We present a revised list of freshwater mussels (order Unionida, families Margaritiferidae and Unionidae) of the United States and Canada, incorporating changes in nomenclature and systematic taxonomy since publication of the most recent checklist in 1998. We recognize a total of 298 species in 55 genera in the families Margaritiferidae (one genus, five species) and Unionidae (54 genera, 293 species). We propose one change in the Margaritiferidae: the placement of the formerly monotypic genus Cumberlandia in the synonymy of Margaritifera. In the Unionidae, we recognize three new genera, elevate four genera from synonymy, and place three previously recognized genera in synonymy. We recognize for the first time two species (one native and one nonindigenous) in the Asian genus Sinanodonta as occurring in North America. We recognize four new species and one subspecies and elevate 21 species from synonymy. We elevate 10 subspecies to species status and no longer recognize four subspecies. We change common names for five taxa, correct spelling for eight species, and correct the date of publication of original descriptions for four species.
INTRODUCTION
During the past 50 yr, there has been considerable interest in freshwater mussels (order Unionida) in the United States and Canada. Much of this interest was brought about by passage of the U.S. Endangered Species Acts of 1966, 1969, and 1973 and the Canadian Species at Risk Act of 2002. These legislative actions and the environmental movement that accompanied them focused conservation attention on all animals and plants, as well as their habitats. This in turn led to assessment of species conservation status and the development of faunal lists for many states and provinces. The task of developing species lists was difficult for most invertebrates, including mussels, because so little attention had been given to the study of their biology, ecology, and systematics. In 1970, only six U.S. states had recent lists or books covering their mussel fauna. The first modem attempt to provide a comprehensive list of freshwater mussels of North America was published by Burch (1973, 1975).
The first comprehensive list of freshwater mussels of the United States and Canada was compiled in Turgeon et al. (1988) and revised a decade later (Turgeon et al. 1998). Williams et al. (1993) was another important resource during this period; although mainly an assessment of species conservation status, this paper also provided a comprehensive and widely used species list similar to those of Turgeon et al. (1988, 1998). These lists standardized and provided taxonomic stability to mussel common and scientific names to an extent that was previously unavailable. However, systematic taxonomy of mussels was poorly known at that time, and classifications at all taxonomic levels were based largely on concepts from the early 1900s.
Since publication of Turgeon et al. (1988, 1998) and Williams et al. (1993), many studies have refined our understanding of mussel systematic taxonomy. Several major publications have addressed systematic relationships within the class Bivalvia, including the order Unionida (Bieler et al. 2010; Carter et al. 2011; Bolotov et al. 2016; Araujo et al. 2017; Combosch et al. 2017). Major studies specific to the Unionida include Graf and Ó Foighil (2000), Hoeh et al. (2001, 2002, 2009), Roe and Hoeh (2003), Campbell et al. (2005), Walker et al. (2006), Graf and Cummings (2007, 2017), Cummings and Graf (2010), and Campbell and Lydeard (2012a, 2012b). In addition, many studies have examined systematic relationships at lower taxonomic levels (e.g., Serb et al. 2003; Jones et al. 2006; Lane et al. 2016). Together, this body of work depicts a view of mussel taxonomy that differs substantially from that of previous lists of the North American fauna.
We present a revised classification and list of the freshwater mussels of the United States and Canada (Tables 1 and 2). The primary purpose of this revision is to provide in a single resource a comprehensive list and taxonomic classification that reflects recent refinement of mussel systematics.
METHODS
We used as a starting point the list of Turgeon et al. (1998). We revised this list and its taxonomic classification based on a review of peer-reviewed mussel taxonomic and nomenclatura! literature produced since 1998, unpublished research by the authors, and discussions with other experts on mussel systematics. We also corrected the spelling of specific epithets and publication dates of original descriptions based on the International Code of Zoological Nomenclature ( http://www.iczn.org/iczn/index.jsp). Species mentioned in the text, but not included in Table 2, have author and date of publication following the name. Author and date of publication for all other species are given in Table 2.
Mussel common names follow Turgeon et al. (1998) with minor exceptions, but they are capitalized as is now the practice for many other animal groups (e.g., birds, reptiles, amphibians, fishes). Capitalization of common names helps avoid confusion by identifying standardized common names. For example, reference to a “fragile papershell” could apply to several thin-shelled species, but the capitalized “Fragile Papershell” is unambiguously recognized as the common name for Leptodea fragilis. We note and explain other instances where we changed common names from those of Turgeon et al. (1998) or where recognition of previously unrecognized species necessitated creation of a new common name.
We provide a rationale for and discussion of all taxonomic changes in the following accounts for each family and genus and in Table 2. There is a degree of uncertainty and subjectivity in our revised list that is unavoidable given our still imperfect understanding of mussel systematics. We attempted to reconcile divergent views regarding mussel systematics based on our assessment of the strength of evidence for these views. In cases where evidence did not allow reconciliation, we attempted to provide a plausible conclusion based on our professional judgment and experience; these conclusions were based on consensus among the authors to the extent possible.
Subspecies is a taxonomic category applied to populations that are morphologically distinct and geographically separated but that exhibit intergradation in contact zones (Mayr et al. 1953; Gilbert 1961). We evaluated morphological and molecular evidence relating to the status of subspecies recognized by Turgeon et al. (1998) and subsequent workers (Jones and Neves 2010). In most cases, recent evidence did not support recognition of subspecies but supported either subsuming subspecies under the nominal species or elevating subspecies to species status; we discuss this evidence for each case. However, strong evidence with which to evaluate their status was lacking for several, mostly extinct, subspecies (see Epioblasma). The designation of subspecies versus species is arbitrary and inconsistent for many animal groups (Huang and Knowles 2016), and this has historically been the case for mussels (e.g., Ortmann 1918, 1920). For subspecies that lacked strong evidence for synonymization or elevation, we recognize all as species to provide more consistent null hypotheses regarding potential diversity in these groups.
This work has been registered with ZooBank and a copy has been archived at Zenodo.org.
RESULTS
Freshwater bivalve higher classification continues to evolve as more data are generated and new techniques are developed. Fossil and modem bivalve higher classification has recently been summarized by Carter et al. (2011), with standardized endings for higher taxa within Bivalvia. Recent evidence supports the order Unionida as a monophyletic clade (Combosch et al. 2017). There have been two recent assessments of the taxonomy for Margaritiferidae (Bolotov et al. 2016; Araujo et al. 2017). Higher level relationships within the Unionidae have recently been reviewed by Lopes-Lima et al. (2017). Based on these publications, we provide our assessment of higher classification of the Unionida and its position in the class Bivalvia (Table 1).
There is general agreement on the three subfamily divisions within the Unionidae in North America and seven subfamilies worldwide, but there remains some uncertainty regarding classification at lower levels. We adopted a subfamily-, tribe-, and generic-level classification for the United States and Canada based on recent phylogenetic research (Table 1). We recognize the Anodontinae as a subfamily with two tribes in the United States and Canada. We recognize the subfamily Gonideinae, containing the genus Gonidea. We recognize the subfamily Ambleminae as consisting of four tribes: Ambiemini, Lampsilini, Pleurobemini, and Quadrulini. The placement of many genera within tribes in the Ambleminae is well supported and consistent among studies, but the placement of others is less certain and varies among studies (e.g., Plectomerus, Campbell et al. 2005). The Mexican and Central American genera Disconaias and Popenaias and North American Reginaia lack sufficient phylogenetic information to be confidently assigned to a classification, and we placed them in Ambleminae incertae sedis (Table 1).
