The February 2026 Mussel of the Month is Lampsilis siliquoidea
But Lampsilis is a Camel
It has been said that, “A camel is a horse designed by a committee.” Instead of serving a singular vision, a group comes up with an unsatisfactory compromise. The freshwater mussel genus Lampsilis Rafinesque, 1820 is a camel.
The gist: Lampsilis is not monophyletic. Mussel of the Month Lampsilis siliquoidea (Barnes, 1823) shouldn’t be a Lampsilis. This isn’t news, but the taxonomically minded mussel-folks out there haven’t done a thing about it. This situation has gone on long enough.
Lampsilis is iconic. Take Lampsilis cardium Rafinesque, 1820, the type species of the genus. Its common name is the “plain pocketbook.” Plain? Look at those phat shells in the figure below: robust, yellow gems decorated with stunning green rays. Pocketbook? Huh?
L. cardium shells are sexually dimorphic. In Lampsilis and other genera of the tribe Lampsilini, male mussels are typically pointed posteriorly while females are often more inflated and bluntly rounded.
The swollen valves of Lampsilis females make room for their distinctive marsupia. The marsupium is the portion of the gills that serve as a brood chamber for larvae. For many mussels, the marsupial parts of the gills appear barely specialized for parental care, and when the spaces within the gills are charged with glochidia (i.e., larvae) the marsupium may not expand much. A typical marsupium occupies the whole gill of all four gills or just the outer two. But, for Lampsilis, marsupia are restricted to portions of each outer gill, and they distend to become puffy pillows of glochidia.
In species of Lampsilis, the mantle edges of the females are elaborated into lures. That is the milkshake that brings all the fish to the yard. The lure of L. cardium is shown below. Those lures mimic the food of piscivorous fish.
See a mantle lure in action on YouTube: Plain Pocketbook mussel (Lampsilis cardium) displaying fish lure!
The female Lampsilis draws a fish to her gaping posterior mantle cavity so that she can let loose her glochidia to infest their fish-host’s gills. Encysted in fish gill tissue, the larvae complete their metamorphosis to free-living juveniles (Cummings & Graf, 2009).
The genus Lampsilis is the amazing avatar of freshwater mussel coolness!
However, Lampsilis has frequently been recovered as not monophyletic. Some species classified as Lampsilis have been shown to be more closely related to (i.e., share a more recent common ancestor with) species in other genera. Our genus isn’t natural; it isn’t a clade. Lampsilis is what’s left over after a century of revisions. It’s a camel.
The trees on the Lampsilis Cladomics page from the MUSSEL Project show the phylogenetic data for the genus. I am going to focus on one tree rather than the forest for this discussion: Inoue et al. (2020: fig. 3).
The cladogram shows 4 clades within Lampsilis. The L. cardium clade — the one with the type species — is the natural Lampsilis clade. The L. higginsii clade (which also includes L. abrupta in other trees; Stodola et al., 2021) is more closely related to Ortmanniana than to L. cardium. A distinct clade of L. siliquoidea and related species is another consistent topology, known for decades (Graf & Ó Foighil, 2000). The fourth clade is represented by L. teres. In this cladogram, L. teres is sister to the L. siliquoidea clade, but those branches move around in other analyses.
In the available phylogenies, Mussel of the Month Lampsilis siliquoidea shares a more recent common ancestor with species in other genera than it does with L. cardium.
The history of classification of Lampsilis explains the path to our current situation. The table below traces the dismantling of Simpson’s (1900) more inclusive conception of Lampsilis. Simpson’s 4 subgenera of Lampsilis were broken up, and genera were carved out to accommodate accumulating soft-part and reproductive data. What we call Lampsilis today is what is left after Potamilus, Hamiota, Ortmanniana, Ligumia, Villosa, etc. were excised. Lampsilis is a compromise. It’s a camel.
When similar inconsistencies between prevailing classification and phylogenetic results have been realized, there were uncomplicated fixes. Last June, I described the case of Lasmigona sensu lato being split into three genera. Genus names were already available for the newly (re)discovered lineages, so the only necessary nomenclatural actions were elevating previously recognized subgenera. Easy peasy. That approach was escalated by the Freshwater Mollusk Conservation Society which split Lasmigona again. The old Lasmigona is now 4 genera: Lasmigona, Pterosyna, Platynaias, and Alasminota.
The FMCS also made moves related to Lampsilis and Ortmanniana (which I also discussed last year), but they left Lampsilis as described above. Unfortunately, there is no genus-group level name waiting in the synonymy of Lampsilis with a type species in the L. siliquoidea clade. The L. cardium clade would obviously still be called Lampsilis. The name Simpsonunio Starobogatov, 1970 is available for the L. teres clade. L. higginsii could get transferred to Ortmanniana Frierson, 1927. But that still leaves Mussel of the Month Lampsilis siliquoidea right where it is currently classified until someone publishes a new genus name. Otherwise, Lampsilis is a camel.
What is it about doing the revisionary work and messing with the taxonomy of such a well-known genus as Lampsilis that is such a turn-off? This seems like the perfect mission for an up-and-comer looking to make a name for themselves in the glamorous world of freshwater mussel taxonomy.
References Cited
Burch, J.B. 1975. Freshwater Unionacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications, Hamburg, Michigan. 204 pp.
Cummings, K.S. D.L. Graf. 2009. Mollusca: Bivalvia. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition. Academic Press-Elsevier, New York. pp. 309-384.
Cummings, K.S. & C.A. Mayer. 1992. Field Guide to Freshwater Mussels of the Midwest. Illinois Natural History Survey Manual (5). 194 pp.
Frierson, L.S. 1917. New genera and species of Central American naiades. Nautilus 31: 47-49.
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.
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.).
Graf, D.L. & D. Ó Foighil. 2000. The evolution of brooding characters among the freshwater mussels (Bivalvia: Unionoidea) of North America. Journal of Molluscan Studies 66: 157-170.
Haas, F. 1969. Superfamilia Unionacea. Das Tierreich, Leif. 88. Walter de Gruyter and Co., Berlin. 663 pp.
Inoue, K., J.L. Harris, C.R. Robertson, N.A. Johnson & C.R. Randklev. 2020. A comprehensive approach uncovers hidden diversity in freshwater mussels (Bivalvia: Unionidae) with the description of a novel species. Cladistics 36: 88-113.
Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365.
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.
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. Bryant Walker, Detroit, Michigan. 1540 pp.
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.
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.
As usual an absolutely splendid read. Thank you very much.