The June 2026 Mussel of the Month is Elliptio complanata
A mussel at the intersection of brooding period & biogeography
Brooding period — the “When?” and “How long?” of the freshwater mussel life cycle — was all the buzz in the Olden Days when the taxonomy was centered over temperate North America and Europe. Today, reproductive phenology figures less prominently in classification, but it still offers insights into freshwater mussel ecology and evolution.
My own study of freshwater mussels began more than 30 years ago with the taxa of Upper Midwest in the USA. That area (including Minnesota and Wisconsin) is situated around the divides separating 3 major watersheds: the Mississippi running south to the Gulf of Mexico, the Red and Rainy rivers that drain north via the Nelson to Hudson Bay, and the Great Lakes flowing east to the North Atlantic. Mussel of the Month Elliptio complanata stands out as a short-term brooder in the Lake Superior watershed. To argue why that is interesting, I need to mussel-splain two seemingly unrelated topics: variation in freshwater mussel reproductive patterns and the post-glacial biogeography of fresh waters in North America.
Brooding Period was fundamental to Freshwater Mussel Classification
Brooding period for freshwater mussels refers to the interval over which females incubate their offspring. Whereas your bog-standard marine bivalve is a broadcast spawner that abandons its gametes to the environment to find their own partners for fertilization and development, freshwater mussels exhibit a remarkable degree of parental care. Females hold on to their eggs within specialized spaces of their gills (known as their marsupium), and it is there that the eggs are fertilized by sperm captured in her respiratory current. The developing embryos are protected from being swept downstream until they are ready for the next life-phase: a parasitic larval stage that completes its metamorphosis encysted in the gill or fin epithelium of its host — almost always a fish. The figure below depicts a freshwater mussel life cycle.
It was noticed early on that most North American mussel species of the family Unionidae could be categorized into one of two brooding periods: summer and winter (Graf, 1997b and references cited therein). Summer brooders like Mussel of the Month Elliptio complanata are gravid with eggs, embryos, or larvae (called glochidia) from spring through late summer, while winter brooders (e.g., Lampsilis species) are found with charged gill marsupia from the late summer into the following spring. With more data and insight, Arnold Ortmann (1911) recognized that the essential distinction wasn’t seasonality but duration. Summer brooders are short-term brooders that spawn gametes in the spring, brood developing embryos, and release glochidia later in the same growing season to infect their hosts. Winter brooders are long-term brooders that spawn near the end of the warm months and brood glochidia through the winter until the next season — even after they are competent for infection.
In technical jargon, long-term/winter brooders are described as bradytictic and short-term/summer brooders are tachytictic. When I was learning this business in the early 1990s, my mnemonic was the Brady Bill — the Brady Handgun Violence Prevention Act of 1993 — that imposed a waiting period on the purchase of handguns in the USA to provide time for background checks (and to maybe let hot-heads cool off). Long-term brooding freshwater mussels require a waiting period before releasing their glochidia. I am an American, so of course my mnemonics are about guns.
The few who have bothered to think about it favored tachytictia and bradytictia as the noun formations. However, since this vocabulary is based on Greek rather than Latin, using tachytixis and bradytixis would be more likely to impress anyone with your polyglot malacological acumen.
The allure of freshwater mussel brooding periods was how well they correlated with other reproductive traits like the morphology of the marsupium and structure of the larvae. Heard & Guckert (1970) whipped up an influential scheme to categorize North American genera, summarized in the table below.
The primary axis of Heard & Guckert’s (1970) classification was the number of marsupial demibranchs, by which they recognized two families. The taxa that brooded their glochidia in only the outer pair of gill demibranchs were classified as the Unionidae, and those with a marsupium composed of all four demibranchs — the inner as well as the outer pair — were Amblemidae. Each family was further divided into subfamilies according to brooding period, whether glochidia had ventral hooks, and other traits like the structure of the water-tubes of the gills. Check out Cummings & Graf (2010) for a more detailed explanation of those characters.
This system for North American freshwater mussels was intuitive and easily applied, but it hasn’t held up into the phylogenetic era. Even though Heard & Guckert (1970) drew a tree to represent their classification (their figure 1), it was decidedly not a cladogram. The proposed system was a nested hierarchy of taxa, but the diagnostic characters were not hierarchical. Moreover, Heard & Guckert apparently ignored the distinction between winter brooding and long-term brooding with regard to the genera Megalonaias and Popenaias. Both are nowadays described as tachytictic, although they may spawn and brood later in year than other short-term brooders — especially in warmer climates. The correspondence between the classification of Heard & Guckert (1970) and the current, phylogeny-based classification from the MUSSEL Project Database is shown below.
Bradytixis evolved convergently in two clades of unionid freshwater mussels: the Lampsilini of North and Central America and the Anodontini, distributed widely across the northern continents. Each has distinctive modifications of their marsupia to accommodate long-term brooding (Cummings & Graf, 2010). In the rest of the world, unionids are presumably tachytictic. For 20th century mussel-heads in North America and Europe, bradytixis vs. tachytixis was a big deal for recognizing freshwater mussel taxa, but for everyone else, it may not seem worthwhile to even bother learning those words.
