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| 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.
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| 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.
