),
USNM119217 (13.5 mm)

The vast majority of these were described by our old buddy Isaac Lea [4], in eight separate papers and monographs published between 1831 and 1868.  Lea described seven of the species we will review over the next two months (as “Melania”) in brief Latinate form in the Proceedings of the American Philosophical Society of 1841 [5], following with more complete English descriptions and figures in the APS Transactions of 1843 [6].  He disgorged an additional dose of seven brief Latinate descriptions (as “Goniobasis”) in the Proceedings of the Academy of Natural Sciences of 1862 [7], following with complete descriptions and figures in the ANSP Journal of 1863 [8].  Lea’s descriptions of the third set of seven species were scattered in other journals at other times.

This catalog may get a bit tedious at times, I’m afraid, involving a lot of rather dry library scholarship, and to be quite frank, is not the kind of thing I am especially good at, not having been blessed with the lawyerly frame of mind necessary to build any reputation in the marble halls of zoological nomenclature.  It’s a service, I suppose.

So, for a spoonful of sugar, last month I traveled up to Washington to see our good friend Ellen Strong of the USNM.  Ellen and her obliging staff set aside for me Isaac Lea’s type specimens [9] for 20 of the species that Goodrich included in his groups of catenaria and laqueata from the greater Ohio drainage.  This is the first time that photos of any of those types have ever been published, as far as I am aware.

And I contacted our good buddy Gonzalo Giribet up at the MCZ Harvard, and he and Ms. Jennifer Trimble agreed to add type specimens of Conrad’s nassula, Anthony’s arachnoidea, and Wetherby’s plicata-striata to their (rather lengthy) “imaging queue.”  And our friends at the ANSP, already on the ball, had previously uploaded and made available to the public a nice photo of Haldeman’s costifera.  Bottom line, over the next two months, we will publish fresh photos of type material for 24 of the 25 catenaria/laqueata group species in habiting the Ohio, Cumberland, and Tennessee River systems.  And offer a coherent, modern hypothesis for both their evolutionary and their taxonomic relationships.  Here we go:

Acuta.  First, we must be very clear about what the pleurocerid snail that Isaac Lea described as Melania acuta is not.  It is not that well-known inhabitant of rivers and streams of the Ohio, Great Lakes, and upper Mississippi drainages described as “Pleurocera acuta” by C.S. Rafinesque [11] in 1831, monographed in loving detail by Dazo [12] in 1965.  Rafinesque’s Pleurocera acuta was lowered to subspecific status under Pleurocera canaliculata by Dillon [13] in 2013.

Completely independent of whatever Constantine Smaltz Rafinesque was discovering and publishing in the early 19th century, on May 7, 1830 Isaac lea read a paper at a meeting of the American Philosophical Society in Philadelphia describing “Melania acuta” from the “Tennessee River, Prof. Vanuxem” bearing a shell whose “delicate form, furnished with undulations and transverse lines, will easily distinguish it.”  Lea’s little 1:1 figure is reproduced below.

From Lea [14], Anthony [20], Lea [22], Reeve [24]

A reading is not a publication, however.  The front page of Volume 4 of the Transactions of the American Philosophical Society, in which Lea’s paper was ultimately published [14], clearly states 1834.  N.P. Scudder [15] argues, however, that Lea’s paper was “issued in the latter end of 1831, and acknowledged by correspondents as received in that year, PANSP 7:243.”  Tryon [16] does not hazard a guess on Lea’s publication date, Goodrich [1] suggests 1830 and both Burch [3] and Graf [10] concur with Scudder’s 1831.

So, the bottom line is that Rafinesque’s acuta and Lea’s acuta seem to have been published simultaneously.  And since Lea’s acuta was reassigned to Goniobasis by Tryon [16], and then re-reassigned to Elimia by Burch [3], and then both Goniobasis and Elimia folded under Pleurocera by Dillon [17], today we have two Pleurocera acutas, both described in 1831, meaning entirely different things.

Rafinesque’s acuta became prominent, however, while Lea’s acuta receded into obscurity.  A big part of the reason is that Isaac Lea’s type locality was vague.  Goodrich [18] speculated that the that the “Tennessee River” from which Vanuxem sampled that first specimen of Lea’s acuta must have been in North Alabama, where specimens matching his description “have been taken at Muscle Shoals by Messrs. Hinkley and Smith.”  That malacologically rich section of the Tennessee River is long inundated and much lamented [19].  Goodrich also reported collections from the Flint River, the Elk River, and Piney Creek, but of course, tributaries are a poor substitute for the main river itself.

