Mussel of the

Page last updated
30 May 2017

Mussel of the Month

The 2017 Mussels of the Month, so far...

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May 2017

ElliptoideusElliptoideus sloatianus (Unionidae, Nearctic)

The May 2017 Mussel of the Month is Elliptoideus sloatianus. Elliptoideus is a monotypic genus endemic to the southeastern United States.

Elliptoideus is another one of those monotypic genera that may require a little scrutiny. As we have discussed previously, monotypic genera have their time and place. But, on the other hand, there are some such taxa that are just hold-overs from the pre-cladistic era.

For much of the 19th century, there were basically only two genera of mussels that concerned most people: Unio (for species with hinge teeth) and Anodonta (for those without). Those genera are still with us, though over the last century, their constituent species have been whittled down by the removal species to their own genera (as well as tribes, subfamilies, and families).

The earliest placement of Unio sloatianus Lea, 1840 into a different genus was accomplished by Conrad (1853): Plectomerus sloatianus. The genus Plectomerus is still in use, but only for P. dombeyanus, rather than the whole slate of original species that have since been removed to Amblema, Megalonaias, and Elliptoideus. The important point is that Conrad’s system of genera was largely ignored, and U. sloatianus remained in Unio – even after it was classified in the "section" Elliptio Rafinesque, 1819 by Simpson (1900).*

Ortmann (1912) raised Elliptio to a genus, and from that Frierson (1927) split out E. sloatianus into its own subgenus, Elliptoideus, because unlike other species of Elliptio, our Mussel of the Month broods in all four demibranchs. Elliptoideus was raised to a full-on genus by Modell (1942), then he sank it back into Plectomerus (Modell, 1964). Haas (1969a) returned Elliptoideus to a subgenus of Elliptio, though Haas (1969b) classified it as a subgenus of Nephronaias. Heart & Guckert (1970) reinstated Elliptoideus has a genus, and that rank as generally stuck ever since.** Given all the taxonomic flux described in this paragraph, it should be emphasized that these changes were not based so much on new data (that largely ended with Ortmann). Rather, these taxonomists were using classification to reflect their own views of how the data could/should be organized.

In the cladistic era, molecular phylogenetic analyses have recovered E. sloatianus in various positions (e.g., Serb et al., 2003; Campbell et al., 2005), and thus the sister lineage remains unclear. Elliptoideus is apparently a good use of a monotypic genus – at least until we get this stuff sorted out.

* Despite the obvious phylogenetic distance between true Unio and Elliptio even on anatomic grounds, the legitimate placement of Unio wasn’t correctly sorted until Graf (2002).

** Oesch (1984) inexplicably synonymized the species E. sloatianus with Plectomerus dombeyanus —the species, not just the genera — and this was followed by Howells et al. (1996), and then we were done with that.

April 2017

ProtunioProtunio messageri (Unionidae, Indotropical)

The April 2017 Mussel of the Month is Protunio messageri. Protunio is a monotypic genus from northern Vietnam.

We chose Protunio messageri as Mussel of the Month because we are busy: something easy for which there isn’t much to say. It seems like a perfectly valid species, and we have 23 lots in our database of specimens, all from northern Vietnam.

Ever since Haas (1912) described the genus (the original description was really just applying the name in a plate caption), the species, Protunio messageri has turned up on all the lists (e.g., Thiele, 1934; Haas, 1969; Dang et al., 1980; Graf & Cummings, 2007). Although we are fuzzy with how to classify it in the Unionidae, this is definitely a thing.

But, P. messageri also has an IUCN Red List entry with this interesting taxonomic note:

“The type specimens of Unio messageri do not match the species recorded from Viet Nam, although they come from the same locality. These are possibly two species under the same heading. This assessment refers to the more recently described of the two species.”

So, P. messageri is apparently endangered but it is not the species that has its type pictured here (which matches the other specimen records in our database)? Well, the specimen figured here is Protunio messageri, so the Red List might be talking about something else. Perhaps the species figured by Dang et al. (1980)?

March 2017

DelphinulusDelphinonaias delphinulus (Unionidae, Neotropical)

The March 2017 Mussel of the Month is Delphinonaias delphinulus. Delphinonaias is a genus of four species found in Central America.

Delphinonaias delphinulus is another one of those Central American mussels about which we know very little. That is about all we can say when we honor mussels from that part of world in our monthly showcase (e.g., Psorula, Sphenonaias).

Delphinonaias caught our eye because it has been so unambiguously classified as a lampsiline since Simpson (1900). Even Martens (1900) — who was apparently unaware that Simpson was simultaneously moving mussel generic classification into the modern era — placed those mussels in Unio (Metaptera). Metaptera is a Rafinesque name for the genus Potamilus (also a Rafinesque genus). Presumably, these early classifications with Lampsilis and Potamilus were based on the alate* shape of Delphinonaias, but we wondered if anyone has actually looked at the anatomy. Does Delphinonaias exhibit the characteristic traits of the Lampsilini, like the marsupium limited to a portion of each outer demibranch and capable of expansion when gravid and long-term brooding?

There really aren’t that many references that mention the classification of Delphinonaias, and our quick scan of those did not produce an explicit description of the anatomy of D. delphinulus or any congener. However, Goodrich & van der Schalie (1937) came close. They lumped D. delphinulus under Leptodea paludosa and stated,

“On the whole, this species has all the characters of a Leptodea, both in soft parts and in shell characters.” (p. 47)

Leptodea (also alate*) is a lampsiline. Frierson (1927) and Haas (1969) even arranged Delphinonaias as a subgenus of Lampsilis. We recognized Delphinonaias as a genus (Graf & Cummings, 2007), and we will continue to do that until we have some data to the contrary.

