Historical Trends in Crustacean Systematics

Using a database that includes nearly half (46%) of the estimated 68,000 described species of extant crustaceans, as well as the names of the authors who described each species and the date of description, we plotted the number of species described vs. year of description to examine rates of, and trends in, crustacean species descriptions over time. Plots were generated for all crustaceans and for selected major taxonomic “subgroups” (currently recognized classes and selected subclasses). The cumulative number of species plotted over time for all crustaceans (pooled) showed no sign of flattening over time; no asymptote was apparent. The same pattern was seen in all subgroups examined (Branchiopoda, Maxillopoda, Copepoda, Malacostraca, Peracarida, Decapoda, and Brachyura). Given that the asymptotes from such analyses have been used to predict species richness in a taxon, our data show that currently there is no reliable way to estimate the number of still-undescribed species of crustaceans through extrapolation. The current estimate of approximately 68,000 described species (Martin & Davis, 2001; Brusca & Brusca, 2003) may represent only half, or far fewer, of the actual number of extant crustacean species. No apparent decrease in the overall rate of discovery (approximated by the rate of subsequent taxonomic description) has been observed since the first crustacean species (in our database) were described in the mid-1700s. Over time, the rate of discovery of some smaller taxa, e.g., copepods, did not differ appreciably from the rate of discovery of larger taxa, e.g., brachyuran crabs, based on best-fit regression lines. Although the rates of species accumulations have differed between groups (4.6 species per year for branchiopods vs. 82.6 species per year for peracarids), the pace appears steady within groups. The taxonomic contributions of notable workers and their published monographs are highlighted and discussed.