Formerly Glaciated Northern Rivers and Lakes were Colonized from Unglaciated Refugia
That brings us to the real reason I have led you this far: tachytictic Mussel of the Month Elliptio complanata in Lake Superior, the uppermost of the Great Lakes in the northern USA.
The freshwater mussel fauna of the Lake Superior watershed is depauperate enough to simply list the species: Anodontoides ferussacianus, Pyganodon grandis, P. lacustris, Strophitus undulatus, Pterosyna complanata, Platynaias compressa, Lampsilis cardium, L. siliquoidea, Ligumia recta, and Elliptio complanata (Graf, 1997a; Sietman, 2003). That is a meager count here in the freshwater mussel Xanadu of North America.
The first six species are bradytictic anodontines. The next three are lampsilines — also long-term brooders. Mussel of the Month Elliptio complanata (tribe Pleurobemini) is the only short-term brooder. Despite their current sympatry in the mussel-poor waters of the Lake Superior basin, these mussels with disparate brooding phenologies arrived at separate times by different routes.
I have hypothesized (Graf, 1997a, b) that those few bradytictic species represent the earliest hardy post-glacial invaders of the Great Lakes and Hudson Bay drainages. As the last glaciers wasted northward about 10,000 years ago to reveal uninhabited lakes and rivers, those habitats were populated by dispersal from the adjacent unglaciated Mississippi Basin. The ice moved out, and mussels, fishes, etc. moved in. Those bradytictic species are widely distributed in the formerly glaciated territory of central North America as well as further south, and the limited fossil record suggests that they may have been the only species present in the area until about 5000 years ago. Other mussel species — bradytictic and tachytictic — colonized the Red River of the North, the central Great Lakes (e.g., Michigan and Erie; Graf, 2002), and Lake Superior via more recent connections.
Mussel of the Month Elliptio complanata is one of those more recent immigrants. It isn’t from the Mississippi Basin. It arrived later from the Atlantic Slope to the east (Clarke, 1973). Brooding period is somehow correlated with the chronology of post-glacial dispersal.
Perhaps bradytictic species capable of splitting their spawning and larval metamorphosis over two summers had an advantage in periglacial habitats. Even as the climate improved, the first few thousand summers may have been too short and prohibited tachytixis in new northern habitats freed of permanent icing. It is interesting to note the generalization that marine invertebrates that spawn in the fall (i.e., as water temperatures are dropping) tend to expand their ranges pole-ward, whereas those that spawn on rising temperatures trend equatorially (Hutchins, 1947). “Summer brooders” like Mussel of the Month Elliptio complanata may have been excluded until the growing seasons in Lake Superior were conducive to mussel reproduction in a single summer.
Or maybe shared brooding phenology of the earliest pioneering post-glacial freshwater mussels is just a coincidence. After all, those anodontine and lampsiline species also share tolerance of lentic habitats. Maybe living in lakes was the key rather than temperature. Or maybe it has more to do with the temperature limits of the mussels’ fish hosts — their primary means of dispersal. Anyway you slice it, the Lake Superior distribution of Mussel of the Month Elliptio complanata demonstrates a synergy between biology and geographical distribution.
References Cited
Clarke, A.H. 1973. The freshwater molluscs of the Canadian Interior Basin. Malacologia 13(1-2): 1-509.
Cummings, K.S. & D.L. Graf. 2010. Mollusca: Bivalvia. [in] Thorp & Covich, eds. Ecology and Classification of North American Freshwater Invertebrates : 309-384.
Graf, D.L. 1997a. Northern redistribution of freshwater pearly mussels (Bivalvia: Unionoidea) during Wisconsin deglaciation in the southern Glacial Lake Agassiz region: a review. American Midland Naturalist 138: 37-47.
Graf, D.L. 1997b. The effect of breeding period on the biogeography of freshwater mussels (Bivalvia: Unionoidea) in the Minnesota Region of North America. Occasional Papers on Mollusks 5: 393-407.
Graf, D.L. 2002. The historical biogeography and late glacial origin of the freshwater pearly mussel (Bivalvia: Unionidae) faunas of Lake Erie. Occasional Papers on Mollusks 6: 175-211.
Heard, W.H. & R.H. Guckert. 1970. A re-evaluation of the recent Unionacea (Pelecypoda) of North America. Malacologia 10: 333-355.
Hutchins, L.W. 1947. The bases for temperature zonation in geographical distribution. Ecological Monographs 17: 325-335.
Ortmann, A.E. 1911. A monograph of the najades of Pennsylvania. Parts I and II. Memoirs of the Carnegie Museum 4: 279-347.
Sietman, B. E. 2003. Field guide to the freshwater mussels of Minnesota. St. Paul, MN: Minnesota Department of Natural Resources.