Holotype of Anodonta wheatleyi, from the Mussel Project.

Belongs within: Unioninae.

Anodonta is a genus of freshwater mussels in which the shell is oblong, thin, and rather compressed behind, and the hinge margin lacks teeth (Gray 1840).

The shells of ducks and swans
Published 18 April 2021

The freshwater environment has been a challenging one for bivalves. Though there is a reasonable diversity of freshwater bivalves around the world, they tend to be dominated by members of a select few lineages. One of the most successful groups of freshwater bivalves is the family Unionidae, and among the more widespread unionids are the freshwater mussels of the genus Anodonta.

Swan mussel Anodonta cygnea, copyright Gail Hampshire.

Anodonta species are found widely across northern Eurasia and North America, commonly referred to as ‘mussels’ in Eurasia and ‘floaters’ in North America. They are relatively large bivalves (one of the largest, the swan mussel Anodonta cygnea of Eurasia, can be up to about twenty centimetres across) with an irregularly elliptical shape and a relatively thin shell. One of their distinguishing features compared to other freshwater bivalves is the teeth of the hinge connecting the shell valves have been lost. Instead, the valves are primarily held together by the dorsal ligament (Moore 1969). Freshwater mussels are most commonly found in mud at the bottom of slow-moving or still waters, such as lakes or slow rivers.

One of the main hurdles to bivalve colonisation of fresh water has been the question of dispersal. In most marine bivalves, populations mostly disperse via their planktonic larvae. But because of the directed flow of water in rivers and the like, passive plankton fare less well in freshwater environments. If you just float along a stream, eventually you’ll be washed out to sea. Anodonta species, like other unionids, solve the problem of getting back upstream through parasitic larvae called glochidia. Female Anodonta have the rear part of the gills modified into a pouch (or marsupium) in which the developing larvae are initially incubated. When they are released by their mother, the glochidia already possess a bivalved, sharp-edged shell. Released glochidia swim towards a suitable host in the form of a passing fish and use the valves of the shell to clamp onto a narrow appendage of the host’s body such as its fins or gills. Eventually, a cyst forms around the attached glochid within with it develops until it is ready to emerge and attain maturity.

Winged floater Anodonta nuttalliana, a North American species, copyright Eric Wagner.

Freshwater molluscs have a history of being subject to taxonomic chicanery, through the Nouvelle École of late nineteenth-century France and other excesses of typological enthusiasm. Anodonta is no exception. The shells of freshwater mussels tend to be very plastic in morphology, their size, shape and appearance being strongly affected by their developmental environment. As a result, they include what were labelled by Riccardi et al. (2020) as “some of the most over-described species on the planet”. The swan mussel A. cygnea alone has had somewhere in the region of 550 different species-group names applied to it at one time or another. Modern estimates of Anodonta diversity are considerably more conservative. Just four species are currently recognised from Eurasia (Riccardi et al. 2020) with the swan mussel and the duck mussel A. anatina being the most widespread (offhand, I don’t know whether the mussels get their vernacular names because they’re eaten by swans and ducks or because the shape of the shell is supposed to look like a swan or duck). Considering the travails of shell-based taxonomy, it is noteworthy that these species often cannot be distinguished with certainty without checking the soft tissue. North America is home to six or seven recognised species with diversity being higher to the west of the continent.

Nevertheless, there are still grounds for questioning the current taxonomy of Anodonta. Molecular studies of the genus by Chong et al. (2008), Bolotov et al. (2020) and Riccardi et al. (2020) have all suggested that Anodonta as currently recognised may be paraphyletic to closely related genera. In particular, there may be a divide between the Eurasian and North American lineages with the North American species closer to taxa found in eastern Asia. Anodonta has been a problem genus in the past and it sees no reason why it should allow itself to be reformed.

Systematics of Anodonta
Anodonta Lamarck 1799G07 (see below for synonymy)
|--A. anatina (Linnaeus 1758)BK20, G07 (see below for synonymy)
`--+--A. cygnea (Linnaeus 1758)BK20, TW07 (see below for synonymy)
`--+--Pyganodon Crosse & Fischer 1893PC11, OE97
| `--P. grandis (Say 1829)SS97
`--Pseudanodonta Bourguignat 1876BK20
`--*P. complanata (Rossmässler 1835)BK20 (see below for synonymy)

