Mussel of the

Page last updated
8 May 2017

Mussel of the Month

The May 2017 Mussel of the Month is Elliptoideus sloatianus. Elliptoideus is a monotypic genus endemic to the southeastern United States.

Elliptoideus sloatianus
AMNH 56104. Chattahoochee River, Georgia. (type).

Elliptoideus is another one of those monotypic genera that may require a little scrutiny. As we have discussed previously, monotypic genera have their time and place. But, on the other hand, there are some such taxa that are just hold-overs from the pre-cladistic era.

For much of the 19th century, there were basically only two genera of mussels that concerned most people: Unio (for species with hinge teeth) and Anodonta (for those without). Those genera are still with us, though over the last century, their constituent species have been whittled down by the removal species to their own genera (as well as tribes, subfamilies, and families).

The earliest placement of Unio sloatianus Lea, 1840 into a different genus was accomplished by Conrad (1853): Plectomerus sloatianus. The genus Plectomerus is still in use, but only for P. dombeyanus, rather than the whole slate of original species that have since been removed to Amblema, Megalonaias, and Elliptoideus. The important point is that Conrad’s system of genera was largely ignored, and U. sloatianus remained in Unio – even after it was classified in the "section" Elliptio Rafinesque, 1819 by Simpson (1900).*

Ortmann (1912) raised Elliptio to a genus, and from that Frierson (1927) split out E. sloatianus into its own subgenus, Elliptoideus, because unlike other species of Elliptio, our Mussel of the Month broods in all four demibranchs. Elliptoideus was raised to a full-on genus by Modell (1942), then he sank it back into Plectomerus (Modell, 1964). Haas (1969a) returned Elliptoideus to a subgenus of Elliptio, though Haas (1969b) classified it as a subgenus of Nephronaias. Heart & Guckert (1970) reinstated Elliptoideus has a genus, and that rank as generally stuck ever since.** Given all the taxonomic flux described in this paragraph, it should be emphasized that these changes were not based so much on new data (that largely ended with Ortmann). Rather, these taxonomists were using classification to reflect their own views of how the data could/should be organized.

In the cladistic era, molecular phylogenetic analyses have recovered E. sloatianus in various positions (e.g., Serb et al., 2003; Campbell et al., 2005), and thus the sister lineage remains unclear. Elliptoideus is apparently a good use of a monotypic genus – at least until we get this stuff sorted out.

* Despite the obvious phylogenetic distance between true Unio and Elliptio even on anatomic grounds, the legitimate placement of Unio wasn’t correctly sorted until Graf (2002).

** Oesch (1984) inexplicably synonymized the species E. sloatianus with Plectomerus dombeyanus —the species, not just the genera — and this was followed by Howells et al. (1996), and then we were done with that.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Family UNIONIDAE Rafinesque, 1820
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe Pleurobemini Hannibal, 1912

Genus Elliptoideus Frierson, 1927

Species Elliptoideus sloatianus (Lea, 1840)

To find out more about Elliptoideus and its taxonomic history, 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.
  • Conrad, T.A. 1853. A synopsis of the family of Naïades of North America, with notes, and a table of some of the genera and sub-genera of the family, according to their geographical distribution, and descriptions of genera and sub-genera. Proceedings of the Academy of Natural Sciences of Philadelphia 6: 243-269.
  • 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. 2002. Molecular phylogenetic analysis of two problematic freshwater mussel genera (Unio and Gonidea) and a re-evaluation of the classification of Nearctic Unionidae (Bivalvia: Palaeoheterodonta: Unionoida). Journal of Molluscan Studies 68: 65-71.
  • Haas, F. 1969a. Superfamilia Unionacea. Das Tierreich, Leif. 88. Walter de Gruyter and Co., Berlin. 663 pp.
  • Haas, F. 1969b. Superfamily Unionacea. pp. N:411-470. in R.C. Moore (ed.) Treatise on Invertebrate Paleontology. Geological Society of America and the University of Kansas. Part N, Vol. 1 (of 3), Mollusca. pp. N411-N470.
  • Heard, W.H. & R.H. Guckert. 1970. A re-evaluation of the recent Unionacea (Pelecypoda) of North America. Malacologia 10: 333-355.
  • Howells, R.G., R.W. Neck, & H.D. Murray. 1996. Freshwater Mussels of Texas. Texas Parks and Wildlife Press, Austin, Texas. 218 pp.
  • Modell, H. 1942. Das natürliche System der Najaden. Archiv für Molluskenkunde 74(5/6): 161-191.
  • Modell, H. 1964. Das natürliche System der Najaden. 3. Archiv für Molluskenkunde 93: 71-126.
  • Oesch, R.D. 1984. Missouri naiades. A Guide to the Mussels of Missouri. Missouri Department of Conservation. Jefferson City, Missouri. 270 pp.
  • Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365.
  • Serb, J.M., J.E. Buhay & C. Lydeard. 2003. Molecular systematics of the North American freshwater bivalve genus Quadrula (Unionidae: Ambleminae) based on mitochondrial ND1 sequences. Molecular Phylogenetics and Evolution 28: 1-11.
  • Simpson, C.T. 1900. Synopsis of the naiades, or pearly fresh-water mussels. Proceedings of the United States National Museum 22: 501-1044.
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