An image of the type specimen (USNM119088) was reproduced way up at the top of this blog post.  Rather than join the speculation on where that shell might have been collected, or whether the population of pleurocerid snails including the individual from the back of which it was snatched almost 200 years ago might have been reproductively isolated from any of the biological species of pleurocerids we recognize today, I will simply suggest that Melania acuta Lea 1831 is a junior homonym of Pleurocera acuta Rafinesque 1831.  RIP Melania (aka Goniobasis, aka Elimia) acuta.

Arachnoidea.  John G. Anthony [20] described Melania arachnoidea from “a small stream emptying into the Tennessee River near Loudon, Tennessee” in 1854.  Goodrich [1] considered it a valid species in his Group of Goniobasis catenaria, as did Burch [3] in his Elimia catenaria Group.  We consider the nomen a junior synonym of Pleurocera troostiana troostiana (Lea 1838).

For our rationale, together with an image of a modern topotype, see Dillon [21] pp 41 – 49 or my essay of [7Jan20].  See above for an image of a lectotype (MCZ50236) and a reproduction of Anthony’s original 1:1 figure.  We measured and scored a sample of N = 30 shells from Anthony’s arachnoidea type locality near Loudon for our troostiana regression analysis two months ago [15Oct24].

Castanea.  Isaac Lea’s brief Latinate description of Melania castanea (Maury County, Tenn. Thomas H. Dutton) was published in 1841 [5], with a more complete English description and figure following in 1843 [6].  Last month [12Nov24] I reproduced Lea’s original 1843 figure, concurring with Goodrich’s [1] suggestion that castanea is a valid subspecies Pleurocera laqueata castanea (Lea 1841), and advancing the hypothesis that pleurocerid populations bearing shells of that distinctive morphology are hybrids between P. laqueata and P. simplex.  A fresh image of the holotype, USNM119217, was reproduced in the figure that opened this essay way up above.

USNM121480 (10.2 mm), ANSP27434 (18.3 mm),
USNM119021 (19.3 mm), USNM118463 (14.1 mm)

Clavula.  Goniobasis clavula was described from “Jackson Co, Alabama, Dr. Spillman” by Isaac Lea [22] in 1868.  That county, in the extreme NE corner of Alabama, lies entirely within the Tennessee River drainage.  The nomen was demoted to subspecific status under Goniobasis acuta by Goodrich [1, 18] and placed in the Group of Goniobasis laqueata.  Burch [3] agreed with Goodrich about the subspecific relationship but transferred clavula along with its parent into a separate Elimia acuta Group.

Both Lea’s [22] original figure and a fresh image of the holotype (USNM121480) are reproduced above.  The type specimen is subadult, very slender, demonstrating both striation and plication, becoming obsolete on the body whorl.  We consider the nomen a junior synonym of the hybrid Pleurocera troostiana perstriata (Lea 1853).

Costifera.   Melania costifera was described in 1841 from “Hennepin, Illinois” by S. S. Haldeman [23].  The nomen was considered to represent a valid species by both Goodrich and Burch, in their Groups of Goniobasis laqueata and Elimia laqueata, respectively.  No figure was provided originally, but Haldeman’s written description “having numerous, spiral, elevated lines, crossing a series of curved ribs, on all the whorls,” together with the slender figure subsequently published by Reeve [24], reproduced above, sound very much like P. troostiana lyonii.

Haldeman’s original type shell (ANSP27434) is still held in the ANSP collection today, however, its image thoughtfully made available online by our friends in Philadelphia.  And that image, as reproduced above, suggests that the “spiral, elevated lines” are negligible, and the body whorl relatively large, as typical for Pleurocera laqueata laqueata.

In such a situation, where the published figure and the type shell are strikingly different, it would be nice to refer to a modern topotypic collection.  Alas, my review of the online catalog at the Illinois Natural History Survey returned no modern records of costifera, laqueata, troostiana, or any pleurocerid bearing a shell with plications or striations of any sort within 250 miles of Hennepin [25].  Absent a tiebreaker, therefore, the actual type shell as held by the ANSP must take precedence over the Reeve’s 1860 figure.  Melania costifera (Hald 1841) would appear to be a junior synonym of Pleurocera laqueata laqueata (Say 1829).

Costulata.  Melania costulata was described in 1841 by Isaac Lea [5] from the “Barren River, Kentucky.”  His 1843 figure [6] is reproduced below.  Goodrich [1] recognized the nomen as a subspecies of Goniobasis laqueata, as did Burch [3] of Elimia laqueata.

