How many branchiopod crustacean species are there? Quantifying the components of underestimation

Aims For the crustacean class Branchiopoda, we estimated global species richness, assessed the reliability of relative diversity estimates among higher taxa and made predictions about the kinds of species remaining to be described. Location Global. Methods We calculated correction factors to account for three different sources of diversity underestimation, analysed taxonomic patterns of species descriptions over time and tested possible correlates of description date. Results Our work suggests that there are c. 2.1 times more branchiopod species in nature than the c. 1180 currently described. The largest correction factor was at the scale of single biogeographical regions, mainly attributed to poor knowledge of species diversity within described morphospecies. Relative diversity estimates among major taxonomic groups are apparently quite reliable, with diversity differences greater than 1.5-fold robust to the heterogeneity of the taxonomic process. Current knowledge of branchiopod diversity is biased towards species with large geographical ranges, whereas body size and higher taxonomic group were only weak predictors of description date. Main conclusions A proper understanding of species boundaries in nature, which is greatly aided by molecular studies, is key in accurately estimating species richness for invertebrate groups. However, we suggest that while many species remain to be described, numbers of presently described species are still useful for macroevolutionary studies of diversification when interpreted with caution.