Species delimitation with limited sampling: an example from rare trapdoor spider genus Cyclocosmia (Mygalomorphae, Halonoproctidae)

The outcome of species delimitation depends on many factors, including
conceptual framework, study design, data availability, methodology
employed, and subjective decision-making. Obtaining sufficient taxon
sampling in endangered or rare taxa might be difficult, particularly when
non-lethal tissue collection cannot be utilized. The need to avoid
overexploitation of the natural populations may thus limit the
methodological framework available for downstream data analyses and bias
the results. We test species boundaries in rare North American trapdoor
spider genus Cyclocosmia Ausserer (1871) inhabiting the Southern Coastal
Plain biodiversity hotspot with the use genomic data and two multispecies
coalescent model methods. We evaluate the performance of each methodology
within a limited sampling framework. To mitigate the risk of species
over-splitting, common in taxa with highly structured populations, we
subsequently implement a species validation step via genealogical
diversification index (gdi), which accounts for both genetic isolation and
gene flow. We delimited eight geographically restricted lineages within
North American Cyclocosmia, suggesting that major river drainages in the
region are likely barriers to dispersal. Our results suggest that
utilizing BPP in the species discovery step might be a good option for
datasets comprising hundreds of loci, but fewer individuals, which may be
a common scenario for rare taxa. However, we also show that such results
should be validated via gdi, in order to avoid over-splitting.