Lea wrote, “In its general characters this species resembles M. laqueata Say.  It may be distinguished in its being of less diameter and being more slender.”  The holotype shell (USNM119021) as freshly figured above is indeed a bit more slender than typical for laqueata.  But it demonstrates strong striations (not noted by Lea) as well as plications, extending down to include the body whorl.  We consider costulata a junior synonym of the hybrid taxon Pleurocera troostiana edgariana (Lea 1841).

From Lea [6], Lea [6], Lea [8].

Crispa.  Isaac Lea [7] described Goniobasis crispa from “Florence, Alabama” in 1862. The nomen was lowered to subspecific status under G. perstriata by Goodrich [1, 18] and placed with its parent in the Group of Goniobasis laqueata.  Burch [3] concurred with the demotion, but not the placement, transferring “Elimia perstriata crispa” to his Elimia catenaria Group.  We consider the nomen a junior synonym of Pleurocera nassula (Conrad 1834).  See Dillon [21] pp 61 – 71 or my essay of [10May20] for a copy of Lea’s [8] original figure.   A fresh image of the holotype (USNM118463) is collected above.

Curreyana.  Just as was the case of Melania costulata, Melania curryana was described by Isaac Lea [5] from the “Barren River, Kentucky” in 1841.  Lea’s 1843 figure of curreyana [6] is reproduced next to his figure of costulata above.  And again, as in M. costulata, Goodrich [1] recognized M. curryana as a valid nomen in his Group of Goniobasis laqueata, as did Burch [3] in his Elimia laqueata Group.

Unlike M. costulata, however, Graf [10] was unable to find any type material for curreyana in the USNM.  It would appear that Lea’s 1841 written description, together with his 1843 figure, are all we have for evidence today.

Lea wrote that the shell of M. curreyana was “Remarkable for large and strong folds,” adding “It is without striae, and the body whorl is smooth, except near the suture.”  Those contemporary observations, together with Lea’s figure of a shell absent any apparent striation, combine to suggest strongly that M. curreyana (Lea 1841) is a simple junior synonym of Pleurocera laqueata laqueata (Say 1829).

Decampii.   Isaac Lea [26] described Goniobasis decampii from “Huntsville, Alab.” in 1866.  He apparently intended to include the Latinate description in his paper of Mayish [27] 1863, because in his follow-up paper of 1866 he stated that his original description had been published three years earlier, but it was not.

The nomen was lowered to subspecific status under G. perstriata by Goodrich [1, 18] and placed with its parent in the Group of Goniobasis laqueata.  Burch [3] concurred with the demotion, but not the placement, transferring “Elimia perstriata decampii” to his Elimia catenaria Group.  We consider the nomen a junior synonym of the hybrid taxon Pleurocera troostiana perstriata (Lea 1853).

For our rationale, together with a copy of Lea’s [26] original figure, see Dillon [21] pp 61 – 71 or my essay of [10May20].   A fresh image of the very slender holotype shell (USNM118967), bearing light striations and plications on its upper whorls only, is collected below.

Edgariana. Isaac Lea [5] described Melania edgariana from “Cany Fork, Tenn.” in 1841.  Tryon [16] synonymized the nomen under Conrad’s (1834) nassula [28] but Goodrich [1] resurrected it as a valid species in his Group of Goniobasis laqueata.  Burch agreed on the specific status but transferred it to his Elimia catenaria Group.  We consider the nomen valid at the subspecific level, Pleurocera troostiana edgariana (Lea 1841), identifying laqueata/troostiana hybrids with strong sculpture on the body whorl.

USNM118967 (17.0 mm), USNM118423 (19.0 mm),
USNM118959 (17.5 mm)

For our rationale, together with a copy of Lea’s [6] original figure, an image of a modern topotypic specimen, and example shells from several additional populations, see Dillon [21] pp 73 – 79 or my essay of [5June20].   A fresh image of Lea’s holotype (USNM118423) is collected below.

Interveniens.  Isaac Lea [7] briefly described Goniobasis interveniens from “North Alabama, Prof. Tuomey” in 1862, with a more complete description and figure following in 1863 [8]. His original 1:1 figure and an image of the holotype (USNM118959) are reproduced above.  Both Goodrich [1, 18] and Burch [3] considered interveniens a valid and distinct species in their Groups of Goniobasis laqueata and Elimia laqueata, respectively.  We are at a loss to find any distinction between Lea’s interveniens and Thomas Say’s laqueata whatsoever, considering Goniobasis interveniens (Lea 1862) a simple junior synonym of Pleurocera laqueata laqueata (Say 1829).

Laqueata.  Melania laqueata was described by Thomas Say in 1829 from “Dr. Troost in Cumberland River” [29].  No original type material seems to have survived, although Say’s written description and figure were sufficient to establish it as the type for both Goodrich’s Group of Goniobasis laqueata and Burch’s Elimia laqueata Group.

