Mussel of the

Page last updated
13 October 2017

Mussel of the Month

The October 2017 Mussel of the Month is Medionidus conradicus. Medionidus is a genus of six species from the southeastern United States.

Medionidus conradicus
USNM 84134. North America. (type).

Medionidus conradicus is the Mussel of the Month because some freshwater mussel has got to do it. We have been working our way through all the genera since 2003, and Medionidus is finally getting its turn. Off the top of our heads, we couldn’t think of anything remarkable about it. A Google Scholar search of recent papers found it mentioned in various stream surveys and toxicology studies. As systematists, we fall back on classification when seeking inspiration. How sturdy is the taxonomic foundation of Medionidus? To regular MotM readers, it will come as no surprise that there are still issues to be resolved.

Our current concept of the genus Medionidus dates largely from Simpson (1914), as updated by Frierson (1927) and Johnson (1977). Ortmann (1912) figured the anatomy of M. conradicus and threw a little fit about Simpson’s (1900) description of the genus, but he apparently got over it. Henry van der Schalie (1939) described M. mcglameriae which turned out to be Leptodea fragilis (Williams et al., 2008), and with some other minor shifting over the last century we have ended up with 6 currently recognized species: M. conradicus from the Cumberland and Tennessee rivers, and the other 5 from the eastern Gulf Coastal Plain (Graf & Cummings, 2007, Haag, 2012). M. conradicus is the only one of these species not listed as endangered on the IUCN Red List.

Only two studies have offered a test of the monophyly of Medionidus: Campbell et al. (2005) and Zanatta & Murphy (2006). These are the only two analyses to include more than one species in the genus,* but their results were inconsistent. M. conradicus and M. acutissimus were recovered in various positions among the lampsilines, sometimes in close association with Lemiox. Medionidus is a genus that could use more attention.

* These two studies my not really constitute independent tests of Medionidus monophyly since they relied upon the same mtDNA sequences.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Family UNIONINDAE Rafinesque, 1820
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe LAMPSILINI Ihering, 1901

Genus Medionidus Simpson, 1900

Species Medionidus conradicus (Lea, 1834)

To find out more about Medionidus, check out:
  • Campbell, D.C., J.M. Serb, J.E. Buhay, K.J. Roe, R.L. Minton & C. Lydeard. 2005. Phylogeny of North American amblemines (Bivalvia, Unionidae): prodigious polyphyly proves pervasive across genera. Invertebrate Biology 124(2): 131-164.
  • Frierson, L.S. 1927. A classification and annotated check list of the North American naiades. Baylor University Press, Waco, Texas. 111 pp.
  • Graf, D.L. & K.S. Cummings. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Journal of Molluscan Studies 73: 291-314.
  • Haag, W.R. 2012. North American Freshwater Mussels: Natural History, Ecology, and Conservation. Cambridge University Press, New York. 505 pp.
  • Johnson, R.I. 1977. Monograph of the genus Medionidus (Bivalvia: Unionidae) mostly from the Apalachicolan region, southeastern United States. Occasional Papers on Mollusks 4(56): 161-187.
  • Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365.
  • Simpson, C.T. 1900. Synopsis of the naiades, or pearly fresh-water mussels. Proceedings of the United States National Museum 22: 501-1044.
  • Simpson, C.T. 1914. A descriptive catalogue of the naiades, or pearly fresh-water mussels. Parts I-III. Published by Bryant Walker, Detroit, Michigan. 1540 pp.
  • van der Schalie (1939)
  • Williams, J.D., A.E. Bogan & J.T. Garner. 2008. Freshwater Mussels of Alabama and the Mobile Basin in Georgia, Mississippi and Tennessee. University of Alabama Press, Tuscaloosa. 908 pp.
  • Zanatta, D.T. & R.W. Murphy. 2006. Evolution of active host-attraction strategies in the freshwater mussel tribe Lampsilini (Bivalvia: Unionidae). Molecular Phylogenetics and Evolution 41(1): 195-208.
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