Abstract
In shelled molluscs, assigning valid species names to independent evolutionary lineages can be a difficult task. Most original descriptions are based on empty shells and the high levels of variation in shape, color and pattern in some groups can make the shell a poor proxy for species-level identification. The deep-sea gastropod turbinid genus Bolma is one such example, where species-level identification based on shell characters alone is challenging. Here, we show that in Bolma both traditional and molecular taxonomic treatments are associated with a number of pitfalls that can lead to biased inferences about species diversity. Challenges derive from the few phylogenetically informative characters of shells, insufficient information provided in original descriptions and sampling artefacts, which at the molecular level in spatially fragmented organisms can blur distinctions between genetically divergent populations and separate species. Based on a comprehensive dataset combining molecular, morphological and distributional data, this study identified several cases of shell-morphological plasticity and convergence. Results also suggest that what was thought to be a set of distinct, range-restricted species corresponds instead to a smaller number of more widespread species. Overall, using an appropriate sampling design, including type localities, allowed us to assign available names to evolutionarily significant units.
Highlights
While soft-tissue anatomy is used for classification and recognition of higher taxonomic ranks in Mollusca, shell features are generally considered adequate and more convenient for species identification (Bouchet & Strong 2010)
The variability of shell features may be interpreted within these species hypotheses to determine diagnostic shell characters, which in turn may be used on post-mortem remains to complement knowledge about species distributions
We developed an integrative approach that follows the formalized approach to taxonomy proposed in Puillandre et al (2012b), in which species hypotheses are primarily built on molecular character sets rather than on shell characters
Summary
While soft-tissue anatomy is used for classification and recognition of higher taxonomic ranks in Mollusca, shell features are generally considered adequate and more convenient for species identification (Bouchet & Strong 2010). Similarity of shell features may result from parallel evolutionary processes occurring within distinct lineages evolving under similar environmental constraints (e.g., Littorina Férussac, 1822; Johannesson et al 2010), or from allopatric speciation and non-adaptive radiation for maintenance of similar shells among divergent taxa within a single genus (e.g., Lunella Röding, 1798; Williams et al 2011) These evolutionary processes include parallel divergent selection in reproductively isolated populations and CASTELIN M. et al, Molecular systematics of the genus Bolma Risso, 1826 the persistence of ancestral polymorphism among closely related species. Taking into account these data, species delimitation in molluscs should not be solely based on shell features but on the integration of complementary sources of data, as discussed in Dayrat (2005) under the umbrella term of ‘integrative taxonomy’. The variability of shell features may be interpreted within these species hypotheses to determine diagnostic shell characters, which in turn may be used on post-mortem remains to complement knowledge about species distributions (as for example applied in Castelin et al 2010, 2012)
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