Our revised list includes many taxonomic changes at the genus, species, and subspecies levels relative to previous lists. We recognize in total 298 freshwater mussel species from the United States and Canada. These comprise the families Margaritiferidae with one genus and five species and Unionidae with 54 genera and 293 species (Table 2). Turgeon et al. (1998) recognized in total 304 taxa: Margaritiferidae with two genera and five species and Unionidae with 49 genera, 286 species, and 13 subspecies. We summarize our changes to Turgeon et al. (1998) as follows. We recognize eight additional genera, including three recently described (Hamiota, Parvaspina, and Reginaia), four elevated from synonymy (Eurynia, Pleuronaia, Theliderma, and Utterbackiana), and one newly reported from North America (Sinanodonta). We place in synonymy four genera, including one in the Margaritiferidae (Cumberlandia) and three in the Unionidae (Arkansia, Lexingtonia, and Quincuncina). We recognize 25 additional species (all Unionidae), including four newly described species and 21 species elevated from synonymy. We place in synonymy 29 species and consider Pleurobema altum a nomen dubium, and we reassigned 41 species to other genera. We corrected the specific epithet spelling for eight species, corrected the date of publication for four, and changed the common names of five. Last, we recognized no subspecies, elevating 10 subspecies to species status and subsuming four subspecies into their nominal species (see Methods).
Table 2.
List of Margaritiferidae and Unionidae of the United States and Canada. Currently recognized taxa are bolded. Taxa preceded by an asterisk and not bolded appeared in Turgeon et al. (1998) but are no longer recognized or reassigned to other genera.
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Margaritiferidae Henderson, 1929
Turgeon et al. (1998) recognized two genera in Margaritiferidae, Cumberlandia (one species) and Margaritifera (four species). On the basis of shell morphology and soft anatomy, Smith (2001) placed Cumberlandia in Margaritanopsis and Margaritifera (in part) in Pseudunio, but this classification was not widely accepted. In a molecular phylogenetic analysis, Huff et al. (2004) considered Cumberlandia a junior synonym of Margaritifera, and this classification was followed by some subsequent authors (e.g., Graf and Cummings 2007, 2017; Cummings and Graf 2010), but others continued to recognize the genus as valid (e.g., Williams et al. 2008; Watters et al. 2009; Haag 2012). A more comprehensive phylogeny of the Margaritiferidae that included eight of 13 currently recognized species (three from North America) retained the use of Cumberlandia (Bolotov et al. 2015). However, based on more recent evidence (Bolotov et al. 2016; Araujo et al. 2017), we consider Cumberlandia a junior synonym of Margaritifera.
Cumberlandia Ortmann, 1912.—Turgeon et al. (1998) recognized one species, Cumberlandia monodonta. We place Cumberlandia in the synonymy of Margaritifera (see Margaritiferidae).
Margaritifera Schumacher, 1816.—Turgeon et al. (1998) recognized four species of Margaritifera. Placement of Cumberlandia in the synonymy of Margaritifera brings the number of recognized species to five (see Margaritiferidae).
Unionidae Rafinesque, 1820
Turgeon et al. (1998) recognized 49 genera, 286 species, and 13 subspecies in Unionidae. We recognize 54 genera, 293 species, and no subspecies. We provide support for and discussion of these changes in the following assessments of genera.
Actinonaias Crosse and Fischer, 1894.—Turgeon et al. (1998) recognized two species, Actinonaias ligamentina and Actinonaias pectorosa. Molecular analyses (e.g., Campbell et al. 2005; Zanatta and Murphy 2006) found that the two species of Actinonaias together did not represent a monophyletic grouping, but the position of each of these lineages within the Lampsilini was unresolved. The type locality for Actinonaias is central Mexico, and 10 recognized species are restricted to this region (Graf and Cummings 2017), but no species attributable to Actinonaias occur between Mexico and the range of ligamentina and pectorosa in the central United States and southern Canada. No phylogenetic research has examined relationships among Mexican Actinonaias and ligamentina and pectorosa, but it is unlikely they are closely related considering the disjunct distribution and lack of precedent for such a geographical pattern in other freshwater taxa (e.g., Miller et al. 2005). Actinonaias ligamentina and pectorosa require placement in two different genera, but at this time we retain these two species in the genus Actinonaias pending the outcome of further phylogenetic research.
Alasmidonta Say, 1818.—Turgeon et al. (1998) recognized 12 species, and recent evidence supports no changes to this classification.
Amblema Rafinesque, 1820.—Turgeon et al. (1998) recognized three species, and recent evidence supports no changes to this classification.
Anodonta Lamarck, 1799.—Turgeon et al. (1998) recognized 10 species. Mock et al. (2004) and Zanatta et al. (2007) found Anodonta to be polyphyletic, with eastern North American species forming a monophyletic clade distinct from the one that includes the type species (Anodonta cygnea, which occurs in Eurasia) and western North American Anodonta. Without discussion, Graf and Cummings (2007) and Cummings and Graf (2010) placed Anodonta couperiana, A. heardi, and A. suborbiculata in Utterbackia, and A. implicata in Pyganodon. Because no supporting evidence was provided, we do not recognize these changes. The next available genus for the eastern North American clade (A. couperiana, A. heardi, A. suborbiculata, and A. implicata) identified as distinct by Mock et al. (2004) is Utterbackiana. Anodonta hartfieldorum Williams, Bogan, and Garner, 2009, was described subsequently and also belongs to Utterbackiana (see Utterbackiana).
In a phylogenetic analysis of western North American Anodonta, Chong et al. (2008) found A. beringiana to be more closely related to the Asian species Sinanodonta woodiana than to North American species. Based on this evidence, we reassign beringiana to Sinanodonta (see Sinanodonta).
We retain the remaining four western North American species within Anodonta (A. californiensis, A. kennerlyi, A. nuttalliana, and A. oregonensis) based on their phylogenetic affinity to Eurasian Anodonta (Mock et al. 2004; Zanatta et al. 2007; Chong et al. 2008). Anodonta dejecta was recognized by Turgeon et al. (1998), Graf and Cummings (2007), and Cummings and Graf (2010). This species is treated as a synonym of A. californiensis by Bequaert and Miller (1973) and the Arizona Game and Fish Department (2017). We do not recognize A. dejecta, which is here placed in synonymy of A. californiensis.
Anodontoides Simpson in Baker, 1898.—Turgeon et al. (1998) recognized two species. One additional species, Anodontoides denigrata, was recognized without discussion by Neves et al. (1997) and Cicerello and Schuster (2003). Haag and Cicerello (2016) recognized A. denigrata on the basis of molecular data showing that upper Cumberland River drainage populations were distinct from A. ferussacianus (Bogan and Raley 2013), and we recognize this species for the same reason. Bogan and Raley (2013) referred to A. denigrata as A. argenteus (Lea, 1840), for which the type locality is Stones River, Tennessee. The Stones River is a tributary of the middle Cumberland River and well downstream of the putative distribution of A. denigrata and other species considered endemic to the upper Cumberland River drainage upstream of the hypothesized original location of Cumberland Falls (Haag and Cicerello 2016). Until further research delineates this species' distribution more precisely, we use A. denigrata, for which the type locality is in the upper Cumberland River drainage (Clear Fork, Campbell County, Tennessee; see Ortmann 1918). Ahlstedt et al. (2016) reported a possibly distinct Anodontoides species from the Powell River, Virginia, but further work is needed to determine its validity and taxonomy.
Arcidens Simpson, 1900.—Turgeon et al. (1998) recognized one species, Arcidens confragosus. Clarke (1981) considered Arkansia (see Arkansia) a junior synonym of Arcidens (see also Graf and Cummings 2007), and this classification was supported by morphological and molecular data (Inoue et al. 2014). We recognize two species of Arcidens.
