The January 2026 Mussel of the Month is Leila blainvilliana
Leila blainvilliana has peculiar posterior “siphons,” distinct from those of typical neotropical Mycetopodidae
The families of the freshwater mussels (order Unionida) can largely be diagnosed by the structures of their posterior apertures, and this post is going to be all about what some vulgarians hungry for clicks might call “clam butts” (or “clam bums” across the pond).
Freshwater mussel posterior apertures are interesting because of their value for diagnosing the extant families. That is, the patterns of mantle fusion (or lack thereof) differ among freshwater mussel taxa. Additional data are necessary to fully differentiate among the six extant families of the Unionida, but if you know the butts, larvae, shells, and arrangement of the brooding demibranchs, you can get the rest of the way there. The apertures (and other characters) of Mussel of the Month Leila blainvilliana (Lea, 1834) appear to buck that system. Graf & Cummings (2006) provided a lot of detail on the morphological characters used for mussel classification. That article is also the source for the classification of posterior apertures applied herein.
Before getting into specifics of the anatomical arrangements of freshwater mussel posterior apertures, it will be helpful to digress on the nature of “clam butts.” The lifestyle of a freshwater mussel (and indeed bivalves generally) is to be stationed in the sediment “downward dog,” with the posterior openings to and from the mantle cavity directed upward to the water-column. Water ladened with food particles and oxygen is drawn through the postero-ventral incurrent aperture to the gills (ctenidia) in the mantle cavity. After passing through the gills, where food particles are captured and respiratory gases exchanged, the water exists the mantle cavity via the postero-dorsal excurrent aperture. This circulation of water is central to the way of the mussel (Cummings & Graf, 2010).
It is not uncommon for inland-dwelling freshwater malacologists to call the posterior apertures “siphons.” If we were talking about clams generally, that would probably be appropriate. However, most clam-talk in the world dwells on marine clams — the seafood we get fried or steamed when visiting the coast. For the most part, freshwater mussels don’t have long, muscular, retractable tubes. A few kinda do (more on that below), but most have less elaborate openings, and we prefer the less specific term, “aperture.”
The apertures of any bivalve are elaborations of the mantle, the soft tissue that lines and secretes the shell. There is a left lobe of the mantle underlying the left valve, and a right lobe on the other side. The two lobes are continuous over the dorsum of the mussel (just as the valves of the shell are joined along the hinge), but the anterior, ventral, and posterior edges of the mantle are broadly separate (just like the margins of the valves). The apertures are formed by various degrees of connection uniting the two lobes of the mantle at specific points.
The simplest aperture arrangement (Type I, see the comparison figures below) is diagnostic of the family Margaritiferidae, like Margaritifera falcata in the photograph above. The mantle around the incurrent and excurrent apertures is exposed, and the openings to and from the mantle cavity are readily distinguishable. With Type I apertures, while the lobes of the mantle are positioned to create discrete openings, there is no tissue-grade fusion across the lobes to delimit them. Even the gills are free of the mantle at their posterior ends, leaving an incomplete diaphragm to divide the parts of the mantle cavity where the water comes in from where it goes out. Instead, there are simple ridges and folds on either side of the mantle to help steer the flow. This is the same condition as seen in Neotrigonia, the closest marine relative of the freshwater mussels.
Members of the family Unionidae have slightly more elaborate Type II apertures. Genera like Ortmanniana still lack fusion of the mantle lobes to delimit the incurrent from excurrent apertures, but the gills are attached to both lobes of the mantle along their entire lengths. This creates what is known as a “slightly incomplete” diaphragm dividing the mantle cavity. It would be complete if the diaphragm was formed by the gills and sealed with a tissue connection between the mantle lobes. Inexplicably, the two sides of the mantle exhibit fusion for a short distance dorsal to the excurrent aperture. This creates a third opening that is diagnostic of the Type II Unionidae: the supra-anal aperture.
The other families — Hyriidae, Etheriidae, Mycetopodidae, and Iridinidae — all have complete diaphragms dividing their incurrent and excurrent apertures. Among mycetopodids like Anodontites and etherids like Etheria (Heard & Vail, 1976), the lobes of the mantle are only fused between the apertures, but otherwise remain broadly open, forming Type III apertures. Type IV apertures are similar except the mantle is also closed dorsal to the excurrent aperture, creating a distinct siphon, while the incurrent aperture is open ventrally. This type of aperture is found among species of the Hyriidae and the iridinid subfamily Aspathariinae. Occasionally, Type IV incurrent apertures also exhibit a very short mantle fusion ventral to the incurrent aperture, so we can make a distinction between Type IVa as seen in the hyriid Echyridella and Type IVb with a short ventral fusion like in Chambardia (Aspathariinae). Both subtypes have been observed among both families (Ortmann, 1921).
In contrast to the short, ventral mantle fusion of Type IVb, Type V apertures have extensive ventral fusion, creating the full-on incurrent and excurrent siphons seen in iridinids like Pleiodon. Among species of the subfamily Iridininae, the ventral edges of the mantle lobes are sealed, with an anterior opening from which the foot can protrude. The mantle is arranged more like a tube than two separate, lateral flaps.
This finally brings us back to Mussel of the Month Leila blainvilliana! Based on its shell, Leila is easily confused with Anodontites, a typical mycetopodid with Type III apertures. But, the soft-parts of Leila look quite different: the apertures are drawn out into cylindrical, retractable “siphons” (Veitenheimer, 1973)!
The “siphons” of the South American genus Leila — as well as other characters like robust pedal elevator scars — have suggested to some a closer evolutionary affinity to the Type V iridinids like Pleiodon in Africa rather than simple Type III mycetopodids like Anodontites in South America (Bonetto, 1963; Heard & Vail, 1976; Graf, 2000). However, close inspection of the figure above reveals that, except for the fusion dividing the two apertures, the mantle remains open. Both the dorsal and ventral views of the “siphons” expose the separation between the left and right sides. Leila blainvilliana has a Type III butt like other genera of the Mycetopodidae — but it’s a fancy Type III butt.
That a fancy, interesting mussel like Leila blainvilliana has never been included in a phylogenetic analysis seems bonkers. The Mycetopodidae in general is low-hanging phylogenetic fruit waiting to be harvested!
References Cited
Bonetto, A.A. 1963. Contribución al conocimiento de Leila blainvilleana (Lea) (Mollusca: Pelecypoda). Physis 24: 11-16.
Cummings, K.S. D.L. Graf. 2010. 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.
Graf, D.L.. 2000. The Etherioidea revisited: a phylogenetic analysis of hyriid relationships (Mollusca: Bivalvia: Paleoheterodonta: Unionoida). Occasional Papers of the Museum of Zoology, University of Michigan (729): 1-21.
Graf, D.L. & K.S. Cummings. 2006. Palaeoheterodont diversity (Mollusca: Trigonioida + Unionoida): what we know and what we wish we knew about freshwater mussel evolution. Zoological Journal of the Linnean Society 148: 343-394.
Heard, W.H. & V.A. Vail. 1976. Anatomical systematics of Etheria elliptica (Pelecypoda: Mycetopodidae). Malacological Review 9: 15-25.
Ortmann, A.E. 1921. South American naiades; a contribution to the knowledge of the fresh-water mussels of South America. Memoirs of the Carnegie Museum 8(3): 451-670, pls. 34-48.
Veitenheimer, I.L. 1973. Contribuição ao estudo do gênero Leila Gray, 1840 — (Mycetopodidae–Bivalvia). Iheringia Série Zoologia 42:6 4-89.
I always look forward to mussel of the month, but I really enjoyed this issue in particular! Keep up the good work