Mussel of the

Page last updated
2 September 2020

Mussel of the Month

The 2020 Mussels of the Month, so far ...

September 2020

DiplodonDiplodon paranensis (Hyriidae, South America)

The September 2020 Mussel of the Month is Diplodon paranensis. Diplodon is a genus of 43 species from South America.

Diplodon, as currently classified, is the sole Recent genus of the Tribe Rhipididontini.* It is by far the most species-rich freshwater mussel genus, with 43 species.*** We have covered Diplodon as Mussel of the Month twice before: D. chilensis and D. parallelopipedon.

The classification of Diplodon is a mess, and different authorities have toyed with splitting the species among two subgenera or genera: Diplodon and Rhipidodonta. Simone (2006) split them at the full-on genus level, whereas Pereira et al. (2014) opted for subgenera. Back in 2007, we followed the former (Graf & Cummings, 2007), and now the MUSSELpdb lines up with the latter (i.e., everything is classified under Diplodon). Back in the day, Parodiz (1968) and Haas (1969) published revisions but without much consensus. There is no phylogeny of Diplodon. In fact, no one has included more than three Diplodon species at the same time in a phylogenetic analysis (Santos-Neto et al., 2016).****

Diplodon paranensis is the type species of Rhipidodonta. So, while today, it represents a tripling-up on Diplodon as Mussel of the Month, someday, when the winds of taxonomy change, this MotM might be reclassified.

* You might think Diplodontini Ihering, 1901 would be the valid name for the tribe, as Parodiz & Bonetto (1963) did, but that name is preoccupied by Diplodontidae Carpenter, 1861 (= Ungulinidae Gray, 1854). Although Diplodon and Diplodonta are different genera, their Latin root conjugates** into the same family-group name.

** I don’t care if that isn’t the correct verb.

*** The top 5 most species-rich genera are rounded out by Elliptio (30 spp.), Epioblasma (28), Lampsilis (26), and Anodontites (26). Pleurobema, Coelatura, and Nephronaias also have more than 20 species each.

**** And their results made no sense.

August 2020

PopenaiasPopenaias berezai (Unionidae, Central America)

The August 2020 Mussel of the Month is Popenaias berezai. Popenaias is a genus of four or five species from Central America, with a single species extending north to the USA.

The last time we looked at Popenaias was 13 years ago this month, but things are changing!

Back in when we made our global checklist (Graf & Cummings, 2007), Popenaias was composed of 3 species: P. popeii, P. metallica, and P. tehuantepecensis. Our interest in the genus Popenaias in North America derived from a recent proposal by the U.S. Fish & Wildlife Service to list the Texas Hornshell, Popenaias popeii, as a federally endangered species. What was unknown at the time of the proposed listing was the taxonomic status of those species referred to as P. popeii in the Rio Pánuco Basin, a Gulf coastal drainage in central México.

KSC and this collaborators conducted fieldwork in the Pánuco basin in México 2017 and 2018, and genetic and morphological analysis revealed that the species previously thought to be P. popeii in central Mexico was a new species, P. bereazi. We also discovered that another species of Popenaias exists in the Pánuco Basin: P. semirasa, which is endemic to the Rio Gallinas above Tamul Falls. P. popeii is now recognized as a Rio Grande endemic and since its status is imperiled, it was formally listed as Endangered by the U.S. Fish & Wildlife Service in late 2018.

“Popenaias” tehuantepecensis requires further scrutiny. It was described in the genus Unio and has bounced around from Sphenonaias and Nephronaias to Popenaias. There are only seven lots of specimens of “Popenaias” tehuantepecensis that we know of — all from Pacific coast drainages in Central America. Given its disjunct distribution and the limited number of specimens in museum collections, the taxonomic validity (including its placement in Popenaias) is uncertain. An examination of the syntypes and other specimens from the Pacifc coast of Central America strongly suggest it is synonymous with Nephronaias rowellii.

July 2020

TrapezoideusTrapezoideus foliaceus (Unionidae, East Asia)

The July 2020 Mussel of the Month is Trapezoideus foliaceus. Trapezoideus is a genus of 2 species found in Myanmar and western Thailand.

The genus Trapezoideus has taxonomically exploded over the last decade. Back in when we made our global checklist (Graf & Cummings, 2007), Trapezoideus was composed of 2 (maybe 4) species. The principle species was the widespread Trapezoideus exolescens. Back in December 2012, T. exolescens was the Mussel of the Month. Then Bolotov et al. (2017) found that at least two species comprised T. exolescens sensu lato, precipitating the first round of revisions. The name-sake of the species, Unio exolescens Gould 1843, was recovered in a clade with Lamellidens, whereas the type species of Trapezoideus was found to share a more recent common ancestor with Contradens. The type species, Unio foliaceus Gould, 1843, had been regarded as a junior synonym (Subba Rao, 1989). With T. exolescens escaping to a new genus (Trapezidens), T. foliaceus has been bumped to seniority.