Pleurocera laqueata (Say 1829) is the oldest name for a distinct, valid, biological species of pleurocerid snail widespread in rivers and streams of Middle Tennessee, North Alabama, and southern Kentucky.  We recognize three subspecies: the big river alveare (Conrad 1834) and the laqueata/simplex hybrid castanea (Lea 1841), as well as the typical (s.s.).  For a complete review, illustrated with a copy of Say’s [29] original figure and images of several modern topotypic shells, see my essay of [18Sept24].

Okay, twelve down.  Many of you, I feel sure, will have heard that saccharine story about the young girl who finds a million starfish washed up on the beach, and begins to toss them back, one at a time.  Along comes a man and asks her how she could possibly hope to save a million starfish.  And she flips another starfish into the sea and replies, “Well, I saved that one.”

So Dan Graf [10] catalogued over 1,000 pleurocerid nomina at the specific or subspecific level, washed up on the beach like starfish.  In this month’s essay we flipped three nomina back into the ocean (castanea, edgariana, laqueata), bagged nine others, and threw them into the dumpster.  Next month we’ll dispatch 14 more, one way or the other.

Notes:

[1] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occasional Papers of the Museum of Zoology, University of Michigan  417: 1-21.

[2] Plus a seventh set of “unknowns” and an eighth set he identified as “invasions” from the Alabama system.

[3] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[4] For a brief biographical sketch of Isaac Lea, and a review of his contribution to our modern understanding of freshwater gastropod evolutionary biology, see:

  • Isaac Lea Drives Me Nuts [5Nov19]

[5] Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings of the American Philosophical Society 2: 11 – 15.

[6] Lea, Isaac (1843) Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society (New Series)  8: 163 – 250.

[7] Lea, Isaac (1862) Description of a new genus (Goniobasis) of the Family Melanidae and eighty-two new species. Proceedings of the Academy of Natural Science of Philadelphia 19: 262 – 272.

[8] Lea, Isaac (1863) New Melanidae of the United States.  Journal of the Academy of Natural Sciences of Philadelphia (New Series) 5: 217 – 356.

[9] All 20 of these specimens are labeled “holotype” in the USNM collection.  Graf [10] considered most of them lectotypes, but I am not going to second-guess the USNM.

[10] Graf, D. L. (2001) The cleansing of the Augean stables.  Walkerana 12(27): 1 - 124.

[11] Rafinesque, C.S. (1831) Enumeration and account of some remarkable natural objects in the cabinet of Prof. Rafinesque, in Philadelphia.  Self-published, 4 pp.

[12] Dazo, B.C. (1965) The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae). Malacologia 3:1-80.

[13] Dillon, R. T., Jr., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127.  [html] [pdf]  For more, see:

  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Pleurocera canaliculata and the process of scientific discovery [18June13]

[14] Lea, I. (1831/34) Observations on the naiads, and descriptions of new species of that and other families.  Transactions of the American Philosophical Society (New Series) 4: 63 – 121.

[15] Scudder, N. P. (1885) Bibliographies of American naturalists – II. The published writings of Isaac Lea, LL.D.  Bulletin of the US National Museum 23: 1 – 278.

[16] Tryon, G. W. (1873) Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

[17] Dillon, R. T., Jr. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque, 1818). Malacologia 53: 265-277. [pdf]  For a review, see:

  • Goodbye Goniobasis, Farewell Elimia [23Mar11]

[18] Goodrich, C. (1930)  Goniobases of the vicinity of Muscle Shoals.  Occasional Papers of the Museum of Zoology, University of Michigan 209: 1 – 25.

[19] The TVA closed Wheeler Dam in 1936 and Pickwick Dam in 1938, creating a pair of reservoirs that covered the North Alabama shoals of the Tennessee River under 100 miles of slackwater and muck.  For my own personal lament, see the latter half of:

  • The Union in Tennessee!  [15Aug23]

[20] Anthony, J.G. (1854) Descriptions of new fluviatile shells of the genus Melania Lam., from the western states of North America.  Annals of the Lyceum of Natural History of New York 6: 80 -132.

[21] Dillon, R.T., Jr. (2023b) The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC. [publications]

[22] Lea, Isaac (1868) New Unionidae, Melanidae, etc., chiefly of the United States.  Journal of the Academy of Natural Sciences of Philadelphia (New Series) 6: 303 – 343.

[23] Haldeman. S. S. (1841) A monograph of the Limniades and other freshwater univalve shells of North America. Volume 2.

