Eastern elliptio Elliptio complanata, copyright Smithsonian Environmental Research Center.

Belongs within: Ambleminae.

Pig’s toes and water nymphs
Published 29 September 2008
The Wabash pigtoe, Fusconaia flava, a widespread species of freshwater bivalve found in both the Mississippi and Great Lakes Basins, and the effective type species of its genus (i.e. the actual type species is a synonym of this one). Photo from Mussels of Illinois.

In contrast to the great diversity of freshwater gastropods, surprisingly few groups of bivalves have really made a go of things in the freshwater environment. The superfamily Unionoidea is therefore very distinctive in not only including the largest number of freshwater bivalves, but in fact being only found in freshwater. Fusconaia is a genus of Unionoidea, usually included in the family Unionidae and subfamily Ambleminae, found in many river basins of North America. Members of Fusconaia are rather unfairly lumbered with the vernacular name of “pigtoe”, evidently a reference to the shape of the shell. For some reason, North American unionids have garnered more than their fair share of memorable names—Motley (1933) gives us “heelsplitter”, “elephant’s ear”, and for one particularly unfortunate Fusconaia species, F. ebena, “niggerhead”. Thankfully, that last name seems to have been universally replaced in the present day by “ebonyshell”. Those more charitably disposed towards the family might refer to them as “freshwater pearly mussels” or “naiads”. “Naiad” derives from a classical Greek term for a class of water spirit, so Fusconaia would mean ‘brown nymph’. ‘Brown nymph’ is arguably a slight improvement over ‘pigtoe’.

Other than their memorable names, unionids are most famed for their distinctive life cycles. After fertilisation, female naiads (unionids have separate males and females) brood their eggs in the gills until they hatch out into bivalved larvae with big teeth on the shells called glochidia. Upon release, glochidia attach themselves to the gills of a passing fish and live as parasites, forming a cyst until they hatch out and develop into a mature mussel. At least some unionids actually develop fleshy outgrowths of the gills that mimic a small fish in order to attract a suitable host for their glochidia – I haven’t established whether Fusconaia is one of the unionids that does so. Speaking speculatively, perhaps the biggest advantage unionids gain from this life cycle is overcoming the hurdle of dispersal in fresh water environments. Most marine bivalves have larvae that disperse as plankton, but this is obviously not a suitable option in fresh water, where currents are either more or less non-existent (lakes) or would inevitably carry the larvae into an unsuitable environment (rivers). [Is this why there are so few freshwater bivalves?] Attaching to a fish potentially allows the unionid larva to disperse upstream. Once they do settle down, most unionids grow very slowly—Bruenderman & Neves (1993) found that Fusconaia cuneolus, the fine-rayed pigtoe of Virginia, grew an average of five millimetres per year for the first ten years of its life, then slowed down to a more sedate two millimetres per year. Because of their slow growth, vulnerability to environmental pollutants and changes (they are sessile filter feeders, after all) and reliance on the presence of suitable fish to act as glochidial hosts, many unionid species are critically endangered. About one-eighth of Recent North American amblemine taxa are believed to have become extinct, and another quarter are endangered or threatened (Campbell et al. 2005).

The longsolid, Fusconaia subrotunda. Photo by Gary Peeples.

Establishing exactly which species are threatened is another matter. Taxonomically, unionids can only be described as evil. Graf & Cummings (2007) listed fifteen species in the genus Fusconaia, but many widespread unionid species are decidedly variable. Prior to the beginning of the 1900s and the revisionary work in North America by Simpson and Oortman, a large number of species and subspecies were erected based on what are now regarded as ecological variants of other species. According to Graf & Cummings (2007), no less than 4955 species-group names are available to unionoids for 840 currently-recognised valid taxa, giving an average of nearly five synonyms for every valid species*. Graf (1997, in an unpublished thesis) listed no less than forty species-group names of relevance to the taxonomy of Fusconaia flava, the widespread Wabash pigtoe. Even after narrowing the field down to valid species only, the confusion is not yet over. Campbell et al. (2005) suggested based on molecular analysis that many, if not most, of the currently recongised North American unionid genera are polyphyletic. Fusconaia, originally established by Simpson in 1900 primarily on the basis that its species incubated their eggs in all four gills, included representatives of no less than three tribes of Unionidae.