Our previous selection of T. exolescens as Mussel of Month is slightly more complicated. We had used the type specimen of Unio siamensis Lea 1866, which was considered a junior synonym (Subba Rao, 1989). But additional splitting has moved U. siamensis under Contradens misellus. As things currently stand, Trapezoideus exolescens sensu lato has been split into nine species in five genera divided among three tribes, including T. foliaceus (Bolotov et al., 2020).

June 2020

BeringianaBeringiana beringiana (Unionidae, East Asia & North America)

The June 2020 Mussel of the Month is Beringiana beringiana. Beringiana is a genus of four species that occurs from eastern Russia to western Canada and USA.

We will get to Beringiana beringiana at the end of this post, but to start, we need to take a step back to look at the whole world.

With the latest influx of new freshwater mussel species in our tally (Lopes-Lima et al. 2020, Bolotov et al. 2020), the balance of species richness has shifted. For a long time, it has been taken for granted that North America is the freshwater mussel diversity hotspot of the world. Our checklist (Graf & Cummings, 2007 and this web site) provided a reference to go with that view. Before that time, we Americans could boast various tallies of our own species (Burch, 1975; Turgeon et al., 1988., 1998; Williams et al., 1993) but to get numbers for the rest of the world, few people had a access to Haas (1969) and Simpson (1914) was grossly outdated. In 2007, we calculated that the 302 North American species represented 36% of the global fauna of 840 spp. — more than any other region.

Nowadays, we still count 302 species in North America*, but East Asia has ticked up to 320 — an increase of 78 species in 13 years! Much of this has to do with the rapid rate of discovery and revision of species in that region over the last few years (a topic we have frequently noted — as recently as last month). Since 2007 East Asia has netted 32 species just through revision (55 elevated from synonymy, 23 sent back down to invalid roster) and 46 species described as new to science. That is 75% of all new species described during that time period!

Another important factor in this shift in regional freshwater mussel species diversity is our revision of biogeographical boundaries. For our previous tallies, we used the traditional physiographic divisions for Eurasia, dividing the continent(s) into Palearctic and Indotropical regions. To better reflect actual areas of freshwater mussel generic endemism, our current tallies divide that part of the Old World into North Eurasia and East Asia. East Asia accounts for the old Indotropics plus the most species-rich parts of the former eastern Palearctic like the Amur Basin and Japan. If we only consider the species flux in the old Indotropics (from the Yangtze through Indochina to India, including the Sunda and Philippine Islands), the new regional total would be 281 species. That represents a gain of 62 species since 2007. So, if we used the old biogeographic regions — that don’t reflect freshwater mussel biogeography as well — the Nearctic region (i.e., North America) would still have the most species.

For those that Americans feel as though we have lost some bragging rights about our freshwater mussel diversity, please consider the following. First, that’s a weird thing to brag about. Second, you might consider that our regional divisions are person-made imaginary boundaries** and the traditional regions are “better.” But, America can rest easy that, in terms of areas of endemism and species richness, the North American Mississippi-Great Lakes and Gulf-Florida subregions each harbor more total and endemic species than any of the Yangtze-Huang, Indochina (including the Mekong), or India-Myanmar subregions that are the richest in East Asia. During this messed up June of 2020, America can at least cling to that.

We would rather look to freshwater mussels as beacons of unity rather than division, and in that light, we have selected Beringiana beringiana as the Mussel of the Month. Only a handful of mussel species have native ranges that extend across regional boundaries, and B. beringiana is one of them, occurring in both East Asia and North America — it gets counted in both tallies!

May 2020

LeoparreysiaLeoparreysia bhamoensis (Unionidae, East Asia)

The May 2020 Mussel of the Month is Leoparreysia bhamoensis. Leoparreysia is a genus of a dozen species from Myanmar.

Leoparreysia is another new genus described or resurrected after the explosion of new research in eastern Asia initiated by Lopes-Lima et al. (2017) and Bolotov et al. (2017). We have used the Mussel of the Month for years now to talk about the proliferation of new taxa. For example, there is a helpful chart in our discussion of Leaunio lienosus. Last Decemeber, we made a list of new genus-group level taxa, but it is already out of date! For 2020 so far, we have gained 18 new species and 5 genera, not to mention all the taxa that have been resurrected from synonymy (Bolotov et al., 2020a, b; Konopleva et al., 2020; Lopes-Lima et al., 2020)

This page was created early in anticpation of being blocked from working for an extended period due to the 2020 Coronavirus pandemic. Rather than missing a monthly entry for the first time in nearly 17 years, we posted this one early. However, since this page was originally drafted in March, we have gained 19 new species and 5 genera! Check out the MUSSELp Database to see the latest additions.