[24] Reeve, L. A. (1860) Conchologia Iconica, or, Illustrations of the shells of molluscous animals. Volume 12, Plate 56.

[25] The INHS collection does hold four historic records of “Elimia” costifera from a creek in Hardin County, bordering the Ohio River about 250 miles south of Hennepin.  The University of Michigan also holds one historic lot of Goniobasis costifera (UMMZ 241604) from Hardin County.  That is simply too far away from Hennepin to have any bearing on this question.

[26] Lea, Isaac (1866) New Unionidae, Melanidae, etc. chiefly of the United States.  Journal of the Academy of Natural Sciences of Philadelphia (New Series) 6: 113 – 187.

[27] Lea, Isaac (1863) Descriptions of fourteen new species of Melanidae and one Paludina.  Proceedings of the Academy of Natural Sciences of Philadelphia 15:154 – 156.  Lea apparently read his paper in May of 1863, and “May” is printed on the bottom of the published pages, but the front of the published volume says, “June and July, 1863.”

[28] Conrad, T. A. (1834) New Fresh Water Shells of the United States, with coloured illustrations, and a monograph of the genus Anculotus of Say; also A synopsis of the American naiades.  Philadelphia, Judah Dobson.  76 pp, 8 plates.

[29] Say, T. (1829) Descriptions of some new terrestrial and fluviatile shells of North America.  New Harmony Disseminator of Useful Knowledge 2(18): 275 – 277.

Tuesday, November 12, 2024

Reticulate Evolution in the North American Pleuroceridae

Last month [15Oct24] we reviewed the evidence that populations of two pleurocerids widespread in the Greater Ohio Drainage, P. laqueata and P. troostiana, hybridize extensively in the rivers and streams of Middle Tennessee.  And our most useful genetic marker was shell plication, a scallop-shaped ridging pattern characteristic of P. laqueata, absent from P. troostiana outside the laqueata range, but variably present in troostiana populations overlapping with laqueata.

Sharing most of those same rivers and streams with both laqueata and troostiana are populations of a third pleurocerid species, P. simplex, our old friend familiar from five previous essays, see footnote [1] below to refresh your memory.  The FWGNA Project recognizes two subspecies of simplex: the typical form found in small streams throughout the greater Tennessee/Cumberland region and a paler, more heavily shelled form common in larger streams of the Cumberland drainage, extending into Central Kentucky, Pleurocera simplex ebenum.

In 1934, Calvin Goodrich [2] published #3 in his “Studies on the gastropod family Pleuroceridae” series, focusing on shell plication.  Here is a verbatim quote from page 5:

G. ebenum (Lea), commonly a smooth species, occurs in the Cumberland River drainage basin. In the upper part of the drainage, material containing plicate shells has been taken. The only lot at hand that can be accepted as a “pure" race of these forms is from New River, Scott County, Tennessee. Of 46 shells from Straight Creek at Pineville, Bell County, Kentucky, 54.4 per cent are plicate. In the Cumberland River a few miles below Pineville, 18 per cent of 72 shells are so sculptured; 74 shells of ebenum taken just above the falls of the Cumberland are 14.8 per cent plicate. The only specimens from the river below the falls which have been seen, taken at Smith's Shoals near Burnside, Pulaski County, Kentucky, are all smooth; so also are shells of all lots of the species ranging as far to the west as streams of Dickson County, Tennessee.”

Yes, all of that is true.  I myself have confirmed at least seven populations of P. simplex ebenum bearing lightly plicate shells scattered about Middle Tennessee, in minor tributaries of the Cumberland, the Harpeth, the Red, and the Duck.  All these populations co-occur with populations of P. simplex bearing normal, smooth shells and populations of (you guessed it) P. laqueata.  The first three shells figured at left below were collected from the backs of pleurocerids inhabiting Brush Creek, a tributary of the Red-Cumberland in Robertson County, NW of Nashville (36.4342, -87.0662): an apparently pure P. simplex, an apparently pure P. laqueata, and what most certainly appears to be a simplex/laqueata hybrid (“castanea”), almost entirely smooth but bearing tiny plications around the apex.

Reticulate evolution in the Pleuroceridae

Exactly as is the case with P. troostiana, P simplex populations inhabiting East Tennessee, where P. laqueata does not occur, never bear plicate shells.  Only where the ranges of P. simplex and P. laqueata overlap in Middle Tennessee does one find pleurocerid populations bearing fat, pear-shaped simplex-looking shells with tiny apical plications.

