MUSSELp
 
Mussel of the
Month
 
 
 
 

Page last updated
5 April 2017

Mussel of the Month

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

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.

 
NSF icon MUSSEL icon
"Making the world a better place, one mollusk at a time."