*More than 1000 of these were introduced for supposed European (mostly French) taxa by the so-called Nouvelle École of Jules-René Bourguignat and his successors in France in the late 1800s (Graf 2007). So extravagant were the efforts of the Nouvelle École that C. T. Simpson, when reviewing the world’s unionid fauna, simply refused to consider them, complaining in 1900 that, “Life is too short and valuable to be wasted in any attempt at deciphering such nonsense“.

Systematics of Pleurobemini
| |--P. beadleanumPM89
| |--P. clava Lam. 1819 (see below for synonymy)V16
| |--P. coccineum (Conrad 1834)OE97
| |--P. collinaPC11
| |--P. conradi Vanatta 1916 [=Unio maculatus Conr. 1834 non U. maculata Rafinesque 1820]V16
| |--P. cordatum (Rafinesque 1820)SS97
| |--P. cyphia (Rafinesque 1820) [=Obliquaria cyphya, Unio cyphia; incl. Pleurobema aesopus Green 1827]V16
| |--P. plenumUSDI77
| |--P. simpsoni Vanatta 1916 [=Unio striatus Lea 1840 non Obovaria striata Rafinesque 1820]V16
| |--P. sintoxia (Rafinesque 1820)BK20
| `--P. strodeanumA64
|--Elliptio Rafinesque 1819D84, B64c
| |--E. arctataD84
| |--E. buckleyiD84
| |--E. chipolaensis (Walker 1905) [=Unio chipolaensis]J68
| |--E. cistelliformis (Lea 1863)G97
| |--E. complanata (Lightfoot 1786)OE97 [incl. Lampsilis virginianaW63]
| | |--E. c. complanataB64a
| | `--E. c. wheatleyi (Lea 1858) [=Unio wheatleyi, U. catawbensis Lea 1862; incl. U. rostrum Lea 1862]B64a
| |--E. congaraeaD84
| |--E. crassidensD84
| | |--E. c. crassidensH64
| | `--E. c. incrassatusH64
| |--E. dilatata (Rafinesque 1820)SS97 (see below for synonymy)
| |--E. fisherianaD84
| |--E. folliculataD84
| |--E. hopetonensisTS65
| |--E. icterinaD84
| |--E. lanceolataD84
| |--E. mcmichaeliD84
| |--E. nigella (Lea 1852) [=Unio nigellus, E. nigellus; incl. U. denigratus Lea 1857, U. purpurellus Lea 1857]J68
| |--E. productaD84
| |--E. shepardianaD84
| |--E. spinosaTS65
| |--E. strigosusH64
| `--E. waccamawensis (Lea 1863)G97
| |--*F. trigonus [=F. undata trigonus]D84
| |--F. cuneolusUSDI77
| |--F. ebena (Lea 1831)WMP89
| |--F. edgarianaUSDI77
| |--F. flava (Rafinesque 1820)SS97
| |--F. ozarkensis (Call 1887)OE97
| `--F. succissaD84
|--U. carolinianusD84
|--U. declivisD84
|--U. excultusD84
`--U. tetralasmus (Say 1831)OE97 [=Elliptio (Uniomerus) tetralasmusB64b]
|--U. t. tetralasmusB64b
`--U. t. sayanus (Conrad 1838) (see below for synonymy)B64b

Elliptio dilatata (Rafinesque 1820)SS97 [=Unio dilatataV16; incl. U. gibbosus Bar. 1823 non Rafinesque 1820V16, Obliquaria sinuata Rafinesque 1820V16, U. sinuataV16]

Pleurobema clava Lam. 1819 [incl. P. cuneata Rafinesque 1820, Unio cuneata, U. elliptica Rafinesque 1820, Elliptio elliptica, Obliquaria scalenia Rafinesque 1820, Unio scalenia]V16

Uniomerus tetralasmus sayanus (Conrad 1838) [=Unio sayanus, Elliptio (Uniomerus) tetralasmus sayana; incl. U. sayi Ward in Tappan 1839]B64b

*Type species of generic name indicated


[A64] Athearn, H. D. 1964. Three new unionids from Alabama and Florida and a note on Lampsilis jonesi. Nautilus 77 (4): 134–139.