Arkansia Ortmann and Walker, 1912.—Arkansia was described as a monotypic genus including A. wheeleri, which was recognized by Turgeon et al. (1998). We place Arkansia in the synonymy of Arcidens (see Arcidens).
Cyclonaias Pilsbry in Ortmann and Walker, 1922.— Turgeon et al. (1998) recognized Cyclonaias, which has long been considered a monotypic genus for C. tuberculata, Cyclonaias tuberculata has been aligned with the Quadrulini based on morphological (e.g., Frierson 1927; Modell 1964) and protein polymorphism data (Davis and Fuller 1981). Heard and Guckert (1971) placed Cyclonaias in the Pleurobemini based on its ectobranchous brooding (see also Graf and Cummings 2007). However, it appears that ectobranchy arose multiple times (Davis and Fuller 1981; Graf 2002; Roe and Hoeh 2003), meaning that this trait does not necessarily exclude Cyclonaias from the Quadrulini, and some female C. tuberculata brood glochidia in all four gills (Frierson 1927).
Recent molecular studies consistently supported inclusion of Cyclonaias in the Quadrulini, but they further show that it is a member of a monophyletic clade including Q. pustulosa and related species (Campbell et al. 2005; Campbell and Lydeard 2012b). Serb et al. (2003) did not support this relationship, but these results were later attributed to an error in sample labeling (Campbell and Lydeard 2012b). However, Serb et al. (2003) as well as Campbell et al. (2005) and Campbell and Lydeard (2012b) support the monophyly of the Quadrula pustulosa clade and its distinctiveness from other species of Quadrula (see Quadrula and Theliderma). In addition to Cyclonaias tuberculata, the Quadrula pustulosa clade identified by these studies includes the following species recognized by Turgeon et al. (1998): Q. asperata, Q. aurea, Q. houstonensis, Q. nodulata, Q. petrina, Q. pustulosa, and Q. refulgens, as well as Fusconaia succissa and Quincuncina infucata (see Fusconaia and Quincuncina).
The name Quadrula is not available for the Q. pustulosa clade because the type species, Q. quadrula, is a member of another distinct, monophyletic clade (see Quadrula). Graf and Cummings (2007) elevated the generic name Amphinaias Crosse and Fischer, 1894, for the Q. pustulosa clade. The type species for Amphinaias (by original designation) is Unio couchianus Lea, 1860, which has a quadrate shell and sulcus (but lacks pustules) similar to the Q. quadrula clade. This morphology is very different from the rounded, pustulose shells of the Q. pustulosa clade. Quadrula couchiana is considered extinct and genetic data are unavailable; however, we do not consider Amphinaias an available name for the Q. pustulosa clade because of the strongly divergent morphology of the type species. Campbell and Lydeard (2012b) proposed Rotundaria Rafinesque, 1820, as a name for the Q. pustulosa clade, presuming its availability based on statements in Valenciennes (1827). However, Valenciennes noted that Rafinesque had confused two species, one for which he kept Rafinesque's name Unio verrucosa and named the other Unio tuberculosa [sic]. As such, Valenciennes's statement cannot be accepted as a subsequent designation of Obliquaria tuberculata Rafinesque, 1820, as the type species of Rotundaria (P. Bouchet, Muséum National d'Histoire Naturelle, Paris, personal communication), and Herrmannsen (1848) later designated Obliquaria subrotunda Rafinesque, 1820, as the type species of Rotundaria. Rafinesque did not select a type species for Rotundaria and because more than one species was included by him in the genus, the type species cannot be fixed by monotypy. Therefore, Rotundaria is not available for the Q. pustulosa clade. Frierson (1927) erected the subgenus Bullata for Q. pustulosa but realized this was preoccupied and created the replacement name Pustulosa with the same type species. Thus, Cyclonaias becomes the oldest available name for this group.
Of the 10 species discussed above as members of Cyclonaias, three were not recognized by Turgeon et al. (1998) (C. archeri, C. kieneriana, and C. kleiniana), and one was considered a subspecies (C. mortoni, as Quadrula pustulosa mortoni). Graf and Cummings (2007) elevated Q. archeri from synonymy with Q. asperata, but they provided no justification for this change. The distinctiveness of C. archeri was recognized by Williams et al. (2008) based on its morphology, absence of intergrades, and isolated and restricted distribution. We recognize C. archeri. The distinctiveness of C. kieneriana was recognized by Williams et al. (2008) based on shell morphology; however, it was not supported by molecular data (Serb et al. 2003), but that study included only one specimen of this putative taxon. We recognize C. kieneriana until additional information becomes available (see Williams et al. 2008). Cyclonaias kleiniana was synonymized under Quincuncina infucata by Clench and Turner (1956), but molecular studies supported the distinctiveness of these species and their inclusion in Cyclonaias (Lydeard et al. 2000; Campbell and Lydeard 2012b).
Molecular data supported the distinctiveness of C. mortoni from C. pustulosa (Serb et al. 2003). In summary, we recognize Cyclonaias as including 14 species: C. tuberculata, seven species recognized by Turgeon et al. (1998) under Quadrula, one subspecies recognized by Turgeon et al. (1998) but now elevated to species status (C. mortoni), two species recognized by Turgeon et al. (1998) in different genera (C. infucata and C. succissa), and three species elevated from synonymy (C. archeri, C. kieneriana, and C. kleiniana).
Cyprogenia Agassiz, 1852.—Turgeon et al. (1998) recognized two species. Subsequent molecular data suggested cryptic species diversity in the genus (Serb and Barnhart 2008; Grobler et al. 2011). The most recent molecular analysis of Cyprogenia identified three independent evolutionary lineages: C. aberti in the Ozark drainages of Arkansas, Missouri, and Kansas; C. stegaria in the Ohio River Basin; and a third lineage in the Ouachita River drainage in Arkansas (Chong et al. 2016). Confusion regarding the type locality of Unio lamarckianus Lea, 1852, requires resolution to determine whether that name is available for the Ouachita River drainage population. We recognize the distinctiveness of this species but defer including it in our list until a specific epithet can be designated.
Cyrtonaias Crosse and Fischer, 1894.—Turgeon et al. (1998) recognized one species, Cyrtonaias tampicoensis, and recent evidence supports no changes to this classification. Five other species are recognized, all of which occur in Mexico or Central America (Graf and Cummings 2017).
Disconaias Crosse and Fischer, 1894.—Turgeon et al. (1998) recognized one species, Disconaias salinasensis Simpson in Dall, 1908, which was subsequently placed in the synonymy of Disconaias fimbriata by Graf and Cummings (2007). Five other species are recognized, all of which occur in Mexico (Graf and Cummings 2017). We recognize Disconaias fimbriata as the only species of the genus occurring in the United States (Rio Grande drainage).
Dromus Simpson, 1900.—Turgeon et al. (1998) recognized one species, Dromus dromas, and recent evidence supports no changes to this classification.
Ellipsaria Rafinesque, 1820.—Turgeon et al. (1998) recognized one species, Ellipsaria lineolata, and recent evidence supports no changes to this classification.
Elliptio Rafinesque, 1819.—Turgeon et al. (1998) recognized 36 species, making it the largest unionid genus in the United States and Canada, but species concepts within this group remain mostly untested, and their highly variable shell morphology precludes traditional approaches for species diagnosis. Recent molecular studies have largely supported the monophyly of Elliptio with two exceptions (Campbell et al. 2005; Campbell and Lydeard 2012b; Perkins et al. 2017). Elliptio dilatata, which is morphologically and anatomically similar to many Elliptio, is not a member of this group; we recognize reassignment of this species to Eurynia (Campbell and Lydeard 2012b). We also recognize reassignment of Elliptio steinstansana to Parvaspina based on molecular data (Perkins et al. 2017). It is important to note that phylogenetic affinities remain unknown for most species that we currently recognize under Elliptio and some may prove to be members of other genera (e.g., Eurynia; Elderkin et al. 2008; Campbell and Lydeard 2012b).
