Cryptic species are widespread across vertebrates

Abstract Many species delimited by morphological data contain two or more species that are distinct based on molecular data (i.e. cryptic species). Cryptic species have important implications for ecology, evolution and conservation. For example, morphology-based species of limited conservation concern can contain cryptic species that are in imminent danger of extinction. Here, we examined the prevalence of cryptic species across vertebrates. We also examined the controversial use of mitochondrial data, and whether they overestimate or underestimate cryptic species relative to nuclear data. We obtained usable estimates of species limits from 373 studies. We found that each morphology-based species contained approximately two cryptic species on average. This number was surprisingly similar across major groups despite dramatic differences in ecology and physiology (fishes versus birds), and similar to comparable estimates in insects. Cryptic species numbers from mitochondrial data alone were often higher than from nuclear data, but were generally not statistically different. Overall, our results show that cryptic species are numerous and widespread across vertebrates. These results suggest that cryptic species should not be ignored in analyses of species richness, speciation and extinction in living and fossil organisms. Furthermore, testing for cryptic species across morphology-based vertebrate species should be an urgent priority for conservation.