There is not a shadow of doubt in my mind that P. simplex hybridizes with P. laqueata, just as P. laqueata hybridizes with P. troostiana.  The two shells at right were sampled from Spring Creek east of Nashville, carried over from last month: an apparently pure P. troostiana and a laqueata/troostiana hybrid (“perstriata.”)  This is reticulate evolution.

Digging back through the classic literature, it turns out that Isaac Lea described a Melania castanea in 1841, the shell of which appears to be a perfect match for the simplex/laqueata hybrid populations I have been referring to here.  Lea’s brief Latinate description appeared in that same early work that featured such notables as clavaeformis, ebenum, and edgariana [3], with a longer English description and figure following in 1843 [4].  Lea’s type locality, “Maury County, Tenn.” is in the upper Duck River drainage, where simplex and laqueata are both common.  Calvin Goodrich [5] lowered Lea’s nomen castanea to subspecific status under Goniobasis laqueata in 1940, giving its range as “Headwaters of the Duck River, Tennessee.”

Melania castanea [4]
OK, fine.  Given that we have recognized three subspecific names for laqueata/troostiana hybrids, I suppose it is only fair to recognize a subspecific name for hybrids between P. laqueata and P. simplex.  So, this week I have added a new (sub)species page to the FWGNA website for Pleurocera laqueata castanea (Lea 1841), with corresponding entries in the gallery and dichotomous key for the Tennessee/Cumberland [6].  This is the 135th species or subspecies of freshwater gastropod we have recognized as valid in our 21-state study region.

I am every bit as certain that P. simplex hybridizes with P. semicarinata in Kentucky and Tennessee, although I have no genetic data or photos to enter into evidence.  The two species are only distinguishable by subtle differences in shell shape, the former bearing fatter shells with a larger body whorl, neither demonstrating any sort of shell sculpture (beyond a carinate upper whorl) that might serve as a discrete marker.  The range of P. semicarinata semicarinata overlaps that of P. simplex broadly in the Cumberland, Green, and Kentucky Rivers, and extends much further north, up into Wisconsin, Michigan and New York, where chubby-shelled populations are referred to the subspecies P. semicarinata livescens.

And I am still amazed [7] by the 1994 allozyme study of Bianchi and colleagues [8] demonstrating hybridization between Great Lakes P. semicarinata livescens and the Hudson River population of Pleurocera virginica through the Erie Canal.  Those two species bear strikingly different shell morphologies, have entirely distinct ranges, and could not have shared a common ancestor in many, many millions of years.  Perhaps since the Appalachian Orogeny?
Hybridizing? [9]

Yes, that is my next point.  The architects of the Modern Synthesis generally seem to have considered hybrid zones an unstable and transitory step toward speciation [11].  I am sympathetic with the Darwinian rationale for such an hypothesis, and admit it could certainly hold in many cases.  But more recently the research emphasis seems to have shifted toward hybrid zones that give evidence of stability and permanence [12].

The photo below comes from the 8Mar24 issue of Science [13].  Here’s the caption: “This fish is the hybrid offspring of an alligator gar and a spotted gar – members of genera that last shared a common ancestor at least 100 million years ago.”

The paper reviewed, by Brownstein and colleagues [14], detailed the results of a survey of 1,105 exons over 481 vertebrate species, demonstrating exceptionally slow rates of molecular evolution in gars and sturgeons.  Yet gar species last sharing a common ancestor no later than the Cretaceous still hybridize naturally in the greater Ohio and southern Mississippi drainages today.

Could some cranky, washed-up old crackpot wading those same rivers and streams, throwing snails into a bucket and measuring them with rusty calipers, achieve the same results as an international team of eight scientists from six different institutions with “massive” DNA data sets and ten different sources of funding?

The distribution of pleurocerid snails in the rivers and streams of North America is whispering a story to us in a language that we do not understand.  It is an ancient story of colliding continents and earthquakes and mountains 10,000 feet high, eroding and shifting and washing into the sea.  Most of the pleurocerids of the Greater Ohio drainage, including P. simplex and P. troostiana, range across the entirety of the state of Tennessee, as well as into Kentucky and North Alabama and even into SW Virginia.  Then why are populations of P. laqueata absent East of Chattanooga?  Is their dispersal capability so much poorer than P. simplex and P. troostiana that they are unable to penetrate Walden’s Ridge?  I simply do not think so.  Here is the story that I hear the pleurocerids whispering to me.

The story I hear is that the crest of the ancient Appalachians, at some point in the millions of years of their orogeny, was approximately where Walden’s Ridge lies today, at the eastern edge of the Cumberland Plateau.  Pleurocera laqueata evolved on the west side of that crest, while P. troostiana and P. simplex evolved on the East.  Then the mountains eroded such that the divide shifted east, opening a hole at Chattanooga, switching the flow of the rivers in which troostiana and simplex evolved from east to west, bringing those pleurocerid populations into secondary contact with laqueata.

