The August 2026 Mussel of the Month is “Unio” cathaicus
We have to look into the past to bring a perfectly good species name back to our present
Marty McFly in the 1985 classic Back to the Future is an allegory for what can happen when an available nominal species slips through the cracks of our collective classification consciousness. Mussel of the Month “Unio” cathaicus was taxonomically Marty McFly’d (OED, take notice!).
Fortunately, the MUSSEL Project Database (MUSSELpdb) Species Inquirenda page is just the bolt of lightning we can use to bring forgotten species back to our attention. (I appreciate that such pop-culture references might be lost on other than Gen X Americans. There aren’t many more… in this article.)
The MUSSEL Project provides resources to help non-taxonomists keep up with freshwater mussel classification as well as a comprehensive database to aid taxonomic researchers. You can subscribe to this Substack to keep up with the latest news from the MUSSELp.
The Problem
Mussel of the Month “Unio” cathaicus was described by Fritz Haas (1930) based on material reportedly from China in the Naturmuseum Senckenberg in Frankfurt am Main, Germany (Nagel, 2025). The genus name is in quotes because, while that was the name Haas used, this species clearly isn’t a Unio in the modern sense. What stands out about this mussel is that no one has dealt with it — not even Haas himself. In his magnum opus aiming to synthesize all the Recent freshwater mussel taxa, Haas (1969) didn’t treat Mussel of the Month “Unio” cathaicus as valid in Unio or any other genus; he didn’t report it as an invalid synonym of some other species; nor did he even include it among the species names of questionable status. It isn’t in the index of Haas (1969). Did he forget his own species?
Haas’s (1930) type specimens were labeled as being from “Hunan, Mittelchina,” but they don’t resemble other species known from China. No reference to the species is made by either Liu et al. (1979) or He & Zhuang (2013). What Mussel of the Month “Unio” cathaicus does look like is a species of the genus Lens (which today includes mussels historically classified as Ensidens, Physunio, Conradens, and Uniandra). It bears a striking resemblance to Lens fultoni from Vietnam and L. contradens, which is widespread and common in tropical southeast Asia.
It should be noticed that Lens fultoni was also described by Haas (1930) in the same work as Mussel of the Month“Unio” cathaicus. The. exact. same. work. L. fultoni is out there in the taxonomic literature, but important revisions from Thailand (Brandt, 1974), Vietnam (Dang et al., 1980), etc. were mum about “Unio” cathaicus.
Why was Mussel of the Month “Unio” cathaicus omitted from Haas (1969)? Maybe he really did just forget about it. I have marked more than 100 nominal species in the MUSSEL Project Database that are missing from Haas (1969). Although his comprehensive synthesis is dated as 1969, many of Haas’s species concepts originated in publications pre-dating 1930 (e.g., Haas, 1924) — although Haas does refer to Contradens semmelincki fultoni Haas 1930.
Maybe he intentionally omitted it. He realized four decades later that he’d been fooled by the wavy umbo sculpture of his paratype. It was in the very same article that Haas (1930) decided that what he had been calling the wavy-umbo’d Nodularia douglasiae (shown in the figure above) should be classified in the genus Unio. Otherwise, we might be talking about “Mussel of the Month Nodularia cathaica”.
“Was mich nun veranlaßte, sie dennoch von Ensidens zu trennen und, wenigstens vorläufig, zu Unio zu stellen, ist die eigentümliche Wirbelskulptur, die ganz von der bei Ensidens ausgebildeten…” (Haas, 1930: 8).
Other shell characters — like those fine hinge teeth shown in the type figures above — are inconsistent with either Unioor Nodularia and raise doubts about the accuracy of the type locality. Perhaps Haas swept Mussel of the Month “Unio” cathaicus under the rug.
If I had to guess, it was the former: forgotten. In any case, Mussel of the Month “Unio” cathaicus was Marty McFly’d. Untethering its past upset its future (or lack thereof).
It isn’t terribly uncommon for available nominal species to become “unclassified.” For example, nomina dubia: names for which the descriptions or type specimens are regarded as inadequate to recognize the species in nature. Those won’t be found in synonymies. “Pleurobema altum” was the name for the valid species today called P. fibuloides, and now the former has been set loose in the taxonomic ether. However, there is at least a published record more extensive than the original description of Unio altus Conrad, 1854 documenting that history. Unrecorded Mussel of the Month “Unio” cathaicus is exceptional.
The MUSSELpdb Solution
Until recently, these species of unknown placement — species inquirenda — were more challenging to locate in the MUSSEL Project Database without specifically searching for them. A curious person can browse to Unio cathaicus and Unio altus on the pages associated with the publications in which they were described: Haas (1930) and Conrad (1854). But, that still probably requires that you know you are looking for them. The MUSSELpdb is organized around valid taxa.
But now, there is a specific page for freshwater mussel species inquirenda: nominal species that are not circumscribed under the synonymy of any valid species in the MUSSELpdb. The link is located under the header at the top of the Unionida browsing hierarchy, and it is also located in the navigation side menu.
This is where you can find an alphabetical list of nomina dubia that we can’t recognize from their descriptions and other names that are in the literature but haven’t been formally described (nomina nuda). The Unionida species inquirenda list is also where we will keep Mussel of the Month “Unio” cathaicus until somebody brings it into the present.
P.S. Species inquirenda records in the MUSSELpdb for other orders are linked with their respective orders (e.g., Venerida).
References Cited
Brandt, R.A.M. 1974. The non-marine aquatic Mollusca of Thailand. Archiv für Molluskenkunde 105: 1-423.
Dang, N.T., T.B. Thai & V.M. Pham. 1980. Ðinh loai dong vat khong xu’o’ng song nu’oc ngot Bac Viet Nam [Identification of Freshwater Invertebrates of North Vientnam]. Sciences and Technology Publishing Co., Ha Noi, Vietnam. 573 pp.
Haas, F. 1924. Beiträge zu einer Monographie der asiatischen Unioniden. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 38: 129-203.
Haas, F. 1930. Beiträge zur Kenntnis ostasiatischer Najaden. Senckenbergiana 12(1): 1-13. Haas, F. 1969. Superfamilia Unionacea. Das Tierreich, Leif. 88. Walter de Gruyter and Co., Berlin. 663 pp.
He, J. & Z. Zhuang. 2013. The Freshwater Bivalves of China. ConchBooks, Harxheim, Germany. 198 pp.
Liu, Y.-Y., Y.-X. Wang & W.-Z. Zhang. 1979. . Freshwater Mollusks. Economic Fauna of China. Science Press, Beijing, Peoples Republic of China. 124 pp.
Nagel, K.-O. 2025.Type catalog of the freshwater mussel (Mollusca: Bivalvia: Unionida) in the collection of the Senckenberg Research Institute and Natural History Museum in Frankfurt am Main, Germany. Arc