A warning for ecologists and conservation biologists using species checklists: How the European marine fauna 'lost' all of its 16 Discodoris species (Mollusca: Gastropoda)

The European marine fauna used to be considered to include 16 species of Discodoris sea slugs until a recent worldwide revision demonstrated that there is not a single Discodoris species in European waters. This exemplary case illustrates the fact that species checklists do not accurately represent biodiversity unless they are based on sound taxonomic work in which (1) the status of every available species name has been addressed, i.e. whether it is valid, synonymous, or of doubtful application, and (2) classification reflects phylogenetic relationships. It is argued that taxonomic revisions are critically needed, because the status of species names can only be addressed properly through revisions. It is discussed that fields which depend on taxonomic data, such as conservation biology and ecology, might be affected deeply if problematic species names (synonyms and nomina dubia) have not been recognized. Consequently, it is proposed that a taxon that has not been revised be red-flagged in checklists, so that non-taxonomists will know which species names should be applied with caution or not at all.