* Alate refers to the presence of hinge wings.

February 2017

PseudodontopsisPseudodontopsis euphraticus (Unionidae, Palearctic)

The February 2017 Mussel of the Month is Pseudodontopsis euphraticus. Pseudodontopsis is a monotypic genus from the Tigris-Euphrates Basin in the Middle East.

P. euphraticus has a big, good-looking shell, but we don’t know very much about the species. It’s distribution in the Fertile Crescent means shells turn up occasionally in the archaeological literature (e.g., Ridout-Sharpe, 2015). But, the biological classification of the genus Pseudodontopsis is a house of cards.

The species was originally described in the 19th century in the genus Unio. Because of its reduced hinge teeth, P. euphraticus was classified for a while as Pseudodon, Leguminaia, etc., until it got its own genus Pseudodontopsis. Kobelt (1913) gave a long bit of hand-waving to make the case that P. euphraticus (and the various synonyms that he described) was distinct from the other genera in the region, and the name Pseudodontopsis has stuck since then.

However, as we explained last month, monotypic genera often seem like a cop-out — a place-holder for species of unknown affinities. Pseudodontopsis is one of those genera.

Today, we classify Pseudodontopsis in the subfamily Gonideinae based on argumentation rather than data. Both Modell (1964) and Starobogatov (1970) classified Pseudodontopsis in a subfamily called Pseudodontinae,* along with other genera like Pseudodon, Leguminaia, and Microcondylaea. Lopes-Lima et al. (2017) recovered those three genera in a clade with Gonidea, and ipso facto we conclude the Pseudodontopsis also belongs in a clade with the Gonideinae. Unfortunately, the transitive property is not a good basis for phylogeny. What we know about the glochidia of P. euphraticus is consistent with this classification (Al-Mahdawi & Al-Dulaimi, 2009).**

* We’ll ignore for this post that Modell and Starobogatov classified the Pseudodontinae in the family Margaritiferidae!

** There was a brief, pre-cladistic period in the late 20th century when some Palearctic mussels (including Pseudodontopsis) in the Nearctic genus Pleurobema, but we digress.

January 2017

PlectomerusPlectomerus dombeyanus (Unionidae, Nearctic)

The January 2017 Mussel of the Month is Plectomerus dombeyanus. Plectomerus is a monotypic genus from the western Gulf Coastal Plain of North America, including the lower Mississippi Basin.

Plectomerus is a monotypic genus. A single species in its own genus separate from all other genera,* and that classification has been widely accepted since Ortmann & Walker (1922). That work was among the earliest formal attempts to reconcile the current nomenclature of freshwater mussels with the International Code of Zoological Nomenclature.

We are generally skeptical of monotypic genera. All species are distinct from other species, but genera should reflect information about relationships — groups of species. Sometimes monotypic genera are helpful, but sometimes they are not (e.g., Arkansia). What makes P. dombeyanus so distinctive that two classical anatomists like Ortmann and Walker thought it should be set apart from genera like Amblema and Megalonaias?

Ortmann (1912) had regarded the species now classified in Amblema and Megalonaias as well as P. dombeyanus to belong in the genus Crenodonta based on their shared soft-anatomical characters and similar shell morphology. At the time, though, Ortmann had noted that Frierson (a source of Ortmann’s specimens) had reported that some females of P. dombeyanus brooded their larvae in only the outer gills. The other species of Crenodonta brooded in all four gills. In technical terms, some P. dombeyanus are apparently ectobranchus while others are tetragenous, like the other species of Crenodonta.

Frierson (1914) — an early apologist of crazy Frenchmen — argued that Amblema Rafinesque, 1820 had priority over Crenodonta of Schlüter, 1838. And then Utterback (1915) sequestered the species with zig-zag umbo sculpture into a new genus, Megalonaias. This splitting of mussels with plicate (i.e., broadly wrinkled) shells into new genera forced Ortmann & Walker (1922) to choose a genus for P. dombeyanus. And, they decided — given Frierson’s claim of possibly different brooding anatomy — that it would be better to resurrect Plectomerus Conrad, 1853** than to make the classify it one of the other genera.

Fortunately, Ortmann & Walker (1922) actually fell backwards into the correct answer — or at least the “correctest” answer to-date. P. dombeyanus is not closely related to Megalonaias (Tribe Quadrulini). In fact, in some analyses, P. dombeyanus shares a more recent common ancestor with Lampsilis (Tribe Lampsilini) than Amblema (Tribe Amblemini) (Chapman et al., 2008). Plectomerus dombeyanus is a species with unclear phylogenetic affinities, and to park it in another genus would be more confusing.

Plectomerus is monotypic, and that is the way we like it.

* Frierson (1927) and others regarded Plectomerus as a subgenus of Amblema, but the sentiment is still the same.

** Conrad (1853) clearly intended Plectomerus as a genus equal in scope to the old Crenodonta, and it included the species now classified as Plectomerus, Amblema, and Megalonaias. However, Conrad neglected to assign a type species, and Ortmann & Walker (1922) applied their ICZN-given right to just pick one. The alternative would have been to pull from obscurity another questionably recognizable Rafinesque† genus, Bariosta Rafinesque, 1820.

† “Rafinesque” is used here as an adjective to describe poorly-described, should-have-been-suppressed taxa. We hope the practice catches on.

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