Genetic Distinction of two closely related unionid bivalve monotypic genera: Arcidens and Arkansia

    

Arcidens confragosus (INHS pic)

Discovering, describing and classifying the world’s species on the basis of evolutionary relationships is a laudable objective.   This is particularly important for taxa that are imperiled.  Unionid bivalves are recognized as one of the most imperiled groups of animals around the world with 70% of the recognized species in North America considered either extinct, endangered, threatened or of special concern (Williams et al., 1993; Neves et al., 1997; Lydeard et al., 2004).  Unionid species and genera have traditionally been diagnosed on characteristics of the shell and soft-parts.  One problem with relying on shell morphology is the fact that some variation has been attributed to environmental factors making it easier to over- or under-estimate species diversity depending on the circumstances. 

Arkansia wheeleri (INHS pic)

     Recently, an interesting paper entitled “Molecular phylogenetics and morphological variation reveal recent speciation in freshwater mussels of the genera Arcidens and Arkansia (Bivalvia: Unionidae)" was published in 2014 in the Biological Journal of the Linnean Society by Kentaro Inoue, Alyssa L. McQueen, John L Harris, and David J. Berg.    The authors examined the evolutionary history and taxonomic status of the two monotypic genera, which some have treated as belonging to separate genera (Turgeon et al., 1998) and others as congeners (e.g., Graf and Cummings, 2007). Phylogenetic analysis of sequences mitochondrial cytochrome oxidase I and NADH dehydrogenase subunit 1 and the nuclear internal transcribed spacer 1 revealed that the two species are reciprocally monophyletic, but very similar genetically.  Indeed, the estimate of divergence time based on mitochondrial data places their speciation event in the Holocene about 5,860 years ago when the streams of the Interior Highlands, where the federally endangered Arkansia wheeleri occurs, was isolated from the more widely distributed sister-species Arcidens confragosus.  Interestingly, traditional morphometric analysis showed distinct shell shapes between the species, but the geometric morphometrics did not thereby indicating that there is still value to conducting traditional morphometric studies of unionid shells to delineate species.  However, shell-shape variation should probably be examined in conjunction with a molecular phylogenetic analysis.  The authors propose that the two species be placed in the nomen Arcidens, which has priority.   

Literature Cited

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

Lydeard, C., R. H. Cowie, W. F. Ponder, A. E. Bogan, P. Bouchet, S. A. Clark, K. S. Cummings, T. J. Frest, O. Gargominy, D. G. Herbert, R. Hershler, K. E. Perez, B. Roth, M. Seddon, E. E. Strong, and F. G. Thompson.  2004.  The global decline of nonmarine mollusks.  BioScience 54:321-330.

Neves, R. J., A. E. Bogan, J. D. Williams, S. A. Ahlstedt, and P. W. Haartfield.  1997.  Status of aquatic mollusks in the southeastern United States: a downward spiral of diversity.  In: G. W. Benz and D. E. Collins (eds.) Aquatic Fauna in Peril: the Southeastern Perspective.  Special Publication 1, Southeast Aquatic Research Institute, Lenz Design & Communications, Decatur, GA, 43-85. 

Turgeon, D. D., J. F. Quinn, Jr., A. E. Bogan, E. V. Coan, F. G. Hochberg, Jr., W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G. Thompson, M. Vecchione, and J. D. Williams.  1998.  Common and scientific names of aquatic invertebrates from the United States and Canada: mollusks.  Speical Publication 26.  Bethesda, MD: American Fisheries Society.

Williams, J. D., M. L. Warren, Jr., K. S. Cumming, J. L. Harris, and R. J. Neves.  1993.  Conservation status of freshwater mussels of the United States and Canada.  Fisheries 18:6-22.