Anodonta incertae sedis:
A. angulata [incl. A. feminalis, A. randalli Trask 1855]C64
A. atra Rafinesque 1820V16
A. benedictiiW79
A. californiensisC64
A. cataractaD84
A. cognataC64
A. cuneata Rafinesque 1820V16
A. cyrea Drouët 1881 (see below for synonymy)G07
A. decora [incl. A. inornata]W79
A. dejectaPF15
A. edentula [incl. A. rhombica]W79
A. euscaphys (Heude 1879) [=Anemina euscaphys]G07
A. ferussacianaW79
A. fluviatilisW79
A. footiana [incl. A. mcneilii, A. opalina]W79
A. fragilis [incl. A. flava, A. glandulosa, A. imbricata, A. irisans, A. pallida, A. subcarinata]W79
A. gibbosaD84
‘Anodon’ gibbumC42
A. glaucaC64
A. grandis (Say 1829)JN90
A. imbecillisT59
A. implicataD84
A. kennerlyi Lea 1860R78
A. marryatana [incl. A. houghtonensis Currier in Walker 1879 (n. n.), A. subinflata]W79
A. modestaW79
A. mutabilis Rafinesque 1820V16
A. nigrescens Rafinesque 1820V16
A. nuttallianaC64
A. ogurae (Kuroda & Habe 1987) [=Oguranodonta ogurae]G07
A. oregonensis Lea 1838 [incl. Beringiana georgiensis Bogatov & Starobogatov 2001]BK20
A. ovata [incl. A. subangulata]W79
A. pepinianaW79
A. planaW79
A. pseudodopsis Locard 1883 [=Gabillotia pseudodopsis]G07
A. ‘radiata’ Rafinesque 1820 non Müll. 1774V16 [=A. cygnea var. radiataH79]
A. salmoniaW79
A. schafferianaW79
A. subcylindraceaW79
A. subglobosaW79
A. swinhoei Adams 1866A66b
A. vescoiana Bourguignat 1856 [=Euphrata vescoiana]G07
A. violacina Rafinesque 1820V16
A. wahlamatensis [incl. A. rotundovata Trask 1855, A. triangularis Trask 1855]C64
A. yubaensis Trask 1855C64
A. (Lamproscapha)A66a
|--A. (L.) ensiformisA66a
`--A. (L.) wheatleyiA66a

Anodonta Lamarck 1799G07 [incl. Colletopterum Bourguignat 1880G07, Euphrata Pallary 1933G07, Gabillotia Servain 1890G07, Goniocypris Brady & Robertson 1870BB61, Oguranodonta Kuroda & Habe 1987G07, Piscinaliana Bourguignat 1881 non Paladilhe 1866G07; Anodontinae, Anodontini]

Anodonta anatina (Linnaeus 1758)BK20, G07 [=Mytilus anatinusG40, Colletopterum (Piscinaliana) anatinumG07; incl. C. apollonicum (Bourguignat 1880)G07, C. baeri (Bogatov, Starobogatov & Prozorova 2005)G07, C. convexum (Drouët 1888)G07, C. (Piscinaliana) depressum (Bourguignat 1881)G07, Anodonta letourneuxi Bourguignat 1870BK20, *C. letourneuxiBK20, C. milaschevichi (Bogatov, Starobogatov & Prozorova 2005)G07, C. (Piscinaliana) nilssonii (Küster 1842)G07, C. ostiarium (Drouët 1881)G07, Anodonta piscinalis Nilsson 1823BK20, Anodon cygneus var. piscinalisG40, Colletopterum (*Piscinaliana) piscinaleG07, BK20, C. (P.) ponderosum (Pfeiffer 1825)G07, C. (P.) rostratum (Rossmässler 1836)G07, C. (P.) sorensianum (Dybowski 1913)G07, C. subcirculare (Clessin 1873)G07]

Anodonta cygnea (Linnaeus 1758)BK20, TW07 [=Mytilus cygneusG40, Anodon cygneusG40; incl. M. avonensisG40, Anodon cygneus var. avonensisG40, M. cellensisG40, Anodon cygneus var. cellensisG40, Anodonta cellensisPV06, Anodonta compressaG40, Anodonta crassa Marks msG40, Mytilus dentatusG40, M. incrassatusG40, Anodonta intermediaG40, M. maculatusG40, *Goniocypris mitra Brady & Robertson 1870BB61, Anodon paludosusG40, Anodonta ponderosaG40, Anodonta rhombeaG40, Anodonta rostrataG40, Anodon cygneus var. rostrataG40, Mytilus stagnalis Gmelin 1791G40, G07, Anodon cygneus var. stagnalisG40, Anodonta stagnalisG07, Anodonta sulcataG40, Anodonta ventricosaG40, Anodonta zellensis (Gmelin 1791)G07]

Anodonta cyrea Drouët 1881 [=Colletopterum (Piscinaliana) cyreum; incl. C. (P.) bactrianum (Rolle 1897), C. (P.) kokandicum Starobogatov & Izzatullaev 1984]G07

*Pseudanodonta complanata (Rossmässler 1835)BK20 [=Anodonta complanataBK20, Anodon cygneus var. complanataG40; incl. Pseudanodonta elongata (Holandre 1836)G07, P. klettii (Rossmässler 1835)G07, P. nordenskioldi Bourguignat 1880G07]

*Type species of generic name indicated


[A66a] Adams, H. 1866a. List of land and freshwater shells collected by Mr. E. Bartlett on the Upper Amazons, and on the River Ucayali, eastern Peru, with descriptions of new species. Proceedings of the Zoological Society of London 1866: 440–445.

