Mussel of the

Page last updated
6 November 2022

Mussel of the Month

The November 2022 Mussel of the Month is Nephronaias aztecorum. Nephronaias is a genus of a couple dozen species in Central America.

USNM 85661. Vera Cruz, Mexico (type of Unio strebelii Lea)

Nephronaias aztecorum is one of the poorly known species of freshwater mussels found in Central America. When we made our first checklist back in 2007 (Graf & Cummings, 2007), the taxonomy of the Unionidae south of the Rio Grande basin was based on pretty sketchy science. A few updates have happened since then (as reflected in our latest checklist and this web site; Graf & Cummings, 2021), but the science isn't much more satisfying.

Nephronaias scamnata was the Mussel of the Month back in September 2004. At that time, our classification for the Central America Unionidae largely followed that of Haas (1969), which in turn was basically a limited update of Frierson (1927). In the pre-cladistic-classification era, Haas (1969) classified Nephronaias and its many subgenera in the "Unioninae." That was a nonsensical taxon that included Unio and Nephronaias in the same subfamily that didn’t include Anodonta or Alasmidonta. The cladistic interpretation of that classification would be that Unio and Nephronaias share a more recent common ancestor than does Unio and Anodonta — contrary to the data available when we made our first global checklist (Graf & Cummings, 2007). As a first approximation, we decided that since Frierson (1927) had treated Nephronaias as a subgenus of Elliptio, and Elliptio is well supported in the subfamily Ambleminae, Nephronaias could be reasonably placed in that subfamily as well. In 2007, we raised all of Haas's subgenera of Nephronaias to genera and classified them as Ambleminae but without specifying a tribe (i.e., "incertae sedis Ambleminae").

Popenaiadini Classification Table

Since then, Popenaias and Psoronaias have both been included in phylogenetic analyses (Pfeiffer et al. 2019; Inoue et al., 2020), and those genera were recovered in their own clade, separate from the other, North American tribes of the Ambleminae. Fortunately, a family-group level name was already available, and the clade was dubbed Popenaiadini. Nephronaias and the other Central American amblemines were also summarily sequestered in the Popenaiadini (except for the ones that could safely be situated among the Lampsilini, e.g., Actinonaias, Delphinonaias, Cyrtonaias).

So, Nephronaias aztecorum is currently assumed to belong in the Popenaidini, and we are sure that someone someday will test that.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily UNIONOIDEA Rafinesque, 1820
Family UNIONIDAE s.s.
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe POPENAIDINI Heard & Guckert, 1970

Genus Nephronaias Crosse & Fischer, 1894

Species Nephronaias aztecorum (Philippi, 1847)

To find out more about Nephronaias and the classification of the Central American Unionidae, check out:

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