Because of our poor understanding of species diversity within Elliptio, we largely retain the classification of Turgeon et al. (1998) with the following exceptions. We stress that this classification is provisional and meant to provide a stable, working hypothesis for diversity within the genus. We elevate from synonymy four species of Elliptio: E. fumata (from E. complanata), E. occulta and E. pullata (from E. icterina), and E. purpur ella (from E. arctata and E. strigosa); these changes are based primarily on differences in shell morphology (Brim Box and Williams 2000; Williams et al. 2008, 2011, 2014). We place eight species into synonymy. Four Atlantic Slope species (E. errans, E. hepatica, E. lugubris, and E. raveneli) were recognized by Turgeon et al. (1998) based on Davis and Mulvey (1993). The research by Davis and Mulvey (1993) was confined almost exclusively to the Savannah River drainage and has no context within the greater Atlantic Coast region. The validity of these species has not been evaluated further. We return these species to synonymy following Johnson (1970) as follows: E. errans is synonymized under E. complanata; and E. hepatica, E. lugubris, and E. raveneli are synonymized under E. icterina. We place Elliptio waccamawensis into the synonymy of E. congaraea based on molecular data (McCartney et al. 2016). We place the following species into synonymy based on examination of shell type material by Clarke (1992) and Williams et al. (2011, 2014): E. waltoni (synonymized under E. ahenea), E. judithae (synonymized under E. roanokensis), and E. buckleyi (synonymized under E. jayensis). After these changes, we recognize 30 species of Elliptio, and it remains the largest unionid genus in the United States and Canada.
Turgeon et al. (1998) listed the common names Flat Spike and Florida Shiny Spike for Elliptio jayensis and E. buckleyi, respectively. We follow the recommendation of Williams et al. (2014) that the common name of E. jayensis be changed to Florida Spike because the species is largely endemic to that state and is neither consistently flat nor shiny.
Elliptoideus Frierson, 1927.—Turgeon et al. (1998) recognized one species, Elliptoideus sloatianus, and recent evidence supports no changes to this classification.
Epioblasma Rafinesque, 1831.—Turgeon et al. (1998) recognized 20 species and five subspecies. Our changes to this classification involve recognition of two newly described cryptic species, elevating one species from synonymy, and elevating subspecies to species status. We recognize Epioblasma ahlstedti Jones and Neves, 2010, a cryptic species formerly included within E. capsaeformis, and we recognize and elevate to species status Epioblasma aureola Jones and Neves, 2010, formerly identified as E. florentina walkeri but described as E. florentina aureola Jones and Neves, 2010.
Epioblasma cincinnatiensis was not recognized by Turgeon et al. (1998), and it has been considered a synonym (e.g., Parmalee and Bogan 1998) or a subspecies (e.g., Morrison 1942) of Epioblasma torulosa. Williams et al. (2008) elevated this species from synonymy based on examination of shell type material. Watters et al. (2009) also recognized this taxon but placed it in the synonymy of Epioblasma phillipsii (Conrad, 1835). However, E. phillipsii is considered a synonym of Obliquaria reflexa (see Williams et al. 2008). We follow Williams et al. (2008) in recognizing E. cincinnatiensis.
Turgeon et al. (1998) recognized eight subspecies of Epioblasma in three nominal species: florentina (three), obliquata (two), and torulosa (three). A conclusive assessment of the taxonomic status of these taxa may be impossible at this time because half are considered extinct (E. florentina florentina, E. f. curtisii, E. torulosa torulosa, and E. t. gubernaculum). Cummings and Berlocher (1990) found no evidence of intergradation between E. t. torulosa and E. t. rangiana and both taxa co-occurred at many sites; based on this evidence, we elevate these subspecies to species status. Epioblasma aureola and E. walkeri represent morphologically and genetically distinct sister taxa (Jones and Neves 2010, as E. florentina aureola and E. florentina walkeri). These taxa appear to be restricted to two different river systems (Tennessee and Cumberland, respectively); based on the low probability of exchange between these populations and their distinctiveness, we recognize and elevate to full species status E. aureola and E. walkeri. There is little information with which to assess the taxonomic status of E. florentina florentina, E. florentina curtisii, E. obliquata obliquata, E. obliquata per obliqua, and E. torulosa gubernaculum, but all have distinctive shell morphology or occupy distinct geographical regions and we recognize all these taxa as distinct species (see Methods).
We recognize 28 Epioblasma species, making it the second largest unionid genus in the United States and Canada.
Eurynia Rafinesque, 1820.—Eurynia was not recognized in Turgeon et al. (1998). Eurynia was elevated from synonymy by Campbell and Lydeard (2012b) to accommodate Elliptio dilatata, which consistently falls outside the Elliptio clade in molecular analyses (see also Perkins et al. 2017). We consider Eurynia monotypic at this time, but more inclusive molecular studies may identify other species that belong to this genus, including some now assigned to Elliptio (Elderkin et al. 2008; Campbell and Lydeard 2012b).
Fusconaia Simpson, 1900.—Turgeon et al. (1998) recognized 13 species. Several studies showed that the genus Fusconaia as portrayed by Turgeon et al. (1998) was polyphyletic (Lydeard et al. 2000; Serb et al. 2003; Campbell et al. 2005; Campbell and Lydeard 2012a, 2012b; Pfeiffer et al. 2016). Based on these results, we reassign three species recognized by Turgeon et al. (1998) to other genera: F. succissa to Cyclonaias, F. barnesiana to Pleuronaia, and F. ebenus to Reginaia. Pleuronaia was resurrected to accommodate F. barnesiana, along with two other species in the clade (Williams et al. 2008; Campbell and Lydeard 2012a, 2012b; see Pleuronaia). Reginaia was described to accommodate F. ebenus and two other species (Campbell and Lydeard 2012a; see Reginaia).
These studies also showed that several species assigned to other genera belonged in Fusconaia. Based on these results, Quincuncina is a junior synonym of Fusconaia, and we reassign Q. burkei and Q. mitchelli to Fusconaia (Lydeard et al. 2000; Serb et al. 2003; Campbell et al. 2005; Pfeiffer et al. 2016; see Cyclonaias, Quadrula, and Quincuncina). Lexingtonia was placed in the synonymy of Fusconaia when its type species, L. subplana, was determined a junior synonym of Fusconaia masoni based on molecular data (Bogan et al. 2003).
Fusconaia chunii was not recognized by Turgeon et al. (1998), but they recognized two other Fusconaia from Texas: F. askewi and F. lananensis. Subsequent molecular data showed that all Fusconaia in Texas drainages from the Sabine River west belonged to a single species (Burlakova et al. 2012). However, Unio chunii Lea, 1861, has priority over Unio askewi Marsh, 1896, and Quadrula lananensis Frierson, 1901, so we place F. askewi and F. lananensis in the synonymy of F. chunii.
We adopt the former common name for F. askewi, Texas Pigtoe, for F. chunii because it is descriptive of the species' range. Turgeon et al. (1988) listed the common name Gulf Pigtoe for Fusconaia cerina, but it was changed to Southern Pigtoe in Turgeon et al. (1998) without comment. However, Turgeon et al. (1998) also used Southern Pigtoe as the common name of Pleurobema georgianum. We designate the common name Gulf Pigtoe for F. cerina.
In summary, we recognize 11 species of Fusconaia, including eight species recognized by Turgeon et al. (1998) under Fusconaia, two species recognized by Turgeon et al. (1998) in other genera, and one species elevated from synonymy.
