The September 2025 Mussel of the Month is Ortmanniana pectorosa
The taxonomy of the genus Ortmanniana is complicated
Unless you have lived your life in the minutia of freshwater mussel nomenclature, you, gentle malacologist, might know our Mussel of the Month as Actinonaias pectorosa, but we call this animal Ortmanniana pectorosa — at least for now. The MUSSEL Project Database has treated the genus name Ortmanniana Frierson, 1927 as valid for a decade, but the Freshwater Mollusk Conservation Society (based in North America, as if the spelling of ‘mollusk’ doesn’t make that obvious) has just now updated the nomenclature in their official list of bivalve names. Ortmanniana back is back in “the news,” and we can jump on that band wagon for this post.
The problem of the classification of Ortmanniana is a sub-problem of a larger issue: the genus Lampsilis Rafinesque, 1820 as currently conceived is not monophyletic. That is, Lampsilis species do not comprise a clade composed of an ancestral species and all of its descendants. Or, to put it another way, some species of Lampsilis are more closely related to species in other genera, such as Ortmanniana, than they are to some other species of Lampsilis. At least, that interpretation is consistent with the available data.
Our saga of the genus-level classification of Ortmanniana pectorosa (Conrad, 1834) (and Lampsilis) begins more than 100 years ago with the monumental works of Charles T. Simpson (1900, 1914). Simpson proposed the first draft of the modern classification of freshwater mussels in 1900, with some updates and more discussion in 1914. The table below shows a skeleton-summary of the genera (and subgenera) that had been classified in the genus Lampsilis over the last 125 years. Obviously, freshwater mussel classification pre-dates Simpson (1900), but the prevailing system would have been more recognizable to Linnaeus and Lamarck than to a modern malacologist.
Simpson’s (1900, 1914) concept of Lampsilis (divided into four subgenera) is today partitioned among 16 genera. Simpson called our Mussel of the Month (highlighted with a red box in the table above) Lampsilis (section Eurynia) pectorosa.
While Simpson’s (1900, 1914) classification delimited most of the scope of this discussion, the next century of the classification of Ortmanniana pectorosa hinged on Simpson’s treatment of Nephronaias sapotalensis (Lea, 1841). Arnold E. Ortmann (1912) expanded on Simpson’s framework and proposed revisions based on his own thorough investigations of freshwater mussel soft anatomy: the foot, the muscles, the gills, the mantle apertures, etc. Ortmann (1912) discovered that the characters of Simpson’s Lampsilis pectorosa and L. ligamentina (Lamarck, 1819) were more like N. sapotalensis than other species of Lampsilis, e.g. L. ovata. For example, the shells were much less sexually dimorphic, and there were no mantle lures to speak of on the females. Figure below contrasts the anatomy of three different mussels, including Nephronaias sapotalensis and Lampsilis ovata.
Based on the congruity of all this anatomy, Ortmann (1912) transferred N. ligamentina and N. pectorosa (both of which Ortmann considered to be the “the same” anatomically, p. 326) to Nephronaias with N. sapotalensis. However, he also noted at the time that he had not seen the type species of Nephronaias Crosse & Fischer, 1894, N. plicatula (Küster, 1856). If that species didn’t resemble N. sapotalensis, N. ligamentina, and N. pectorosa, then the latter would need to be reclassified.
L.S. Frierson (1917) didn’t describe the anatomy of Nephronaias plicatula, but he did the next best thing. Frierson recognized two species groups within Simpson’s (1900, 1914) conception of the genus Nephronaias: one group of with their morphology more like Elliptio (e.g., shells with duller periostracum and not sexually dimorphic, whole outer demibranchs of the gills gravid while females brooded their larvae), and other group with traits like N. sapotalensis (dimorphic shells with rayed periostracum, larvae brooded in only the posterior part of the outer demibranchs of the gills).The difference in the brooding gills is shown in the anatomical drawings above.