April 2020

PisidiumPisidium amnicum (Sphaeriidae, Widespread)

The April 2020 Mussel of the Month is Pisidium amnicum. Pisidium is a globally distributed freshwater bivalve genus of more than 50 valid species.

This isn’t the first time that we have chosen a non-mussel as the freshwater mussel of the month. As recently as last February, we selected Monodacna colorata for Valentines Day, and for April Fools 2013, it was Corbicula fluminea. The selection of a sphaeriid clam (= fingernail or pea clam) this month is to highlight (once again) how much information we have on this site about freshwater bivalves generally —although our bias admittedly leans heavily toward the Unionoida.

This month, we want to call attention to a bit of data entry we have recently completed. Thanks to the generosity of our friend, Yuri Kantor, we have recently captured the bivalve data in Vinarski & Kantor (2016). That means, we have captured their combinations of genera and species, and those data are served in the Mussel Project Database. This includes freshwater mussels, the Sphaeriidae, and the other freshwater bivalve taxa found in the former Soviet Union. We have written quite a bit about Russian bivalve taxonomy over the years, most recently in the context of Anodonta anatina.

In the MUSSELpdb, you can access those taxonomic opinions at the level individual species. For example, in the synonym of Pisidium amnicum, at the very top is Tellina amnica Müller, 1774. That is the original combination of the nominal species. Our data suggest that the species has been regarded as valid since it was described (true for the minority of freshwater bivalves). Each of those opinions comes with a link to relevant references — e.g., Vinarski & Kantor (2016).

If you click the link to that publication (or any publication), you can get a list of genera and/or species introduced therein, as well as a list of species regarded as valid and their synonyms.

As of the instant of typing this sentence, we have 76084 taxonomic opinions about species from 1400 different works. Some entries are more complete that others, but we hope that these data provide an entree to the freshwater bivalve literature.

March 2020

IndonaiaIndonaia caerulea (Unionidae, East Asia)

The March 2020 Mussel of the Month is Indonaia caerulea. Indonaia is a genus of 14 species found from India east to Thailand.

Back in 2007, when we published our first global checklist of freshwater mussels (Graf & Cummings, 2007), we didn’t treat the genus name Indonaia as valid. Instead, we followed the going taxonomy of Subba Rao (1989), Ramakrishna & Dey (2007) and others to lump those species under Parreysia (Radiatula). Then (as now) we were not fans of subgenera, but the concept of the genus Parreysia at the time was unwieldy, and we made an exception to acknowledge that other contemporary classifications recognized Radiatula as a distinct genus (e.g., Nesemann et al., 2007). Nowadays, with the availability of molecular phylogenies, the tendency has been to recognize the few species of Indonaia that have been analyzed as a clade distinct from the rest of Radiatula (Bolotov et al., 2017; Konopleva et al., 2019). And so, here we are with Inodonaia caerulea, the type species of the genus.

Among the difficulties of studying freshwater mussel systematics is making sense of the history of name changes and re-classifications over the decades and centuries. The rules of the ICZN require us to care about priority and first revisers, and as such the past is our business. Fortunately, the MUSSEL Project Database is available to assist in those endeavors, and we are pleased to announce a new feature of available on this website. For each and every freshwater bivalve name (valid or not), we provide the history of citations we have captured from the literature.

For example, on the specimens page for the valid species Indonaia caerulea, we not only provide a map of specimen records but also the taxonomic history since 2007. That history documents the various names and combinations used since our 2007 checklist for this species and all of its synonyms, and links are provided to the individual references.

By clicking the link to the nominal species, you can find the synonymy for Indonaia caerulea. Typically, the nomina in a synonymy are listed strictly chronologically. On our site, we list the names by how recently they were regarded as valid. This moves the most interesting names to the top of what can be a long list (e.g., the synonymy of Anodonta anatina). For each name we provide a complete cresonymy — the history of name usage for that nomen.

So there it is. For any freshwater bivalve species on our web site, you can see not only how we classify it and circumscribe the often-numerous synonyms. And, you get the taxonomy that is the basis for our usage as well as potential alternative classifications. We don’t know of another mollusk database or any source since Simpson (1900) that provides such comprehensive taxonomic data on freshwater mussels!

February 2020

MonodacnaMonodacna colorata (Cardiidae, North Eurasia)

The February 2020 Mussel of the Month is Monodacna colorata. Monodacna (Family Cardiidae) is not actually a mussel taxon. It is a genus of mostly brackish-water species found in the Caspian and Black Seas with only one or two species living in freshwater.