Glebula Conrad, 1853.—Turgeon et al. (1998) recognized one species, Glebula rotundata, and recent evidence supports no changes to this classification.
Gonidea Conrad, 1857.—Turgeon et al. (1998) recognized one species, Gonidea angulata, and recent evidence supports no changes to this classification.
Hamiota Roe and Hartfield, 2005.—Hamiota was described subsequent to Turgeon et al. (1998) to accommodate a monophyletic clade of four species that produce super-conglutinates (Roe et al. 2001). They were previously recognized under Lampsilis: L. altilis, L. australis, L. perovalis, and L. subangulata (Roe and Hartfield 2005). We recognize all four of these species under Hamiota.
Hemistena Rafinesque, 1820.—Turgeon et al. (1998) recognized one species, Hemistena lata, and recent evidence supports no changes to this classification.
Lampsilis Rafinesque, 1820.—Turgeon et al. (1998) recognized 28 species and four subspecies. Molecular data indicated that Lampsilis, as presented by Turgeon et al. (1998), is polyphyletic (Graf and Ó Foighil 2000; Campbell et al. 2005). There are likely unrecognized taxa in the genus Lampsilis (e.g., in Arkansas; Harris et al. 2009). The genus Hamiota was described to accommodate a monophyletic clade of four species, Lampsilis altilis, L. australis, L. perovalis, and L. subangulata (Roe and Hartfield 2005), and we recognize reassignment of these species from Lampsilis to Hamiota. We also recognize reassignment of Lampsilis haddletoni to Obovaria (Williams et al. 2008; see Obovaria). In addition to Hamiota, molecular data suggested the existence of at least two other paraphyletic clades within Lampsilis as recognized by Turgeon et al. (1998). Lampsilis cardium, L. ornata, and L. ovata formed a monophyletic clade sister to Hamiota, and L. siliquoidea and L. teres were members of a clade sister to the latter two groups; however, these groupings were not consistently or strongly supported, and the analyses did not include other species of putative Lampsilis (Campbell et al. 2005). Additional generic-level changes regarding Lampsilis will likely occur in the future, but we retain traditional use of this genus for all species except those reassigned to Hamiota and Obovaria.
Lampsilis floridensis was not recognized by Turgeon et al. (1998), and formerly it was recognized as a subspecies (Clench and Turner 1956) or synonym (Burch 1975) of Lampsilis teres. We recognize L. floridensis as a full species based on shell morphology, unpublished molecular data, and its allopatric distribution (Williams et al. 2008).
Turgeon et al. (1998) recognized nominal Lampsilis reeveiana along with two subspecies, L. r. brevicula and L. r. brittsi. Molecular data showed that brittsi populations from the Missouri River drainage formed a well-supported monophyletic clade separate from nominal reeveiana, but there was no morphological or genetic distinction between nominal L. reeveiana and L. r. brevicula (Harris et al. 2004). Based on these data, we follow McMurray et al. (2012) in recognizing L. brittsi and L. reeveiana as species and placing L. reeveiana brevicula into the synonymy of L. reeveiana.
Turgeon et al. (1998) recognized nominal Lampsilis radiata and one subspecies, L. r. conspicua. However, molecular and shell morphology data did not support the distinctiveness of L. r. conspicua (Stiven and Alderman 1992), and we place this taxon into the synonymy of Lampsilis radiata. Turgeon et al. (1998) also recognized Lampsilis fullerkati, but we recognize placement of that species into the synonymy of L. radiata based on molecular data (McCartney et al. 2016).
Turgeon et al. (1998) recognized nominal Lampsilis straminea and one subspecies, L. s. claibornensis. Lampsilis straminea straminea is restricted to the Black Belt Prairie region of Alabama and Mississippi and is characterized by a profusion of fine, concentric ridges on the shell, which are absent in L. s. claibornensis. However, concentric ridges are present in some other mussels inhabiting streams in the Black Belt Prairie region and are most likely environmentally induced and not due to genetic differences (Williams et al. 2008). We do not recognize the taxonomic validity of these shell forms and place L. s. claibornensis in the synonymy of Lampsilis straminea. The common name of Lampsilis s. straminea, Rough Fatmucket (Turgeon et al. 1998), is descriptive of individuals in only a small portion of its range (i.e., the Black Belt Prairie). Therefore, we retain the common name for L. straminea claibornensis, Southern Fatmucket, for L. straminea.
In summary, we recognize 24 species of Lampsilis including one species elevated from synonymy and two species elevated from subspecies. Lampsilis is the third largest genus in the family Unionidae following Elliptio (30) and Epioblasma (28).
Lasmigona Rafinesque, 1831.—Turgeon et al. (1998) recognized six species and one subspecies. Williams et al. (2008) elevated Lasmigona complanata alabamensis to species status based on examination of museum shell material, and molecular data supported the distinctiveness of this taxon (King et al. 1999). Williams et al. (2008) also recognized Mobile Basin populations of Lasmigona holstonia as a distinct species based on unpublished molecular data and the occurrence of these populations in two different river systems. They resurrected from synonymy Lasmigona etowaensis to refer to Mobile Basin populations and retained L. holstonia to refer to Tennessee and Ohio River drainage populations. We recognize all three of these species.
Molecular studies showed that Lasmigona is polyphyletic: L. alabamensis, L. complanata, and L. costata formed a monophyletic clade, and L. compressa and L. subviridis represented another monophyletic clade more closely related to Alasmidonta (King et al. 1999). However, this study did not include all species of Lasmigona, and a broader study within the context of the tribe Anodontini is needed to clarify these relationships. Populations of Lasmigona costata in the Ozark Highlands represented a monophyletic clade strongly differentiated from populations east of the Mississippi River, suggesting the presence of at least one cryptic species within this taxon; additional investigation across the range of L. costata is needed to better understand these patterns (Hewitt et al. 2016). An endemic form of Lasmigona in the Barrens region of the upper Caney Fork drainage in Tennessee was recognized by Layzer et al. (1993), but the status of this putative taxon has not been evaluated further.
Lemiox Rafinesque, 1831.—Turgeon et al. (1998) recognized one species, Lemiox rimosus, and recent evidence supports no changes to this classification.
Leptodea Rafinesque, 1820.—Turgeon et al. (1998) recognized three species, and recent evidence supports no changes to this classification. Smith (2000) proposed moving Leptodea ochracea into the genus Ligumia based on mantle margin pigment and size of glochidia. We do not accept this proposal due to the limited number of taxa (four species in two genera) in that analysis, and we retain ochracea in Leptodea.
Lexingtonia Ortmann, 1914.—Turgeon et al. (1998) recognized two species. However, the type species, Lexingtonia subplana, was subsequently relegated to the synonymy of Fusconaia masoni based on Johnson (1970) and Bogan et al. (2003). As such, Lexingtonia is a junior synonym of Fusconaia. The other species recognized by Turgeon et al. (1998), Lexingtonia dolabelloides, did not group with Fusconaia in molecular analyses but formed a monophyletic clade with two other species (Campbell et al. 2005; Campbell and Lydeard 2012a, 2012b). Pleuronaia was resurrected by Williams et al. (2008) to accommodate this clade (see Pleuronaia).
Ligumia Swainson, 1840.—Turgeon et al. (1998) recognized three species. Subsequent molecular studies indicated the genus is not monophyletic, but further research is needed to fully elucidate these patterns (Campbell et al. 2005; Kuehnl 2009). We retain the classification of Turgeon et al. (1998), but as additional information becomes available taxa assigned to this genus will likely change (see Raley et al. 2007). Gangloff et al. (2013) identified a genetically divergent clade of Ligumia recta from the Mobile Basin that may warrant recognition as a distinct taxon.
