The December 2025 Mussel of the Month is Mutela dubia
Mutela dubia (Iridinidae) from Africa, like so many freshwater mussels, has been “super-nominated”
Our Mussel of the Month, Mutela dubia (Gmelin, 1791), has been formally described 26 times. That means that since the 10th edition of Systema Naturae (Linnaeus, 1758), the criteria of the International Code of Zoological Nomenclature (ICZN) have been satisfied more than two dozen times for that one species. Mytilus dubius Gmelin, 1791 was the first, attaching a binomial genus and species to a specimen figured earlier by Adanson (1757, pl. 17, fig. 21) as “le mutel.” The species is now classified in Mutela Scopoli, 1777, with 13 other Recent species in Africa. Mutela dubia was given a name many more times than it needed to be — it was over-named, or super-nominated. Sorting through species names is exactly the sort of problem for which the MUSSEL Project was started.
Why did Mutela dubia get named so many times?
There are a couple good reasons for the super-nomination of Mussel of the Month Mutela dubia. One reason that is easy to understand is that the species as we know it today is distributed across multiple river basins including the Nile, the Niger, parts of the Congo, and the several separate river systems of West Africa. And, shell morphologies within and among rivers can vary with habitat and age. To explorers in the field and the “closet naturalists” in museums that received their specimens, isolated shells with disparate shapes from far-flung localities were assumed to represent different distinct “kinds” meriting a name. The concept of “species” has meant different things to workers of different eras with different philosophies. During the formative years of malacology, species were more narrowly circumscribed — they were more “split.”
Another reason that there are excess names for Mutela dubia is that taxonomists were applying names to “varieties,” “forms,” and “subspecies.” They were putting a label on a particular shell shape or subpopulation so they could talk about them. Louis Germain himself provided nine of the available names for M. dubia, and five of those were trinomials like Mutela angustata var. ponderosa Germain, 1906, described from Lake Chad. The Principle of Coordination of the ICZN (Article 46.1) says, “A name established for a taxon at either rank in the species group is deemed to have been simultaneously established by the same author for a taxon at the other rank in the group… .” Meaning, if you create an available subspecies name, you are simultaneously creating an available species name as well.
The task of the modern taxonomist is to discover the correspondence between the named species (i.e., nominal species) and the evolutionary entities in nature (the valid species). Fortunately, nominal species are tied to specific specimens and the localities from which they were collected. These are the “type specimens.” Once a biologist has a hypothesis about the geographical range and morphological diversity of a species (in the biological, evolutionary, phylogenetic, etc. sense), the type specimens will dictate the correct name to apply to the species. The ICZN’s Principle of Priority (Art. 23) says that, “The valid name of a taxon is the oldest available name applied to it… .” Mutela dubia is the correct name because it was the earliest one of the available options.
That sounds simple enough — as long as you have access to all the literature and natural history collections relevant to the species problem that you are trying to solve. You just need to examine the available specimens scattered in the museums of the world and study the scintillating descriptions of nominal species from the last few centuries. Easy peasy! Actually, this provides further insight into the basis of over-naming: some species’ authors were probably either unaware of the previous work or didn’t have access to the relevant shells and books.
This is where the MUSSEL Project and our database comes in. For any species of freshwater mussel in the database (and for many freshwater bivalves, generally), a synonymy lists the nominal species circumscribed under each valid species. For example, all 26 available synonyms of Mutela dubia are listed chronologically (priority!), many with associated images from the literature or our own collections work. Previous misspellings, literature references of taxonomic significance, and other names that have not met the criteria of the ICZN (i.e., nude names) are also listed. Images of museum specimens that form the basis of our modern understanding of Mutela dubia are also available and arranged geographically. Moreover, the same types of data are available for all of the species of Mutela to explore species boundaries and phylogenetic relationships. The MUSSEL Project Database provides a useful starting place for revisionary work on freshwater mussels.
The General Pattern of Super-Nomination Among the Freshwater Mussels
We first started developing the MUSSEL Project Database (MUSSELpdb) to get a handle on the known global diversity of freshwater mussels. On average, there are five nominal species for each of the 999 valid species of freshwater mussels. Wrangling those 5000 freshwater mussel species names was job-one for the MUSSEL Project. We started by surveying the secondary literature, beginning with Simpson (1900, 1914) and Haas (1969). Those three works were the only ones of the 20th century that attempted to list all the then-known species and genera of freshwater mussels of the world and provide comprehensive synonymies. From there, we moved on to other sources that dealt with the malacofaunas of more localized geographical areas (e.g., countries, basins) or specific taxa (references too numerous to list; see Graf & Cummings, 2021).
Our modern, comprehensive accounting of how nominal species have been treated over time provides a useful perspective on freshwater mussel taxonomy. For example, while there is a 5:1 ratio of nominal-to-valid species for freshwater mussels in general, the distribution around that mean is highly skewed. The median number of available nomina synonymized under each valid species is only 2, and 46% of valid species (458/999) have only one available name.
The 10 most super-nominated species account for more than 26% of all available freshwater mussel names (1334/5105). Anodonta anatina and A. cygnea each have so many synonyms, 329 and 278, respectively, that they are umbos-and-hinges above the 134 of Unio mancus. Those tallies make Mussel of the Month Mutela dubia seem downright reasonable for only having been over-named 25 extra times.
A close look at the more than 5000 names for fewer than 1000 freshwater mussels reveals yet another factor contributing to their super-nomination: some people just really like naming freshwater mussels. More than half of all the names (2625/5105) were introduced by only 12 taxonomists, mostly working in 19th century and the early 20th. Isaac Lea of Philadelphia, PA himself named 863 of those, and the rest of the top twelve are: Locard (359), Bourguignat (344), Rafinesque (176), Drouët (174), Heude (130), Kobelt (117), Conrad (108) Germain (93), von Martens (92), Servain (86), and Simpson (83). Those numbers are minimum estimates because my back-of-the-spreadsheet calculation only counted the species names when they were the sole authors.
Among the challenges of modern freshwater mussel taxonomy is figuring out if the “new” species you have discovered is actually new to science or if an available name already exists, buried somewhere in synonymy. By accounting for the available nominal species (and genus- and family-group taxa), the MUSSEL Project is here to serve the People as an invaluable taxonomic utility.
References Cited
Adanson, M. 1757. Histoire naturelle du Sénégal: Coquillages: avec la relation abrégée d’un voyage fait en ce pays, pendant les années 1749, 50, 51, 52 & 53. 275 pp., 19 pls.
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.).
Haas, F. 1969. Superfamilia Unionacea. Das Tierreich 88: 1-663.
Linnaeus, C. 1758. Systema Naturae per Regna Tria Naturae, Secundum Classes, Ordines, Genera, Species, cum Characteribus, Differentiis, Synonymis, locis 1 (10th ed.): 823, emendanda.
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. 1540 pp.