This month, especially for Valentine’s Day, we are featuring Monodacna colorata, a cockle in the veneroid family Cardiidae. “Cardium” is greek for “heart,” so the tie-in with 14 February isn’t too much of a stretch. This “Mussel” of the Month can also serve as a reminder that the MUSSELp database serves information about freshwater bivalves generally. Those data are based on the review by Graf (2013). Obviously, the site is biased toward freshwater mussels, but the less consequential bivalves are there as well.

With the online version of the MUSSELpdb, the easiest way to find out what taxa we serve — besides using the site-specific Google Search — is to follow the “all genera” link.

January 2020

UtterbackianaUtterbackiana suborbiculata (Unionidae, North America)

The January 2020 Mussel of the Month is Utterbackiana suborbiculata. Utterbackiana is a genus of five species from eastern North America.

The genus Utterbackiana was described by Frierson (1927), but it was largely forgotten until Williams et al. (2017) brought it back for U. suborbciulata (type sp.), U. couperiana, U. hartfieldorum, U. heardi, and U. implicata. There has been no classic revision (evaluating synonyms, etc.), but the use of Utterbackiana is supported by recent molecular phylogenetic work.

(This is a long post with lots of Dillon-esque footnotes, so you might want to pause to fill up your coffee before you continue.)

It is worth reviewing how we got to this generic revision for a group of relatively well-studied North American mussels. The story technically begins at the end of the 18th century when Lamarck (1799) described the genus Anodonta, and the name was widely applied around the world. Then a bunch of stuff happened ... blah blah blah ... and Hoeh (1990) took up the problem of Anodonta sensu lato in Europe, western North America, and eastern North America. Using allozyme data (reanalyzed by Zanatta et al., 2007), it was reported that those species belonged to three separate clades that were recognized as genera: Pyganodon (like P. grandis) and Utterbackia (e.g., U. imbecillis) in eastern North America, and Anodonta in Europe (e.g., A. cygnea), western North America (A. kennerlyi), and Eastern North America (“A.” suborbiculata and “A.” couperiana). This new taxonomy was soon taken up by the freshwater mussel illuminati (e.g., Cummings & Mayer, 1992; Williams et al., 1993; Turgeon et al., 1998), and that taxonomy carried us into the 21st century.*

By the early 2000’s, hindsight had revealed that the Hoeh (1990; Zanatta et al., 2007) analyses were flawed. The ingroup consisted solely of species from Europe and North America then classified as Anodonta (like the species listed above), and Lasmigona (from North America) comprised the outgroup. As now understood, the eastern North American anodontines (including Pyganodon, Utterbackia, and Lasmigona) form a clade (Graf & Ó Foighil, 2000; Chase et al., 2018). Thus, a monophyletic Anodonta that includes species from eastern and western North America and Europe was only achieved by biased taxon sampling and a misplaced root on the cladograms.** Biogeographically, that kind of a distribution for Anodonta isn't well supported either since few genus-level clades are known to be shared across both sides of the Rocky Mountains in North America and Europe.***

This brings us to 2007 and the Graf & Cummings checklist (like so many of these tales often do). When considering only the species of North America — as Williams et al. (1993) and Turgeon et al. (1998) did — the difficulties with the taxonomy and biogeography of the local Anodonta species are somewhat smoothed over.**** But taking on the scope of all freshwater mussels and new knowledge gained from phylogenetic studies, it was evident to us (Graf & Cummings, 2007) that the “Anodonta” of eastern North America shared a more recent common ancestor with mussels in eastern North America than with Anodonta species in either the American West or Europe.

Fortunately, previous taxonomic opinions provided a solution. Hoeh (1990) didn’t invent the genus-level taxonomy of the former Anodontas. Rather, those names had been used previously as subgenera (Johnson, 1972, 1980; Heard, 1975) — e.g., A. (Pyganodon) grandis, A. (Utterbackia) imbecillis, A. (Utterbackia) couperiana, and A. (Utterbackia) suborbiculata. Putting U. couperiana and U. suborbiculata in Utterbackia solved the nonsensical eastern Anodonta problem, allowing the taxonomy to reflect a clade of eastern North American anodontines (Alasmidontina), although the relative phylogenetic positions of U. imbecillis and U. suborbiculata were not well understood at the time.

The relative positions of U. imbecillis and U. suborbiculata are better understood now: Utterbackia imbecillis, U. peggyae, and U. peninsularis comprise a clade (frequently sister to Pyganodon), and Utterbackiana suborbiculata and the other species represent a distinct clade (Breton et al., 2011; Smith et al. 2018). And thus explains the resurrection of Frierson’s (1927) long unused genus name.*****

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