Medionidus Simpson, 1900.—Turgeon et al. (1998) recognized seven species. We no longer recognize Medionidus mcglameriae, which was placed in the synonymy of Leptodea fragilis based on examination of the type specimen (Williams et al. 2008). Campbell et al. (2005) found some evidence for polyphyly of Medionidus, but this evidence was not conclusive and we make no other changes to this genus.
Megalonaias Utterback, 1915.—Turgeon et al. (1998) recognized one species, Megalonaias nervosa, and recent evidence supports no changes to this classification.
Obliquaria Rafinesque, 1820.—Turgeon et al. (1998) recognized one species, Obliquaria reflexa, and recent evidence supports no changes to this classification.
Obovaria Rafinesque, 1819.—Turgeon et al. (1998) recognized six species. Molecular data showed that Obovaria as depicted by Turgeon et al. (1998) is polyphyletic (Campbell et al. 2005). Notably, Obovaria rotulata was not a member of this group, and it was later reassigned to Reginaia (Campbell and Lydeard 2012b); we recognize this reassignment. In an analysis by Campbell et al. (2005), O. olivaria fell outside the clade containing other Obovaria and Epioblasma, but this conclusion was not consistently supported. We retain olivaria within Obovaria, but further work on this species is needed to resolve its generic assignment.
Evidence also supports reassignment to Obovaria of species recognized by Turgeon et al. (1998) under other genera. We reassign Villosa arkansasensis and V. choctawensis to Obovaria based on molecular data (Kuehnl 2009; Inoue et al. 2013) and marsupial morphology (Williams et al. 2011, for choctawensis). We also recognize reassignment of Lampsilis haddletoni to Obovaria based on shell morphology of the type lot (Williams et al. 2008, 2011), but this species is considered extinct and there are no available soft parts for anatomical or molecular study. Obovaria jacksoniana was recognized in Turgeon et al. (1998) but is synonymous with Villosa arkansasensis (Inoue et al. 2013). Unio jacksoniana Frierson, 1912, is a junior synonym of Unio arkansasensis Lea, 1862, and we place O. jacksoniana in the synonymy of Obovaria arkansasensis. There is also potential for unrecognized taxa within O. arkansasensis in central Gulf Slope drainages (Inoue et al. 2013).
In summary, we recognize seven species of Obovaria, including four species recognized by Turgeon et al. (1998) and three species reassigned from other genera, one from Lamp silis and two from Villosa.
Parvaspina Perkins, Gangloff, and Johnson, 2017.— Parvaspina was described subsequent to Turgeon et al. (1998) to accommodate a monophyletic clade of two species previously recognized as Elliptio steinstansana and Pleurobema collina (Perkins et al. 2017). We recognize these species as Parvaspina steinstansana and Parvaspina collina.
Pegias Simpson, 1900.—Turgeon et al. (1998) recognized one species, Pegias fabula, and recent evidence supports no changes to this classification.
Plectomerus Conrad, 1853.—Turgeon et al. (1998) recognized one species, Plectomerus dombeyanus, and recent evidence supports no changes to this classification.
Plethobasus Simpson, 1900.—Turgeon et al. (1998) recognized three species, and recent evidence supports no changes to this classification.
Pleurobema Rafinesque, 1819.—Turgeon et al. (1998) recognized 32 species, making it one of the largest unionid genera. Molecular data largely support the monophyly of Pleurobema as depicted by Turgeon et al. (1998) with two exceptions (Campbell et al. 2005, 2008; Campbell and Lydeard 2012b). These studies support reassignment of P. collina to Parvaspina and P. gibberum to Pleuronaia (Campbell et al. 2005, 2008; Campbell and Lydeard 2012b; see Parvaspina and Pleuronaia). However, Campbell et al. (2008) and Campbell and Lydeard (2012b) provided evidence that Pleurobema includes two distinct lineages, one including P. sintoxia, P. cordatum, P. plenum, P. riddellii, and P. rubrum and the other including all other species. Further research is needed to elucidate these relationships; we retain traditional use of Pleurobema.
Pleurobema rivals Elliptio in its large number of described species and the intractability of many species concepts, particularly in the Mobile Basin, but these problems are compounded for Pleurobema because many putative taxa are considered extinct. Based on a comprehensive comparison of shell type specimens and other available material, Williams et al. (2008) placed into synonymy nine species of Mobile Basin Pleurobema recognized by Turgeon et al. (1998): P. chattanoogaense (into P. decisum); P. murrayense (into P. stabile); P. nucleopsis and P. troschelianum (into P. georgianum); P. flavidulum and P. johannis (into P. perovatum); and P. avellanum, P. furvum, and P. hagleri (into P. rubellum). Some of these synonyms are further supported by molecular data (e.g., P. chattanoogaense, P. furvum; Campbell et al. 2008), and we recognize all of these changes. We do not recognize Pleurobema altum since it was deemed a nomen dubium because it is not identifiable due to incomplete description, vague type locality, and lack of type material (Williams et al. 2008). One Ohio River drainage species, Pleurobema bournianum, was placed into the synonymy of Pleurobema clava based on shell morphology (Watters et al. 2009), and we recognize this change.
We recognize four additional Mobile Basin species of Pleurobema not recognized by Turgeon et al. (1998). Williams et al. (2008) recognized three species based on examination of shell type specimens: P. fibuloides, P. hartmanianum, and P. stabile. We correct the spelling of P. stabilis as used by Williams et al. (2008) to stabile based on Lee (2008). Pleurobema athearni Gangloff, Williams, and Feminella, 2006, was described subsequent to Turgeon et al. (1998) based on morphological data (Gangloff et al. 2006). In addition, preliminary findings identified an undescribed species in the upper Tennessee River drainage (Schilling 2015).
In summary, we recognize 23 species of Pleurobema, including 19 species recognized by Turgeon et al. (1998), three species elevated from synonymy, and one newly described species.
Pleuronaia Frierson, 1927.—Pleuronaia was not included in Turgeon et al. (1998). This was the senior available name for a monophyletic clade of three species—Fusconaia barnesiana, Lexingtonia dolabelloides, and Pleurobema gibberum—identified in a molecular study by Campbell et al. (2005). We recognize resurrection of Pleuronaia to accommodate this group and reassignment of these three species to Pleuronaia as proposed previously (Williams et al. 2008; Campbell and Lydeard 2012a, 2012b). There are likely cryptic taxa of Pleuronaia in the upper Tennessee River drainage (Schilling 2015). We correct the gender agreement of the specific name of Pleuronaia gibberum to gibber (H. Lee, Jacksonville, Florida, personal communication).
Popenais Frierson, 1927.—Turgeon et al. (1998) recognized one species, Popenais popeii, and recent evidence supports no changes to this classification.
Potamilus Rafinesque, 1818.—Turgeon et al. (1998) recognized six species. One additional species, Potamilus metnecktayi Johnson, 1998, was described subsequently, and we recognize this species. Potamilus inflatus was referred to as the Inflated Heelsplitter by Turgeon et al. (1988) but was changed to Alabama Heelsplitter by Turgeon et al. (1998) without comment. Alabama Heelsplitter is the established common name for Lasmigona alabamensis, and we adopt the original common name Inflated Heelsplitter for P. inflatus. Roe and Lydeard (1998) found the Amite River population of P. inflatus to be genetically divergent, and it may warrant recognition as a distinct taxon.
Ptychobranchus Simpson, 1900.—Turgeon et al. (1998) recognized five species. Ptychobranchus foremanianus was elevated from the synonymy of Ptychobranchus greenii (in part) by Williams et al. (2008) based on shell morphology and periostracum color. A molecular analysis of this genus included insufficient material to resolve the relationship between these two taxa (Roe 2013), but we recognize both species. We correct the gender agreement of Ptychobranchus subtentum to P. subtentus following Lee (2008).
