Redescription and Description of Threatened Thiarids (Gastropoda: Cerithioidea) from Argentina-Paraguay

A. brunneum holotype

Although knowledge of the composition and phylogenetic placement of Thiaridae has increased due to recent studies (Strong et al., 2011 and citations therein), modern taxonomic and systematic treatment is still needed within the family.  Recently, Roberto Vogler, Ariel Beltramino, Juana Peso, and Alejandra Rumi provided a redescription of one species and a description of a new species from Argentina and Paraguay in an attempt to provide useful data towards future comparative studies.  The paper entitled “Threatened gastropods under the evolutionary genetic species concept: redescription and new species of the genus Aylacostoma (Gastropoda: Thiaridae) from High Paraná River (Argentina-Paraguay)” begins with an introduction to thiarids and a history of available genus-group names that are available for Neotropical thiarids.  The authors describe that in Argentina and Paraguay three species of Aylacostoma were recorded in the High Paraná River at the border area between Argentina and Paraguay, but regrettably, their highly oxygenated habitat disappeared due to the construction of a dam that formed the Yacyretá Reservoir.  The authors suggest that Aylacostoma guaraniticum and A. stigmaticum could now be categorized as extinct while A. chloroticum and the newly described species exist due to an ex situ conservation program.

Vogler et al. examined specimens from the Museo de La Plata and Museo Argentino de Ciencias Naturales and live specimens from the ongoing ex situ conservation program taking place at the Universidad Nacional de Misiones.  They took seven shell measurements and examined the protoconchs, operculate, and radulae using scanning electron microscopy.  They also obtained mitochondrial Cyt b sequences from five individuals and conducted phylogenetic analyses of mitochondrial COI data of 37 A. chloroticum and six A. new sp. as well as six outgroup taxa.    

The authors used the Evolutionary Genetic Species Concept to determine if well-supported sister clades could be different species.  The method, which has been developed for asexual species states that if phylogenetic analysis shows that small samples from two populations are reciprocally monophyletic , and if the mean sequence difference between them is more than four times θ=2N_eµ estimated from the within-sample variation, then the samples came from different species (Birky et al., 2010).  The authors found support for the recognition of the redescribed A. chloroticum and new species, which they named A. brunneum Vogler & Peso.  Aylacostoma brunneum could also be distinguished from A. chloroticum by its distinctive coloration pattern being dark brown with alternating lighter brown bands, while A. cloroticum is greenish-yellow to mid greenish-brown.

Unfortunately, only a single relict population of A. chloroticum is known in the wild, and wild populations of A. brunneum are probably extinct (although further survey work is necessary).  It is a sad state of affairs that as scientists begin to gain knowledge of this poorly studied group, they are disappearing from the wild.

      

Literature Cited

Birky, C. W., Jr., J. Adams, M. Gemmel, and J. Perry.  2010.  Using population genetic theory and DNA sequences for species detection and identification in asexual organisms.  PLoS ONE 5: e10609. Doi:10.1371/journal.pone.0010609.

Strong, E. E., D. J. Colgan, J. M. Healy, C. Lydeard, W. F. Ponder, and M. Glaubrecht.  2011.  Phylogeny of the gastropod superfamily Cerithioidea using morphology and molecules.  Zool. J. Linnean Society 162:43-89.

Vogler, R. E., A. A. Beltramino, J. G. Peso, and A. Rumi.  Threatened gastropods under the evolutionary genetic species concept: redescription and new species of the genus Aylacostoma (Gastropoda: Thiaridae) from High Parana River (Argentina-Paraguay).  Zool. J. Linnean Society 172:501-520.