Aim In this paper, we compare species description rates to predict the numbers of undescribed species. These data are used to discuss the merits of various attempts to estimate species richness in the oceans. Location European marine areas. Methods Predictions of how many species may exist on Earth have lacked an inventory of how many have been described, except for a few small taxa. The ocean is a good place to start an inventory because it includes all but one of the phyla and most classes of life on Earth. The European Register of Marine Species (ERMS) was compiled by taxonomic experts, covered all marine taxa, and accounted for synonyms. Reflecting taxonomic history, Europe's species are the best described in the world. Results ERMS listed 29,713 species of animals, plants and protists, but excluded bacteria and viruses. An estimated 6500 described species were not included. The best prediction of the number of species remaining to be described was 5613. Plots of years when species were first described showed no decrease in the rate of description for any taxa except birds, mammals and krill. If taxonomic effort has increased, whether due to more resources globally or greater efficiencies of productivity, then description rates per unit effort may be declining and the number of undescribed species may be lower than predicted. However, apart from reduced rates of description during the World Wars, there were no changes in description rates that could be easily attributed to such factors. Conclusions There are about 36,000 species described from European seas, and we predict that 40,000 to 48,000 may exist. This comprises 15% of the estimated 230,000 described marine species. However, this area is well known compared with other seas and the proportion of species yet to be discovered will be higher elsewhere.