Pyganodon Crosse and Fischer, 1894.—Turgeon et al. (1998) recognized five species. Graf and Cummings (2007) without comment moved Anodonta implicata to Pyganodon and omitted P. fragilis and P. lacustris. However, molecular data demonstrated the validity of P. fragilis and P. lacustris (Doucet-Beaupré et al. 2012). Based on these results and the lack of justification for movement of A. implicata to Pyganodon, we retain the classification of Turgeon et al. (1998) for Pyganodon.
Quadrula Rafinesque, 1820.—Turgeon et al. (1998) recognized 18 species and two subspecies. Molecular studies generally supported the monophyly of Quadrula as depicted by Turgeon et al. (1998), but they also showed that it is composed of three deeply divergent monophyletic clades plus Tritogonia verrucosa, each of which warranted generic recognition (Serb et al. 2003; Campbell et al. 2005; Campbell and Lydeard 2012b). The type species for Quadrula is Q. quadrula, and the clade containing this species also includes Q. apiculata, Q. fragosa, Q. nobilis, and Q. rumphiana. Quadrula nobilis was elevated from synonymy based on shell morphology and unspecified genetic data (Howells et al. 1996) but not recognized by Turgeon et al. (1998). Relationships among species in the Q. quadrula group were not clearly resolved by Serb et al. (2003), but we recognize all five species. We also recognize within this group Q. couchiana on the basis of its shell morphology, which is similar to that of Q. quadrula (see Cyclonaias).
Based on molecular data, we reassign to Cyclonaias 10 taxa recognized by Turgeon et al. (1998) under Quadrula, and we reassign 5 species to Theliderma (Serb et al. 2003; Campbell et al. 2005; Campbell and Lydeard 2012b; see also Graf and Cummings 2007). We also synonymize two taxa recognized by Turgeon et al. (1998) under Quadrula (see Theliderma). In summary, we recognize six species of Quadrula, including five recognized under this genus by Turgeon et al. (1998) and one elevated from synonymy (Q. nobilis).
Quincuncina Ortmann, 1922.—Turgeon et al. (1998) recognized three species. Molecular data showed that the type species, Quincuncina burkei, belongs in Fusconaia (Lydeard et al. 2000; Serb et al. 2003; Campbell et al. 2005). As such, Quincuncina is a junior synonym of Fusconaia, and we reassign to this genus Q. burkei and Q. mitchelli (see also Pfeiffer et al. 2016). Based on these findings, we also reassign Q. infucata to Cyclonaias (see Cyclonaias).
Reginaia Campbell and Lydeard, 2012.—Reginaia was described subsequent to Turgeon et al. (1998) to accommodate a monophyletic clade of two species identified in a phylogenetic analysis of Ambleminae (Campbell and Lydeard 2012b). The two Reginaia species were included in Turgeon et al. (1998) as Fusconaia ebena and Obovaria rotulata (Campbell and Lydeard 2012b); we recognize assignment of these species to Reginaia. We follow Watters et al. (2009) in correcting the spelling of the species name ebena to ebenus. A third species, Fusconaia apalachicola Williams and Fradkin, 1999, was described subsequent to Turgeon et al. (1998) from archaeological material; we reassign this species to Reginaia based on its shell characters, which are similar to those of R. ebenus and R. rotulata.
Simpsonaias Frierson, 1914.—Turgeon et al. (1998) recognized one species, Simpsonaias ambigua, and recent evidence supports no changes to this classification.
Sinanodonta Modell, 1945.—Sinanodonta was not included in Turgeon et al. (1998). This genus was previously considered to be confined to Asia and not part of the North America fauna. Molecular data showed that A. beringiana is more closely related to the Asian species Sinanodonta woodiana than to other western North American Anodonta (Chong et al. 2008; see Anodonta). Based on this evidence, we reassign beringiana to Sinanodonta. In 2010 S. woodiana, Chinese Pondmussel, was found in Wickecheoke Creek, a tributary of the Delaware River, New Jersey (Bogan et al. 2011a). Several known glochidial host fishes, native and introduced species, occur in the watershed (Bogan et al. 2011b). The species appears to have become established in that stream despite eradication efforts (J. Bowers-Altman, New Jersey Division of Fish and Wildlife, personal communication). We recognize S. woodiana as established in New Jersey (Table 2). This is the only nonindigenous unionid mussel known to have become established in the United States or Canada.
Strophitus Rafinesque, 1820.—Turgeon et al. (1998) recognized three species, and recent evidence supports no changes to this classification. Strophitus undulatus, one of the most wide-ranging species in the United States and Canada, likely contains unrecognized cryptic taxa (Watters et al. 2009).
Theliderma Swainson, 1840.—Theliderma was not recognized by Turgeon et al. (1998). This genus was resurrected from synonymy by Graf and Cummings (2007) to accommodate a monophyletic clade of five species recognized by Turgeon et al. (1998) under Quadrula (Q. cylindrica, Q. intermedia, Q. metanevra, Q. sparsa, and Q. stapes; see Serb et al. 2003). Theliderma is the oldest available name for this clade and has T. metanevra as its type species. We recognize placement of all five of these species in Theliderma. No molecular data are available for Theliderma stapes, but its shell morphology is very similar to that of other Theliderma, and we include it in this genus following Graf and Cummings (2007).
Turgeon et al. (1998) recognized Quadrula tuberosa, but we place this taxon in the synonymy of Theliderma metanevra following Parmalee and Bogan (1998, as Q. metanevra). However, the relationship of tuberosa to other species is uncertain, and if it represents a valid species, it is considered extinct (see Haag and Cicerello 2016). Quadrula cylindrica was recognized in Turgeon et al. (1998) as containing two subspecies, Theliderma cylindrica cylindrica and T. cylindrica strigillata. These subspecies traditionally were distinguished from each other based on shell morphology and distribution, with strigillata being confined mainly to the upper Tennessee River system in Tennessee and Virginia (Parmalee and Bogan 1998). However, the distributional limits of strigillata have never been clearly defined as it grades into typical T. c. cylindrica in larger streams, suggesting that the shell forms represent ecophenotypic variation (Ortmann 1920), and molecular data provide no support for recognition of T. c. strigillata (Serb et al. 2003; Sproules et al. 2006). Based on this evidence, we do not recognize subspecies within T. cylindrica. Both T. c. cylindrica (threatened) and T. c. strigillata (endangered) are federally protected taxa. Synonymizing strigillata under T. cylindrica will not remove the protection provided by the Endangered Species Act but may impact the status of populations formerly recognized as strigillata.
Toxolasma Rafinesque, 1831.—Turgeon et al. (1998) recognized eight species. Recent evidence supports no changes at the genus level, but species boundaries within Toxolasma remain uncertain. Howells et al. (1996) placed Toxolasma mearnsi in the synonymy of Toxolasma texasiense based on electrophoretic analysis, a change overlooked by Turgeon et al. (1998); we recognize placement of T. mearnsi in the synonymy of T. texasiense. Undescribed species of Toxolasma have been recognized (e.g., Gulf Lilliput) but have yet to be formerly described (Williams et al. 2008, 2014).
Lee (2006) concluded that Toxolasma has a neuter gender, which necessitates correction of spellings from lividus to lividum, parvus to parvum, and paulus to paulum, without change to corvunculus, cylindrellus, or pullus; we recognize these spelling changes. Lee (2006) provided an incorrect spelling of Toxolasma texasiense (as texasense), but we correct it based on the spelling presented in the original description.
