Mussel of the

Page last updated
7 September 2023

Mussel of the Month

The September 2023 Mussel of the Month is Pressodonta viridis. Pressodonta is a monotypic genus found in the Laurentian Great Lakes and the adjacent Ohio and Mississippi basins in North America.

USNM 86261a. Ohio (Neotype).

Pressodonta viridis has been difficult to classify for more than 100 years. Most of the confusion stemmed from differing opinions about what actual species had been referred to under the name Unio (Elliptio) viridis Rafinesque, 1820. Some considered it to be related to Lasmigona (Platynaias) subviridis and L. compressa (e.g., Frierson, 1927; Haas, 1969), but Morrison (1969) and others, in their enthusiasm for stirring the pot of Rafinesque names, treated viridis as what most authorities called Alasmidonta calceola (Lea, 1828). Clark (1981) solved this nomenclatural issue by selecting a neotype. That specimen is the one we used in the picture above.

For almost all of the last 40 years, this month's Mussel of the Month has gone by Alasmidonta viridis, and for most people, it probably still does. Until Whelan et al. (2023), the evolutionary relationships among most of the species of Alasmidonta had never been tested with a phylogenetic analysis. Now that it has, there is an Alasmidonta clade (which includes mussels like Alasmidonta marginata), but two traditional species were not recovered with the other alasmidontas.

One of those rebellious species was Pressodonta viridis. Fortunately, a genus name was already available, and we have updated our classification to reflect Whelan et al.'s (2023) results.

The other species in need of reclassification was Prolasmidonta heterodon. That species wasn't part of the Alasmidonta clade with Alasmidonta marginata, but neither did it pair with Pressodonta viridis. We again lucked into an existing available genus name.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily UNIONOIDEA Rafinesque, 1820
Family UNIONIDAE s.s.
Subfamily UNIONINAE s.s.
Tribe ANODONTINI Rafinesque, 1820
Subtribe ALASMIDONTINA Rafinesque, 1820

Genus Pressodonta Simpson, 1900

Species Pressodonta viridis (Rafinesque, 1820)

To find out more about the phylogenetics and classification of Pressodonta, Prolasmidonta, and Alasmidonta, check out:
  • Clarke, A.H. 1981. The tribe Alasmidontini (Unionidae: Anodontinae), Part I. Pegias, Alasmidonta, and Arcidens. Smithsonian Contributions to Zoology (326). iii + 101 pp.
  • Frierson, L.S. 1927. A Classified and Annotated Check List of the North American Naiades. Baylor University Press, Waco, Texas. 111 pp.
  • Haas, F. 1969. Superfamilia Unionacea. Das Tierreich 88. Walter de Gruyter and Co., Berlin. 663 pp..
  • Morrison, J.P.E. 1969. The earliest names for North American naiads. American Malacological Union, Inc. Annual Reports (1969): 22-24.
  • Whelan, N.V., N.A. Johnson, A.S. Williams, M.A. Perkins, C.E. Beaver & J.W. Mays. 2023. Hidden in the hills: phylogeny of the freshwater mussel genus Alasmidonta (Bivalvia: Unionidae) and description of a new species. Zoological Journal of the Linnean Society 198(2): 650-676.

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