Mussel of the Month

The freshwater mussel world has gotten a little more harmonious. This Mussel of the Month post is about the people of the world unifying under a single vision (more or less) for freshwater mussel classification. What we are talking about is an ending for the Soviet Comparatory species concept in favor of species as evolutionary entities.

We all know that freshwater mussels are a challenge for the uninitiated to identify due to high intraspecific shell variation and the lack of meristic characteristics. Keys are not that helpful except in localized geographical areas and are seldom constructed unless forced by editors (e.g., Cummings & Graf, 2009). This subjective deference to authority has created a couple movements over the centuries to make species identification more "objective" using shell measurements. One of these was the French Nouvelle École of Bouguignat, Locard, and others. In the last quarter of the 1800’s they had the idea that mollusk species could be recognized by making lots of measurements and defining species as shells differing from others by some arbitrary set of dimensions. Unfortunately, the bar for distinctiveness was set very low, and one of the dimensions was absolute length — so just being a different size at the time of measurement contributed to taxonomic splitting. And split they did! Instead of around a dozen French species, Arnould Locard recognized more than 300. Most of these were ignored by Simpson (1900) in his assessment of global mussel diversity because, in his words, “life is too short.” The Nouvelle École was closed, and the species and (especially) genera they concocted remained in the unsettled taxonomic ether until Graf (2010, 2011) put the final nails in their coffins.

The 20th century (and later) effects of all this 19th century splitting were eventually sorted out, but it has taken a while. In our global checklist (Graf & Cummings, 2007), we followed Haas (1969) by placing Anodonta anatina as a junior synonym of A. cygnea. There were plenty of references at the time recognizing A. anatina and A. cygnea as distinct, but those works did not unambiguously account for all the synonymous nominal species (more that 550 of them between the two). Haas did, and since accounting for synonymy was part of our goal, his species circumscription won out.

More recently, the corpse of the Nouvelle École had been resurrected by Starobogatov and other malacologists of the former Soviet Union (Shikov & Zatravkin, 1991). They used a single characteristic, the curve of the profile of a valve, to distinguish species. This was known as the Comparatory Method, and, as would be expected, it led to a proliferation of new species and genera. As a rule of thumb, Comparatory genera are largely equivalent to Western species (Korniushin, 1998; Kantor et al., 2010). DLG dealt with those taxa in a separate paper (Graf, 2007) so they wouldn’t gum up our checklist (Graf & Cummings, 2007).

The Comparatory School of Malacology persisted in parallel with the Western system into the Cladistic Era (Startobogatov et al., 2004). As we discussed back in November 2013, some Russian scientists, like Ivan Bolotov et al. (2013), have been disrupting the practice of equating shell curvatures with freshwater mussel species, but others, like Victor Bogatov (2013), continued to argue in favor of the old ways.

But recently, there have been two papers applying molecular characters to Russian freshwater mussels that have convincingly demonstrated that, at least for Anodonta anatina, mussels with slightly different shell shapes belong to the same biological populations (Klishko et al., 2018; Bogatov et al., 2018). Perhaps that comes as little suprise. It is significant (to us, anyway) that Bogatov — who described 40 Comparatory species and 5 genera since 1987 — was the lead author on one of those articles. We hope this begins a global reconciliation of freshwater mussel taxonomy.

As our friend, Yuri Kantor, pointed out to us, a shift in taxonomy in Russia can have consequences for mussel conservation. The Red Data Book of the Russian Federation is a national counterpart to the Red List produced by the International Union for the Conservation of Nature. Widespread abandonment of split Comparatory species for lumped biological species could result in freshwater mussel populations bureaucratically shifting from one conservation status to another and sowing confusion for the people that study and manage those mollusks.** But perhaps some extra paperwork is worth taxonomic détente.

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* Obviously, the distribution of Anodonta is interesting — in the sense that it makes no biogeographical sense. It is the among last remnants of the 19th century system wherein all known mussels we basically classified as either Unio, Anodonta, or Margaritifera. Since Simpson (1900), those unwieldy, unnatural genera have been whittled down as new genera were described/recognized. The type species of Anodont is A. cygnea, so Anodonta sensu strico is certainly part of the European fauna. "Anodonta" has finally been scrubbed from eastern North American mussel fauna and replaced by Pyganodon, Utterbackia, and Utterbackiana, but you don't need too be too gray to remember when Anodonta grandis, A. imbecillis, and A. suborbiculata were all in common usage. The taxonomy of western American anodontas remains to be modernized.

** Yuri has also pointed out that in the latest (as yet unapproved) Red Book, the Comparatory Species have already been removed.