Tritogonia Agassiz, 1852.—Turgeon et al. (1998) recognized one species, Tritogonia verrucosa. Molecular data clearly supported inclusion of T. verrucosa within the tribe Quadrulini, but its placement within that group was unresolved, and Serb et al. (2003) recommended its placement within Quadrula (sensu lato) until relationships were better understood (e.g., see Williams et al. 2008; Haag and Cicerello 2016). Regardless of its relationship to other clades within the Quadrulini, Tritogonia represents a deeply divergent lineage (Serb et al. 2003; Campbell et al. 2012b), and our recognition of three other genera within this tribe (Cyclonaias, Theliderma, and Quadrula sensu stricto) warrants retention of Tritogonia as a monotypic genus (e.g., see Watters et al. 2009; Sietman et al. 2012).
Truncilla Rafinesque, 1819.—Turgeon et al. (1998) recognized four species, and recent evidence supports no changes to this classification.
Uniomerus Conrad, 1853.—Turgeon et al. (1998) recognized three species. Recent evidence supports no changes at the genus level, but species concepts within Uniomerus are uncertain (see Williams et al. 2008). Uniomerus columbens is was not recognized by Turgeon et al. (1998) but was elevated from synonymy by Williams et al. (2008) based on unpublished molecular data and shell morphology; we recognize this change. Species boundaries for other taxa (e.g., Uniomerus declivis) remain unresolved.
The inappropriate and misleading common name for Uniomerus carolinianus, Florida Pondhom, was changed to Eastern Pondhom by Williams et al. (2014) because the species occurs not only in Florida but northward along the Atlantic Coast; we recognize this change.
Utterbackia Baker, 1927.—Turgeon et al. (1998) recognized three species and recent evidence supports no changes to this classification.
Utterbackiana Frierson, 1927.—Utterbackiana was not recognized by Turgeon et al. (1998). We resurrect this genus as the senior available name for a monophyletic clade of four eastern North American species included in Turgeon et al. (1998) under Anodonta (A. couperiana, A. heardi, A. implicata, and A. suborbiculata; Mock et al. 2004; Zanatta et al. 2007; see Anodonta). The type species for the genus is Anodonta suborbiculata Say, 1831. In addition to the four taxa mentioned above, a new species was described subsequent to Turgeon et al. (1998), Anodonta hartfieldorum (Williams et al. 2009). We also place this species in Utterbackiana because it appears closely related to U. suborbiculata and was formerly associated with that species.
Venustaconcha Frierson, 1927.—Turgeon et al. (1998) recognized two species. Molecular data showed that Villosa perpurpurea and Villosa trabalis also are members of Venustaconcha (Kuehnl 2009; Lane et al. 2016). Molecular data further showed that Venustaconcha perpurpurea is a junior synonym of Venustaconcha trabalis, and populations of this species in the Tennessee River drainage are genetically and morphologically distinct from those in the Cumberland River drainage (Lane et al. 2016). Based on the type locality of trabalis, Flint River, Alabama, this name is applicable to the Tennessee River drainage species. Unio troostensis Lea, 1834, is the oldest available name for the Cumberland drainage species (type locality is Stones River, Tennessee), and we recognize this species as Venustaconcha troostensis (see Haag and Cicerello 2016; Lane et al. 2016). Cumberland Bean was the common name used for V. trabalis by Turgeon et al. (1998), but Lane et al. (2016) proposed Tennessee Bean for Venustaconcha trabalis and Cumberland Bean for Venustaconcha troostensis; we follow this use. Venustaconcha sima was not included in Turgeon et al. (1998) but was elevated from synonymy by Gordon (1995) based on shell coloration and conchological characters, and its distinctiveness is supported by molecular data (Kuehnl 2009). This species was synonymized under Villosa iris by Parmalee and Bogan (1998), and molecular data support its relationship to Villosa (Kuehnl 2009). We recognize sima as a species of Villosa.
Villosa Frierson, 1927.—Turgeon et al. (1998) recognized 17 species and one subspecies. Molecular data show that Villosa, as depicted by Turgeon et al. (1998), is wildly polyphyletic, with species occurring in as many as seven different clades within the Lampsilini (Kuehnl 2009). These and other data support reassignment of Villosa trabalis to Venustaconcha, synonymization of Villosa perpurpurea under Venustachoncha trabalis (see Venustaconcha), and reassignment of Villosa choctawensis and V. arkansasensis to Obovaria (see Obovaria). Most other species will require reassignment to existing genera (e.g., V. vaughniana to Ligumia; Raley et al. 2007; Kuehnl 2009) or resurrected or newly described genera, potentially with only Villosa amygdala and V. villosa remaining in Villosa (Kuehnl 2009). However, these relationships are not fully understood, and currently synonymized or newly described generic names have not been proposed. With the exception of Villosa trabalis, V. perpurpurea, V. choctawensis, and V. arkansasensis, we retain all other species recognized by Turgeon et al. (1998) in Villosa.
Villosa vanuxemensis umbrans was elevated to species status by Williams et al. (2008) based on shell characters and preliminary molecular data, and subsequent molecular data support this change (Kuehnl 2009); based on this evidence, we recognize V. umbrans. There are several undescribed taxa within Villosa (Kuehnl 2009; Harris et al. 2009). We recognize correction of gender agreement for Villosa amygdala, as given by Turgeon et al. (1998), to Villosa amygdalum following Williams et al. (2011, 2014). We recognize fifteen species of Villosa.
DISCUSSION
Changes in mussel taxonomy compared to Turgeon et al. (1998) reflect our better understanding of mussel phylogenetic relationships obtained mainly from molecular genetic data (e.g., Serb et al. 2003; Campbell and Lydeard 2012a, 2012b; Inoue et al. 2013, 2014; Pfeiffer et al. 2016). Molecular genetics continues to be one of the most important tools for understanding unionoid relationships and taxonomy, but other data sets (e.g., life history, host use, soft anatomy, shell morphology, zoogeography) are informative and should not be overlooked when constructing phylogenies and conducting taxonomic studies (e.g., Roe et al. 2001; Jones and Neves 2010; Lane et al. 2016).
We recognize a larger number of genera than Turgeon et al. (1998; 56 vs. 49), but the number of currently recognized species is similar. However, recent studies show that considerable cryptic biodiversity exists in the Unionidae (e.g., Cyprogenia, Lampsilis, Villosa). Most of this biodiversity remains to be discovered, and its future recognition may result in increased numbers of species in the United States and Canada (see Haag 2012). Currently unrecognized species may be narrowly distributed (e.g., one river system) and in need of conservation measures. Development of additional molecular markers, more inclusive taxon sampling, advancements in phylogenetic analyses, and other techniques for species delineation are facilitating taxonomic recognition of species. More thorough understanding of life histories with improved husbandry techniques should also help facilitate species recognition.
Future research will most likely reveal unrecognized taxa. Conversely, additional synonymy may be warranted for some currently recognized species. Much more research is needed to delineate true diversity of the mussels of the United States and Canada.
ACKNOWLEDGMENTS
We thank the following individuals who were always very responsive to our questions regarding names of freshwater mussels: John Alderman, Gerry Dinkins, Mike Gangloff, Dan Graf, Jordan Holcomb, Bob Howells, Sarina Jepsen, Paul Johnson, Stephen McMurray, Terry Myers, Charles Randklev, Kevin Roe, Tim Savidge, Daniel Schilling, Brian Watson, and Jason Wisniewski. We acknowledge Harry G. Lee (Jacksonville, Florida) for providing expert advice on the proper terminations for numerous species names. We also thank Sherry L. Bostick for assistance in preparation and review of several drafts of the manuscript. Although the individuals mentioned here provided assistance and input, we bear full responsibility for any errors. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of their agencies and institutions. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