[B64a] Baker, H. B. 1964a. Elliptio complanata wheatleyi. Nautilus 78 (2): 67.

[B64b] Baker, H. B. 1964b. E. (Uniomerus) tetralasmus sayana. Nautilus 78 (2): 67.

[B64c] Baker, H. B. 1964c. Elliptio feminine. Nautilus 78 (1): 33.

Bruenderman, S. A., & R. J. Neves. 1993. Life history of the endangered fine-rayed pigtoe Fusconaia cuneolus (Bivalvia: Unionidae) in the Clinch River, Virginia. American Malacological Bulletin 10 (1): 83–91.

Campbell, D. C., J. M. Serb, J. E. Buhay, K. J. Roe, R. L. Minton & C. Lydeard. 2005. Phylogeny of North American amblemines (Bivalvia, Unionoida): prodigious polyphyly proves pervasive across genera. Invertebrate Biology 124 (2): 131–164.

[D84] Davis, G. M. 1984. Genetic relationships among some North American Unionidae (Bivalvia): sibling species, convergence, and cladistic relationships. Malacologia 25 (2): 629–648.

Graf, D. L. 1997a. Morphology, Zoogeography, and Taxonomy of Fusconaia flava (Rafinesque) (Mollusca: Bivalvia: Unionidae) in the Upper Mississippi, Great Lakes, and Nelson River Basins. MSc thesis, Northeastern University, Boston.

[G97b] Graf, D. L. 1997b. Sympatric speciation of freshwater mussels (Bivalvia: Unionoidea): a model. American Malacological Bulletin 14 (1): 35–40.

Graf, D. L. 2007. Palearctic freshwater mussel (Mollusca: Bivalvia: Unionoida) diversity and the Comparatory Method as a species concept. Proceedings of the Academy of Natural Sciences of Philadelphia 156: 71–88.

Graf, D. L., & K. S. Cummings. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Journal of Molluscan Studies 73 (4): 291–314.

[J68] Johnson, R. I. 1968. Elliptio nigella, overlooked unionid from Apalachicola River system. Nautilus 82 (1): 22–24.

Motley, H. L. 1933. Histology of the fresh-water mussel heart with reference to its physiological reactions. Journal of Morphology 54 (2): 415–427.

[OE97] Obermeyer, B. K., D. R. Edds, C. W. Prophet & E. J. Miller. 1997. Freshwater mussels (Bivalvia: Unionidae) in the Verdigris, Neosho, and Spring River basins of Kansas and Missouri, with emphasis on species of concern. American Malacological Bulletin 14 (1): 41–55.

[PM89] Payne, B. S., A. C. Miller, P. D. Hartfield & R. F. McMahon. 1989. Variation in size demography of lotic populations of Corbicula fluminea (Müller). Nautilus 103 (2): 78–82.

[PC11] Plazzi, F., A. Ceregato, M. Taviani & M. Passamonti. 2011. A molecular phylogeny of bivalve mollusks: ancient radiations and divergences as revealed by mitochondrial genes. PLoS One 6 (11): e27147.

[SS97] Schloesser, D. W., R. D. Smithee, G. D. Longton & W. P. Kovalak. 1997. Zebra mussel induced mortality of unionids in firm substrata of western Lake Erie and a habitat for survival. American Malacological Bulletin 14 (1): 67–74.

[TS65] Thomas, G. J., & D. C. Scott. 1965. Note on Elliptio spinosa in Georgia. Nautilus 79 (2): 66–67.

[USDI77] United States Department of the Interior. 1977. Endangered and threatened wildlife and plants—republication of list of species. Federal Register 42: 36420–36431.

[V16] Vanatta, E. G. 1916. Rafinesque’s types of Unio. Proceedings of the Academy of Natural Sciences of Philadelphia 67 (3): 549–559.

[WMP89] Way, C. M., A. C. Miller & B. S. Payne. 1989. The influence of physical factors on the distribution and abundance of freshwater mussels (Bivalvia: Unionidae) in the lower Tennessee River. Nautilus 103 (3): 96–98.

[W63] Wheeler, M. J. 1963. Type of Unio luteolus Lamarck, 1819. Nautilus 77 (2): 58–61.

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