I have said it many times [15], but I will say it again.  A step off the creek bank in the Southern Appalachians is a step back millions of years.  Look around you, colleagues, look!  Those banks are covered with mosses and liverworts, horsetails and ferns.  The waters team with dragonflies and stoneflies, gars and hellbenders.  And pleurocerid snails jostle each other to graze across every square inch of substrate.

Why does this entire ecosystem seem frozen in time?  My hypothesis calls on three independent sets of factors: environmental, genetic and historical.

First, the freshwater environment is more stable than that of the land.  Water temperatures lag behind and buffer air temperatures.  That buffer is not just seasonal, it is climatological.  The temperature in smaller streams, in particular, typically remains very close to that of the ground, 10 – 15 degrees Centigrade year round.  Such environments are not simply protected from hot Julys, they are protected from ice ages.  And the lower the temperature of the environment, I might add, the slower the generation times of its poikilothermic biota.

Rock Island State Park, TN [16]

Rainfall and storm are similarly buffered.  Droughts obviously have less effect on rivers than on the surrounding land, ditto wind and fire.  The ecosystems of many (especially smaller) bodies of water are based on allochthonous input, rather than primary productivity, and life could more easily survive (let us say) a cometary impact, and a period of worldwide darkness.

Most of the above, it must be admitted, could also be said for the marine environment as well as the freshwater.  This calls upon a second set of factors, which are population genetic.

In two words, marine populations are gigantic and panmictic.  Almost all the mollusks, for example, retain a planktonic larval stage lasting at least a couple weeks, facilitating dispersal over very long distances.  Here on the Atlantic side, the population of commercially important eastern quahogs (“cherrystone” or “littleneck” clams), demonstrates no significant allelic frequency differences at multiple allozyme-encoding loci from Canada to Florida [17].  Ditto oysters, ditto oyster drills, ditto whelks, ditto periwinkles [18].

Consequently, when a beneficial mutation arises in a marine population, it spreads quickly in evolutionary time.  Diseases, predators, and other riders of the apocalypse spread as quickly as the angels.  Speciation is quick, extinction is quick, evolution is quick.  The marine molluscan fauna of the Virginia Pliocene does not look like the marine molluscan fauna of the Virginia Recent.

But for better or worse, freshwater populations are small and fragmented.  Evolution does not stop, of course; the molecular clock keeps ticking [19].  But when adaptations evolve (such as reproductive isolation, for example) they do not spread [20, 21].  The outward appearances of such populations, then, will give the impression of morphological stasis.

So, freshwaters are more environmentally stable than the land, and the populations inhabiting those freshwaters more genetically stable than those inhabiting the sea.  There is a third factor.  History.

The land mass that we today identify as the “Appalachians,” together with the freshwaters that drain those mountains to the ocean, is really, really old.  It is clear that several orogenies have taken place, beginning with the Grenville over one billion years ago, proceeding through the Taconic (500 mybp) and the Acadian (400 mybp), culminating with the Alleghanian Orogeny at the formation of Pangaea 300 mypb.

Did Cerithiacean gastropods crawl from the sea at that time, evolve into the first pleurocerids, disperse and diverge across drainage systems as they existed in the ancient Appalachians hundreds of millions of years ago, and then sit in evolutionary stasis as the mountains wore down around them?  Yes, I think so.

Next month… taxonomic implications.

Notes:

[1] See the following essays for a review of the biology of Pleurocera simplex, its sibling gabbiana and its subspecies ebenum:

  • The cryptic Pleurocera of Maryville [13Sept16]
  • The fat simplex of Maryville matches type [14Oct16]
  • CPP Diary: Yankees at The Gap [4Aug19]
  • CPP Diary: What is Pleurocera ebenum? [3Oct19]
  • CPP Diary: The spurious Lithasia of Caney Fork [4Sept19]

[2] Goodrich, C. (1934)  Studies of the gastropod family Pleuroceridae – III.  Occasional Papers of the Museum of Zoology, University of Michigan 300: 1 – 11.

[3] Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings of the American Philosophical Society 2: 11 – 15.

[4] Lea, Isaac (1843)  Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society 8: 163 – 250.

[5] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occasional Papers of the Museum of Zoology, University of Michigan  417: 1-21.

[6] Alas, Pleurocera laqueata castanea cannot be retroactively included in the hardcopy FWGNA Volume 5, which came off the presses in the fall of 2023.  In our next edition, however, castanea will be FWGNA species Number 103.2.

