North American Freshwater Mussels
 The freshwater mussels of eastern North America comprise the most diverse and best studied unionid assemblage in the world. Historically, interest in American mussels focused on the economic value of the pearl button industry. However, as stocks became depleated, considerable resources were turned to the problem of mussel propagation. It is from that work in the early 20th century that the basics and range of variation of the freshwater mussel life cycle was illucidated.
As freshwater mussel research continued in the first half of the 20th century, malacologists like Arnold Ortmann tried to put the taxa thay they recognized into a global context. That work has continued into the modern era of phylogenetic systematics, and has led to two important conclusions:
 Several higher-level lineages exist among Recent freshwater mussels, and these are reflected in the six families and numerous subfamilies of the Unionoida. - The North American freshwater mussel fauna, while species rich and morphologically and behaviorly diverse, is derived primarily from only two major radiations (and an indeterminate number of smaller radiations).
The Nearctic unionoid assemblage has been well sampled in recent phylogenetic analyses of freshwater mussel relationships. However, the most robust studies have been based upon molecular characters, so while the principle North American freshwater mussel lineages can be identified, some are not well characterized by morphological characters. But no matter — the lineages are still relatively clear.
Based upon the traditional classification of freshwater mussels, there are at least three distinct lineages of freshwater mussels in North America. In this context, we think of a lineage as being a monophyletic group — all the descendents of a particular ancestor. These lineages are the Margaritiferidae, Anodontini and Ambleminae. Based upon the evidence currently available, three lineages seems like an underestimate.
 Margaritiferidae
The family Margaritiferidae has a patchy distribution across Eurasia and North America. There are only five Nearctic margaritiferid freshwater mussels.
Anodontini
 The family Unionidae is divisible into (at least) two subfamilies, one of them being the Unioninae, diagnosed by the presence of subtriangular glochidia with large, medial hooks. Among the Unioninae, we recognize two tribes, the strictly Old World Unionini and the Anodontini, which is widespread in both North America and Eurasia. In eastern North America, there are 37 anodontine species, and recent phylogenetic work is consistent with those taxa being part of a distinct, New World lineage. In the Pacific drainages, on the western side of the Rocky Mountains, seven more species are known. Those presumably have affiliations to Asian anodontines.
 
Ambleminae
A second family of the Unionidae is the Ambleminae, very diverse with around 340 species and apparently endemic to North and Central America. Based upon most molecular studies, the Ambleminae is divisible into five tribes. One of these, Gonideini, is monotypic and distributed in Pacific drainages of westen North America. The other four, all from eastern North and Central America, are known informally as the Amblemini Tribe Group.
The four tribes that comprise the Amblemini Tribe Group are the Amblemini, Quadrulini, Pleurobemini and Lampsilini. The species of these groups present a wide variety of morphological characters and behaviors of interest to both specialists and lay-people alike.
 Amblemini. — As far as is known, the Amblemini consists only of the genus Amblema (3 species) in Canada and the USA.
Quadrulini. — Until the advent of molecular phylogenetic analyses, the Amblemini and Quadrulini were united into a single taxon. However, studies of mitochondrial DNA consistly recover Amblema as a lineage apart from Quadrula, Megalonaias, etc. (27 species) Members of these two tribes brood their larvae in all four demibranchs, and they tend to have shells sculptured with ridges, pustules or both. The Quadrulini is distributed from the Great Lakes and Hudson Bay drainages south into Central America.
 
 Pleurobemini. — The principle genera are Pleurobema, Elliptio and Fusconaia. The tribe is reasonably diverse (101 species) and has a distribution that includes the Canadian Interior Basin, Great Lakes, Atlantic Slope, Mississippi Basin, Gulf Coastal Basins and Central America. In general, the shells are unsculptured and larvae are brooded only in the outer demibranchs (with exceptions). For example, Fusconaia spp. are tetragenous like Amblema and Quadrula.
A number of Central American amblemine taxa such as Nephronaias and Psoronaias (65 species) can not be assigned with any certainty to a specific tribe. This is because the tribes are not unambiguously diagnosed morphologically, and even if they were, the anatomy of those Central American species is only incompletely understood.
 
 Lampsilini. — Unlike the three preceeding tribes, the Lampsilini is easily diagnosed and has been consistently recognized in traditional classifications. All lampsiline species are distinguished from those of the Amblemini, Quadrulini and Pleurobemini in possessing a highly modified marsupim in the brooding demibranchs. Among the lampsilines, larvae are brooded in only a portion of the outer demibranchs, and the ventral edge of the marsupium is capable of great expansion. Many species have sexually dimorphic shells, and the group employes a wide array of strategies to attract host fish — e.g., conglutinates, super-conglutinates and mantle lures. These are wonderfully illustrated and discussed on Chris Barnhart's Unio Gallery web site.
There are 148 species assigned to the Lampsilini, and they range from Canada south to Central America and even Cuba!
 
 Gonideini. — Including Gonidea in a discussion of Nearctic amblemines demonstrates how similar the members of the Amblemini Tribe Group are to each other. Gonidea broods in all four demibranchs but the septa dividing the gills into water-tubes are perforated. Those and other characteristics, including the distribution of the monotypic genus in the Pacific drainages of North America have suggested an affinity with certain genera of southeastern Asia, but that problem requires further investigation.
While it is indisputable that the North American freshwater mussel fauna is diverse and biologically intereting, it can't be over emphasized that those species only represent a single slice through the evolutionary tree of the Unionoida. The mussels of other areas are diverse and interesting as well!! Check out the Mussel of the Month and the MOM archive to experience more of the global mussel diversity. Click here to see complete family-group level classification of the Unionoida applied on this web site.
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