Mussel of the

Page last updated
28 May 2019

Mussel of the Month

The 2019 Mussels of the Month, so far...

May 2019

PletholophusPletholophus tenuis (Unionidae, Indotropical)

The May 2019 Mussel of the Month is Pletholophus tenuis. Pletholophus is a monotypic genus widespread in southeastern Asia.

We have recently added the genus Pletholophus to the MUSSELp Database as a valid taxon. Pletholophus is not a new genus — it was described more than a century ago. Simpson (1900, 1914) split Cristaria into three subgenera:Cristaria, Pletholophus, and Crassitesta. Cristaria sensu stricto for things like C. plicata that are large, alate (i.e., winged), somewhat thick, with weak hinge teeth. Pletholophus was imagined for P. discoidea and other shells that were thinner, “scarcely” alate, and with hinge teeth that are “very feeble, often nearly wanting.” Crassitesta was just for C. radiata that was more solid, also “scarcely winged,” brightly rayed on the exterior, with hinge teeth “reduced to the merest vestiges.” Apparently, the differences (or the lack thereof) in soft-anatomy had nothing to do with all this.

Haas (1969) basically followed this classification, although he synonymized Simpson’s dozen or so species down to three —one in each subgenus. Since every genus could be uninformatively represented by as many monotypic subgenera as there are constituent species, little insight was gained from the revision. Perhaps beyond the lumping, Haas (1969) didn’t really give the subgenera that much thought.

That was basically the taxonomy we inherited entering the 21st century. Some authors had recognized Pletholophus as a full genus (e.g., Modell, 1945, Dang et al., 1980), but that didn’t seem to catch on at the time. A couple new species were also described. In our global checklist, we listed four species, all in the genus Cristaria (Graf & Cummings, 2007).

Since then, there have been some progress — or, at least changes. First, Petit & Coan (2008) delved deeply into Griffith & Pidgeon’s update of Cuvier's "Le règne animal..." . Until then, we had all been toiling under the misconception that G. & P. had published their work in 1834. Nope — it was 1833. How is that possibly relevant? Cristaria discoidea (Lea, 1834) (the type species of Pletholophus) was now a junior synonym of Cristaria tenuis (Gray in Griffith & Pidgeon, 1833). After more than a century, C. discoidea became C. tenuis.

Then, a couple years ago, two different studies included both C. plicata and C. tenuis in phylogenetic analyses that found C. tenuis shared a more recent common ancestor with Sinanodonta woodiana than C. plicata (Lopes-Lima et al., 2017; Sano et al., 2017). That is, Cristaria was not monophyletic. Those results should not be too shocking. It happens all the time.*

The challenge for any systematist in that situation is how to handle it, especially for a post-hoc interpretation of what could be an artifact of the particular taxa and characters that were being analyzed. In the case of Lopes-Lima et al. (2017), they simply applied the next available genus name: Pletholophus tenuis. Bogatov & Prozorova (2017), on the other hand, recognized the species as a Sinanodonta based on their assessment of the figure of the type of Symphynota discoidea Lea, 1834 in He & Zhang (2013).** That classification is also consistent with the phylogenetic results and avoids our general beef with monotypic genera (e.g., Arkansia).

The trend in the literature (specifically those papers that use the same Genbank sequences as Lopes-Lima et al., 2017), has been to use the name Pletholophus tenuis, and the MUSSELpdb has jumped on the band-wagon.

* Raise your hand if you have seen a phylogenetic genetic tree with a topology that implied a different evolutionary history than another tree from a different analysis.
** Bogatov & Prozorova (2017) neglected to list P/.”S.” tenuis as a valid species in Sinanodonta. The figure in He & Zhang (2013) was reproduced from the MUSSEL Project Web Site.

April 2019

PachynaiasPachynaias spheniopsis (Unionidae, Neotropical)

The April 2019 Mussel of the Month is Pachynaias spheniopsis. Pachynaias is a genus of only two species endemic to Central America.

Pachynaias spheniopsis is from Central America, and if you are a frequent reader of the Mussel of the Month, you know it is our longstanding beef that we don’t have much to say about those mussels (e.g., Nephronaias, Martensnaias, Sphenonaias, Micronaias, Arotonaias, Barynaias, Friersonia). Those genera apparently exist because there are shells in drawers labeled as species assigned to them (Frierson, 1927; Haas, 1969). That streak of nearly total ignorance ends this month! Pfeiffer et al. (2019) have published the first phylogeny with multiple Central American mussel genera, and they have advanced our knowledge quite a bit. The two results we will focus on here are that 1) Pachynaias is a Central American lampsiline and 2) Psorula and Psoronaias are molecularly indistinguishable.