[A66b] Adams, H. 1866b. Descriptions of six new species of shells, and note on Opisthostoma de-Crespignii. Proceedings of the Zoological Society of London 1866: 445–447.

[BB61] Benson, R. H., J. M. Berdan, W. A. van den Bold, T. Hanai, I. Hessland, H. V. Howe, R. V. Kesling, S. A. Levinson, R. A. Reyment, R. C. Moore, H. W. Scott, R. H. Shaver, I. G. Sohn, L. E. Stover, F. M. Swain & P. C. Sylvester-Bradley. 1961. Systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt Q. Arthropoda 3: Crustacea: Ostracoda pp. Q99–Q421. Geological Society of America and University of Kansas Press.

[BK20] Bolotov, I. N., A. V. Kondakov, E. S. Konopleva, I. V. Vikhrev, O. V. Aksenova, A. S. Aksenov, Y. V. Bespalaya, A. V. Borovskoy, P. P. Danilov, G. A. Dvoryankin, M. Y. Gofarov, M. B. Kabakov, O. K. Klishko, Y. S. Kolosova, A. A. Lyubas, A. P. Novoselov, D. M. Palatov, G. N. Savvinov, N. M. Solomonov, V. M. Spitsyn, S. E. Sokolova, A. A. Tomilova, E. Froufe, A. E. Bogan, M. Lopes-Lima, A. A. Makhrov & M. V. Vinarski. 2020. Integrative taxonomy, biogeography and conservation of freshwater mussels (Unionidae) in Russia. Scientific Reports 10: 3072.

[C42] Cantor, D. 1842. Letter containing a list of a collection of shells from Chusan, presented to the society. Proceedings of the Zoological Society of London 10: 124.

[C64] Carpenter, P. P. 1864. Supplementary report on the present state of our knowledge with regard to the Mollusca of the west coast of North America. Report of the British Association for the Advancement of Science 33: 517–686.

Chong, J. P., J. C. B. Box, J. K. Howard, D. Wolf, T. L. Myers & K. E. Mock. 2008. Three deeply divided lineages of the freshwater mussel genus Anodonta in western North America. Conserv. Genet. 9: 1303–1309.

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

[G07] Graf, D. L. 2007. Palaearctic 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.

[G40] Gray, J. E. 1840. A Manual of the Land and Fresh-water Shells of the British Islands, with figures of each of the kinds. By William Turton, M.D. A new edition, thoroughly revised and much enlarged. Longman, Orme, Brown, Green, and Longmans: London.

[H79] Hey, W. C. 1879. Contributions to a better knowledge of the land and freshwater shells of Yorkshire. Journal of Conchology 2: 310–314.

[JN90] Jirka, K. J., & R. J. Neves. 1990. Freshwater mussel fauna (Bivalvia: Unionidae) of the New River Gorge National River, West Virginia. Nautilus 103 (4): 136–139.

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 1. The Geological Society of America, Inc. and The University of Kansas.

[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.

[PF15] Pilsbry, H. A., & J. H. Ferriss. 1915. Mollusca of the southwestern states, VII: the Dragoon, Mule, Santa Rita, Baboquivari, and Tucson Ranges, Arizona. Proceedings of the Academy of Natural Sciences of Philadelphia 67 (2): 363–418, pls 8–15.

[PV06] Pimenova, E. A., & A. A. Varaksin. 2006. Putative nitroxidergic cells in the digestive system of some mytilids (Mollusca: Bivalvia: Mytilidae) revealed by NADPH-diaphorase histochemistry. Malacologia 49 (1): 61–77.

[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.

[R78] Reid, R. G. B. 1978. The systematic, adaptive and physiological significance of proteolytic enzyme distribution in bivalves. Veliger 20 (3): 260–265.

Riccardi, N., E. Froufe, A. E. Bogan, A. Zieritz, A. Teixeira, I. Vanetti, S. Varandas, S. Zaccara, K.-O. Nagel & M. Lopes-Lima. 2020. Phylogeny of European Anodontini (Bivalvia: Unionidae) with a redescription of Anodonta exulcerata. Zoological Journal of the Linnean Society 189: 745–761.

[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.

[TW07] Taylor, J. D., S. T. Williams, E. A. Glover & P. Dyal. 2007. A molecular phylogeny of heterodont bivalves (Mollusca: Bivalvia: Heterodonta): new analyses of 18S and 28S rRNA genes. Zoologica Scripta 36 (6): 587–606.

[T59] Thomas, G. J. 1959. Self-fertilization and production of young in a sphaeriid clam. Nautilus 72 (4): 131–140.

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

[W79] Walker, B. 1879. Catalogue of the shell-bearing Mollusca of Michigan. Journal of Conchology 2: 325–337.

Leave a comment

Your email address will not be published. Required fields are marked *