The Shells of Ducks and Swans

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.

REFERENCES

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.

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.

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.

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.

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