Although he hadn’t seen the soft-anatomy of N. plicatula specifically, it’s shell characteristics placed it squarely in the former, Elliptio-like group. Frierson (1917) split the latter species into Actinonaias Crosse & Fischer, 1894, of which A. sapotalensis is the type species. That was good enough for Ortmann & Walker (1922) when they updated Simpson’s (1914) classification for North America, Actinonaias ligamentina and A. pectorosa were reclassified, and that scheme remains popular today.
Frierson (1927), in his checklist of North and Central American freshwater mussels, didn’t follow Ortmann & Walker’s (1922) view of Actinonaias. While he proceeded without much explanation, it is easy to reverse-engineer the justification for a new genus-group taxon, Ortmanniana Frierson, 1927. Actinonaias sapotalensis and other congeners are known only from Mexico, whereas A. ligamentina and A. pectorosa are from the Interior Basin of North America. That is a major, unprecedented biogeographical disjunction that could have easily been remedied by moving the North American species to Lampsilis (Ortmanniana) (as Frierson classified it), and retaining Actinonaias for the Central American species. Perhaps if those moves were precisely what Frierson (1927) had done, his system might have had an earlier impact on classification.
While Frierson’s (1927) checklist had Lampsilis (Ortmanniana) carinata (Barnes, 1823) (= ligamentina), it also had L. (Ortmanniana) higginsii (Lea, 1857), L. (Ortmanniana) abrupta (Say, 1831), and — out of the blue — a reclassification of L. (Lampsilis) pectorosa. This new arrangement was hard to reconcile with the well-argued scheme of Ortmann & Walker (1922). Fritz Haas (1969) applied Frierson’s system for North America, but he did so uncritically with few updates. Jack Burch’s (1975) synthesis and key made no mention of Ortmanniana, and we followed that view in our first global checklist (Graf & Cummings, 2007): Actinonaias pectosa, A. ligamentina, and A. sapotalensis.
However, the biogeography problem remained: the genus Actinonaias was split between Central America (A. sapotalensis) and eastern North America (A. ligamentina and A. pectorosa). Why had no one fixed this? Probably because there were lots of people working on Actinonaias in North America from “official” lists with only the two North American species. With fewer researchers working on Central American mussels, most malacologists must have simply been unaware of the other half dozen or so Mexican species classified in the same genus. Well, we were aware (as was Watters et al., 2009). So, we wrote about it in a Mussel of the Month post from July 2015 and updated the MUSSEL Project Database accordingly, applying Ortmanniana pectorosa and O. ligamentina.
Epilogue
When the FMCS recently updated their list of freshwater mussel names for 2025, the nomenaclature was updated to Ortmanniana ligamentina and Lampsilis pectorosa. Again — out of the blue — O. pectorosa has been classified as Lampsilis. What are the data behind that taxonomic hypothesis?
The Ortmanniana Cladomics page on the MUSSEL Project Web Site shows the published cladograms that included either species of that genus. As can be seen in the topologies on that page (author’s note: I am going to keep talking about that page like you have it open in another window), whenever both species have been included in the same analysis, Ortmanniana has not been recovered as monophyletic — i.e., O. pectorosa and O. ligamentina are not sister to each other. O. ligamentina groups with available Lampsilis higginsii and L. abrupta Genbank sequences (Stodola et al., 2021), and O. pectorosa has been placed as sister to Lampsilis fasciola Rafinesque, 1820 (Campbell et al., 2005; Zanatta & Murphy, 2006).
Including Lampsilis higginsii and L. abrupta in Ortmanniana with O. ligamentina (the type species) would agree nicely with Frierson’s (1927; Haas, 1969) long-ignored classification. However, that proposed revision was rejected by the FMCS, and so we too will keep those two Lampsilis in Lampsilis — for now. However, that sweeps under the rug the observation that the available COI sequences for L. higginsii, L. abrupta, and O. ligamentina are remarkably similar to each other, as shown in the table below.
The FMCS new species list reclassified Ortmanniana pectorosa as Lampsilis based on a hypothesized relationship with Lampsilis fasciola. It is interesting to note that only two individuals of O. pectorosa have ever been analyzed in phylogenetically: UAUC 880 from Campbell et al. (2005) and UF 439496 from Smith et al. (2019). At the nucleotide level, they are very similar (COIs differ by < 0.5%).
