Mussel of the Month

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.

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* 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.