That Pachynaias is classified under the Tribe LAMPSILINI is not a tremendous mind-blow. It has been arranged there for about as long as we have known there was such a thing as lampsilines (e.g., Haas, 1929; Thiele, 1934). And, based on brooding morphology, we have known that the Lampsilini is represented in Central America (e.g., Actinonaias, Disconaias). Pfeiffer et al. (2019) used DNA sequence analyses to show that, in fact, there were multiple lampsiline lineages south of the Rio Grande.

In addition to Pachynaias and other Central American lampsilines, Pfeiffer et al. (2019) included one species of Psoronaias and multiple species of Psorula from that part of the Neotropics, spanning the range of conchological variation found in those taxa. What they found is that all that morphological variation among those mussels is unrelated to nucleotide variation — it was all just one shallow clade looking more like a population than a genus of multiple species. As such, we have collapsed those two genera on our list, with Psoronaias Fischer & Crosse, 1894 having priority. The formal lumping of species should come with additional study. Moreover, the Psoronaias clade grouped with Popenaias in a lineage distinct from the other amblemine tribes we know from North America. For this clade, they proposed resurrecting the name POPENAIADINI Heard & Guckert, 1970.

These results were part of one chapter in John Pfeiffer’s Ph.D. dissertation. Last month, he successfully defended the work he has scienced at the University of Florida since finishing his M.Sc. at the University of Alabama. Congratulations John on a job well done! It is nice to have stories of a “Florida man” that don’t end in wacky misfortune.

March 2019

UtterbackiaUtterbackia peggyae (Unionidae, Nearctic)

The March 2019 Mussel of the Month is Utterbackia peggyae. Utterbackia is a genus of three species from eastern North America.

Utterbackia imbecillis was the Mussel of the Month almost five years ago. There are still lots of genera we haven’t even gotten to yet, but we wanted to honor our friend Richard Johnson this month with a freshwater mussel that he named, so we chose U. peggyae. Richard has a birthday coming up, and we were recently reminded that he is probably the longest surviving member of the American Malacological Society/Union (Mikkelsen, 2010). Richard Johnson attended his first AMU meeting in his teens before World War II (Boss, 1998)!

During Richard Johnson’s career in the Mollusk Department at Harvard’s Museum of Comparative Zoology, he contributed to our knowledge of freshwater mussel systematics in important and lasting ways. He published type catalogues for many of the most important collections (e.g., Lamarck, Lea, MCZ, UMMZ, ANSP) and biographies of significant malacologists (e.g., Ortmann, Frierson, Call). He took on Elliptio and other genera on the Atlantic Slope, post-Pleistocene biogeography (Johnson, 1980), and a revision of Epioblasma. Richard also described new species, and he did so with memorable flair. There are the three he named for the women in his life, Haasodonta fannyae, Utterbackia peggyae, and Margaritifera marrianae. That is probably the most romantic thing we have ever heard of. He also named three more by mildly Latinizing concatenations of the names of his contemporary unio aficionados. Samuel L.H. Fuller, Pieter W. Kat, Carol B. Stein, David H. Stansbery, Artie L. Metcalf, Raymond W. Neck, and Dwight W. Taylor have been immortalized as Lampsilis fullerkati, Parvaspina steinstansana, and Potamilus metnecktayi. It should be remembered that much of this work was done before freshwater mussels attracted the attention they do today. Back in the day, Unionidae was just another obscure family of bivalves.

We also know Richard Johnson as a generous friend and mentor, serving on DLG’s M.Sc. thesis committee. Thank you, Mr. Johnson, for your contributions to freshwater malacology!

February 2019

MonodontinaMonodontina vondembuschiana (Unionidae, Indotropical)

The February 2019 Mussel of the Month is Monodontina vondembuschiana. Monodontina is a genus of four species from southeastern Asia, from Indochina to the Sunda Islands.

Monodontina vondembuschiana is the type species of the recently recognized genus, Monodontina. M. vondembuschiana has traditionally been classified as a Pseudodon (Zieritz et al., 2018), but that genus has been blown-up by the phylogenetic work of Bolotov et al. (2017).