These sequence data conflict with the anatomical characters described by Ortmann (1912) and the broader consensus of character evolution in Lampsilis. According to Ortmann (1912) and others (e.g., Williams et al., 2008), Ortmanniana pectorosa lacks the conspicuous mantle lures known from Lampsilis fasciola, L. ovata (see anatomy figure above), and other species of the Lampsilis cardium clade. Maybe O. pectorosa is a true Lampsilis that has lost its mantle lure. Maybe Ortmann misidentified his specimens when he described the anatomy of O. pectorosa and no one noticed it until now. Or, maybe we should pay attention to the fact that Smith et al. (2019) did not recover O. pectorosa in the Lampsilis clade.
Revising taxonomy based on post hoc interpretations of phylogenetic analyses still makes the hair on the back of our necks stand up. We are going to remain conservative in our MUSSEL Project classification and keep Ortmanniana pectorosa where it is — for now.
Lampsilis is still a mess…
References Cited
Burch, J.B. 1975. Freshwater Unionacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications, Hamburg, Michigan. 204 pp.
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. https://www.jstor.org/stable/3701492
Frierson, L.S. 1917. New genera and species of Central American naiades. Nautilus 31: 47-49. https://www.biodiversitylibrary.org/page/45818659
Frierson, L.S. 1927. A Classification and Annotated Check List of the North American Naiades. Baylor University Press, Waco, Texas. 111 pp. https://babel.hathitrust.org/cgi/pt?id=coo.31924002812125&view=1up&seq=3
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. https://doi.org/10.1093/mollus/eym029
Graf, D.L. & K.S. Cummings. 2021. A ‘big data’ approach to global freshwater mussel diversity (Bivalvia: Unionoida), with an updated checklist of genera and species. Journal of Molluscan Studies 87(1): eyaa034 (36 pp.). https://doi.org/10.1093/mollus/eyaa034
Haas, F. 1969. Superfamilia Unionacea. Das Tierreich, Leif. 88. Walter de Gruyter and Co., Berlin. 663 pp.
Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365. https://www.biodiversitylibrary.org/page/9480224
Ortmann, A.E. & B. Walker. 1922. On the nomenclature of certain North American naiades. Occasional Papers of the Museum of Zoology, University of Michigan (112): 1-75. https://www.biodiversitylibrary.org/page/12241592
Simpson, C.T. 1900. Synopsis of the naiades, or pearly fresh-water mussels. Proceedings of the United States National Museum 22: 501-1044. https://www.biodiversitylibrary.org/page/32021246
Simpson, C.T. 1914. A Descriptive Catalogue of the Naiades, or Pearly Fresh-water Mussels. Parts I-III. Bryant Walker, Detroit, Michigan. 1540 pp. https://www.biodiversitylibrary.org/page/11344898
Smith, C.H., N.A. Johnson, K. Inoue, R.D. Doyle & C.R. Randklev. 2019. Integrative taxonomy reveals a new species of freshwater mussel, Potamilus streckersoni sp. nov. (Bivalvia: Unionidae): implications for conservation and management. Systematics and Biodiversity 17(4): 331-348. https://doi.org/10.1080/14772000.2019.1607615
Stodola, A.P., C. Lydeard, J.T. Lamer, S.A. Douglass, K.S. Cummings & D. Campbell. 2021. Hiding in plain sight: genetic confirmation of putative Louisiana fatmucket Lampsilis hydiana (Mollusca: Unionidae) in Illinois. Freshwater Mollusk Biology and Conservation 24: 59-86. https://doi.org/10.31931/fmbc-d-19-00040
Watters, G.T., M.A. Hoggarth & D.H. Stansbery. 2009. The Freshwater Mussels of Ohio. The Ohio State University Press, Columbus. 421 pp.
Williams, J.D., M.L. Warren, K.S. Cummings, J.L. Harris & R.J. Neves. 1993. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18(9): 6-22. https://doi.org/10.1577/1548-8446(1993)018%3C0006:CSOFMO%3E2.0.CO;2
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. https://doi.org/10.1016/j.ympev.2006.05.030