Nature
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We’re only in the early stages of 2026, and one massive change from last year may have already occurred: there may actually be twice as many vertebrate species on the planet as was previously assumed. The error is not due to a miscount, but neither have we just had some miraculous population boom (unfortunately, we are still experiencing an overall mass die-off across the natural world). Instead, something far more subtle is going on.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.According to a new study, our current understanding of how many species exist on the planet is significantly underestimated due to our reliance on morphology – in essence, how animals look. But this focus has obscured the presence of thousands of species that were otherwise hiding under our noses. These are known as cryptic species that may appear similar to others but are genetically different.
As our planet continues to undergo the current ecological crises, it is more important than ever that we keep track of just how many species are alive for conservation purposes. However, ever since the 17th century, scientists have mostly identified species based on their morphology. This took into account their size, their bone structure, and their color. But this is not a reliable method for classification when cryptic species enter the picture.
The issue here, from an ecological, evolutionary, and conservation perspective, is simple. Cryptic species – not to be confused with cryptid species – may appear like other species but are greatly different. So, it is possible that one morphologically identified species that is not considered in imminent danger of extinction could actually contain a cryptic species that is.
In this latest study, researchers from the Department of Ecology and Evolutionary Biology at the University of Arizona conducted a meta-analysis of 373 studies to find potential cryptic species that have been overlooked to this point. Their analysis focused purely on vertebrate species: fish, amphibians, reptiles, birds, and mammals.
The team found that for every one species currently recognized for its morphology, there are two distinct cryptic species, on average.
“We found that cryptic species are consistently widespread and common across major vertebrate groups. In every major group of vertebrates, we estimated (on average) approximately two cryptic species per morphology-based species,” they explain in their study.
There was a striking consistency to these results across the vertebrate groups. The lives of fish are very different from those of birds, and yet the prevalence of cryptic species was similar regardless. If the results of this study are applied to the whole planet, then it means the total number of vertebrate species is effectively doubled.
The study also addressed a significant debate in biology related to whether mitochondrial DNA (mtDNA) – which comes from a small, circular chromosome found in the mitochondria of cells that is only passed down from the mother – is as effective at discerning species as nuclear DNA (nucDNA) – the primary genetic material found in the cell’s nucleus. The former DNA is often used to distinguish between closely related species because it evolves faster than nuclear DNA.
Due to the speed at which mutations occur, scientists use this DNA to detect any genetic difference that accumulates over a time period between two groups of animals.
“There has been an extensive literature arguing for the limitations of mtDNA data in species delimitation and phylogeny estimation, and many case studies showing mtDNA giving problematic results. Yet, this literature may be somewhat biased,” the team explains.
“Here, we tested for systematic bias in the species delimited by mtDNA alone as opposed to nucDNA alone. When comparing matched sets of species, we found that mtDNA tended to estimate more species than nucDNA. However, this bias was limited (e.g. mean ratios for nucDNA approximately 2, mtDNA approximately 2.5 for the largest groups) and was only significant in ray-finned fishes.”
The team does stress that the results do not guarantee that nucDNA and mtDNA delimit identical species. Instead, they only show that the overall numbers are similar.
The team concludes with a call to arms. This is not a simple naming issue – it has significant conservation implications.
“These results raise the possibility that there might be hundreds or thousands of cryptic vertebrate species that are in danger of extinction but are presently hidden by their current taxonomic status based on morphology. Furthermore, our results suggest that many analyses of species richness, speciation, extinction and diversification may be compromised by the presence of large numbers of cryptic species.”
Basically, we cannot protect what we haven’t identified, and we need to change this urgently, as our current conservation maps are likely missing half of the vertebrate species on Earth.
The study is published in Proceedings of the Royal Society B: Biological Sciences.
