At least one-third of the species that inhabit the world's oceans may remain completely unknown to science, according to an exhaustive, international study of the seas.
And that's despite the fact that more species have been described in the last decade than in any previous one, according to a new report that details the first comprehensive register of marine species of the world.
The massive collaborative undertaking conducted by 270 taxonomists from 32 countries found the current number of marine species identified is roughly 230,000.
However, the researchers estimate that the ocean may be home to as many as one million species in all - likely not more. Around 226,000 of those species have so far been described and there are another 65,000 species awaiting description in specimen collections.
"For the first time, we can provide a very detailed overview of species richness, partitioned among all major marine groups. It is the state of the art of what we know—and perhaps do not know—about life in the ocean," says Ward Appeltans of the Intergovernmental Oceanographic Commission (IOC) of UNESCO.
The findings provide a reference point for conservation efforts and estimates of extinction rates, the researchers say. They expect that the vast majority of unknown species - composed disproportionately of smaller crustaceans, molluscs, worms, and sponges - will be found this century.
Earlier estimates of ocean diversity had relied on expert polls based on extrapolations from past rates of species descriptions and other measures. Those estimates varied widely, suffering because there was no global catalogue of marine species.
Of the roughly 230,000 marine species known, about 200,000 belong to the kingdom Animalia, 7,600 to Plantae, 19,500 to Chromista, 550 to Protista; and 1,050 to Fungi. The research has only counted with eukaryote organisms, i.e. those whose genetic information is enclosed in a cell nucleus, which has left out bacteria, viruses, and archaea.
The World Register of Marine Species (WoRMS) is an open-access, online database and is now 95% complete and is continually being updated as new species are discovered.
"Building this was not as simple as it should be, because there has not been any formal way to register species," Costello says.
A particular problem is the occurrence of multiple descriptions and names for the same species - so called "synonyms," Costello says. For instance, each whale or dolphin has on average 14 different scientific names.
As those synonyms are discovered through careful examination of records and specimens, the researchers expect perhaps 40,000 "species" to be struck from the list. But such losses will probably be made up as DNA evidence reveals overlooked "cryptic" species.
While fewer species live in the ocean than on land, marine life represents much older evolutionary lineages that are fundamental to our understanding of life on Earth, Appeltans says. And, in some sense, WoRMS is only the start.
"This database provides an example of how other biologists could similarly collaborate to collectively produce an inventory of all life on Earth," Appeltans says.