[7] See my essay of 3Mar22 for rankings of a broad selection of freshwater gastropod papers by international amazingness units. The paper of Bianchi et al [8] scored a whopping 93.2 iau, good for first place in the population genetics subdivision:

  • The third-most amazing research results ever published for the genetics of a freshwater gastropod population. And the fourth-most amazing, too. [3Mar22]

[8] Bianchi, T. S., G. M. Davis, and D. Strayer 1994.  An apparent hybrid zone between freshwater gastropod species Elimia livescens and E. virginica (Gastropoda: Pleuroceridae).  Am. Malac. Bull. 11: 73 – 78.

[9] From left to right.  Pleurocera simplex simplex from Brush Creek, Robertson Co, TN (see above).  Pleurocera simplex ebenum from the Falls of The Cumberland, Whitley Co, KY [see 3Oct19].  Pleurocera semicarinata semicarinata from Harrison Ck, Nelson Co, KY [see 6Sept17]. Pleurocera semicarinata livescens from Portage Ck, Washtenaw Co, MI [10]. Pleurocera virginica, an especially chubby shell from Deer Ck, Harford Co, MD courtesy R. Aguliar.

[10] “Station 2” of Dazo, B. C. (1965)  The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae).  Malacologia 3: 1 – 80.

[11] Dobzhansky, T. (1940) Speciation as a stage in evolutionary divergence. American Naturalist 74: 312 – 321.

[12] Barton, N.H. and G.M. Hewitt (1985) Analysis of hybrid zones.  Annual Review of Ecology and Systematics 16: 113-148.

[13] Heidt, A. (2024) Gars truly are “living fossils,” massive DNA data set shows.  Science 383 (6687): 1041.

[14] Brownstein, Chase B, Daniel J MacGuigan, Daemin Kim, Oliver Orr, Liandong Yang, Solomon R David, Brian Kreiser, and Thomas J Near (2024) The genomic signatures of evolutionary stasis.  Evolution 78: 821 – 834. https://doi.org/10.1093/evolut/qpae028

[15] Dillon, R T. and J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11.  (Rosemary Mackay Award)  [pdf].  For a review, see:

  • The snails the dinosaurs saw [16Mar09]

[16] The Caney/Collins River system, impounded below Rock Island State Park, was home to at least eight species of pleurocerid snails, including P. simplex [4Sept19], P. troostiana edgariana [5June20] and the pleurocerid megafauna hung in Cousin Bob Winter’s prehistoric necklace as depicted [5Apr22].

[17] The population genetic literature on Atlantic coastal bivalves is very large.  For a review of the Mercenaria case, see:

  • Dillon, R.T. and J.J. Manzi (1992) Population genetics of the hard clam, Mercenaria mercenaria, at the northern limit of its range.  Canadian Journal of Fisheries and Aquatic Sciences 49:2574-2578. [pdf]

[18] For reviews of the genetics of marine gastropod populations on the Atlantic coast, see:

  • Wise, J., M. G. Harasewych, and R. T. Dillon. (2004)  Population divergence in the sinistral Busycon whelks of North America, with special reference to the east Florida ecotone.  Marine Biology 145:1167-1179. [pdf]
  • Dayan, N.S., and R.T. Dillon (1995) Florida as a biogeographic boundary: Evidence from the population genetics of Littorina irrorata. The Nautilus 108: 49-54. [pdf]

[19] An inexorable (but not especially clocklike) accumulation of neutral mutations yields the startlingly high levels of mtDNA sequence divergence often recorded among pleurocerid populations.  And the crazy distribution patterns of those crazy mtDNA sequence markers come from rare long-distance dispersal events which, given hundreds of millions of years of birds wading through these streams and flying off elsewhere, do happen.  For more about my Jetlagged Wildebeest Model of mitochondrial superheterogeneity, see:

  • Mitochondrial superheterogeneity: What we know [15Mar16]
  • Mitochondrial superheterogeneity: What it means [6Apr16]
  • Mitochondrial superheterogeneity and speciation [3May16]

[20] The absence of any correlation between genetic divergence and environmental difference in isolated populations of Pleurocera proxima, together with strong correlations between genetic divergence and geographic distance, supports this hypothesis.  See:

  • Dillon, R.T. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82. [pdf]

[21] Evidence from Pleurocera proxima transplant experiments is also consistent with the hypothesis that beneficial genomes may be prevented from spread by the isolated character of southern Appalachian streams.  See:

  • Dillon, R.T. (1988) Evolution from transplants between genetically distinct populations of freshwater snails. Genetica 76: 111-119. [pdf]