In our global checklist (Graf & Cummings, 2007), we divided the almost two dozen species of Pseudodon among nine subgenera. There were only a handful genera that caused us to worry about subgenera back then — Alasmidonta, Arcidens, Lasmigona, Parreysia, Pseudodon, Prisodon, and Hyridella. These were generally taxa with many species and either some suggestion that they might be excessively lumped or just in need of clarification how the taxonomy we followed corresponded to other classifications. Some of those lumpings have been rectified, such as Prisodon sensu latu (= Prisodon sensu stricto and Triplodon), Parreysia s.l. (= Parreysia s.s., Leoparreysia, Indonaia, and Radiatula), and Pseudodon s.l.

The former species of Pseudodon s.l. are now split among Pseudodon s.s., Monodontina, and Bineurus based on the non-monophyly of the old genus (Bolotov et al., 2017). Either Pseudodon needed to be broken up or the species of Pilsbryoconcha had to be folded in. So, where there were two genera, we now have four. The phylogenetic relationships among the other six subgenera of Pseudodon have yet to be fully explored.

January 2019

GibbosulaGibbosula rochechouartii (Margaritiferidae, Indotropical)

The January 2019 Mussel of the Month is Gibbosula rochechouartii. Gibbosula is a genus of six species in southeast Asia, from Myanmar to China.

Gibbosula rochechouartii is a new genus for this freshwater mussel species. If you have been keeping track like we have, then a mussel species being reclassified from one genus to another won't blow your mind. Based on the data in the MUSSELpdb, each valid freshwater mussel species has, on average, been classified in more than 4 different genera (4.46 to be precise). Among the "champions"* in this regard is Ortmanniana ligamentina. Over the last couple hundred years, that species and its synonyms have been classified under 14 different genera (e.g., Unio, Obliquaria, Lampsilis, Nephronaias, Actinonaias, Ellipsaria, Venustaconcha, Ligumia) — and that number goes even higher if we consider subgenera.

The classification of G. rochechouartii had been pretty straight forward until very recently. It was originally described in Unio (like most freshwater mussels of the 19th century). Simpson (1914) classified it as Quadrula (Lamprotula), and then Haas (1969) moved it to Lamprotula.** And there it stayed for decades. "L." rochechouartii held its position even after Lamprotula was up-ended and split to move 7 species to Aculamprotula in a different subfamily because not all bumpy, thick-shelled Asian freshwater mussels necessarily belong to the same genus (Zhou et al., 2007; Pfeiffer et al., 2013).

But then last year, some fresh phylogenetic work by Huang et al. (2018) recovered our Mussel of the Month in the family Margaritiferidae, rather than Unionidae where it and the rest of Lamprotula (and Aculamprotula) had always been. Recent phylogenetic work had confirmed that all the Recent margaritiferids can be classified as Margaritifera (Graf & Cummings, 2007; Araujo et al., 2016; Bolotov et al., 2016), so our mussel became Margaritifera rochechouartii.

And then again last year, a more comprehensive analysis by Lopes-Lima, Bolotov et al. (2018) examined a wider array of taxa and data, and the classification of margaritiferids changed course. Not only did they advocate splitting the Recent Margaritiferidae back into multiple genera (as had been done by Smith, 2001 and almost everyone before), but we even have subfamilies now. "M." rochechouartii became Gibbosula rochechouartii in the subfamily Gibbosulinae. G. rochechouartii sat for more than 100 years in the genus (or subgenus) Lamprotula in the family Unionidae, and then WHAM, BAM, thank-you CLAM — two more different genera and a change in family.

Fortunately, the MUSSELpdb is available to help you keep score with freshwater bivalve taxonomy! As we have discussed in various posts over the last few months, freshwater mussel taxonomy has been changing at an almost unprecedented pace. This can be attributed to:

  • the increase in the availability of molecular sequences,
  • the increase in the number of people working on these questions, and
  • a greater willingness to make post hoc revisions based on phylogenetic tree topologies — every node on a cladogram apparently gets a name now.

The table summarizes the history of the taxonomic shifts described in this post.

* If confusion = winning.
** The MUSSELpdb currently holds no references to this species dating between 1914 and 1969. As we have recently noted, those years were slow ones for mussel taxonomy. There may be data out there, but we haven't captured it yet.

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