Most are probably crustaceans, mollusks and other small organisms, but there may also be up to eight species of whales and dolphins yet to be identified.
Between 700,000 and one million species live in the world's oceans, according to a thorough new analysis, which also estimated that between one-third and two-thirds of those species have yet to be named and described.
The new numbers are far smaller than previous estimates, which had put the tally of marine species as high as 10 million or more. By coming up with a more accurate picture of what we know and what we don't yet know about marine life, the study should help scientists better focus conservation efforts where they're needed most.
"You can only love something if you know it," said Ward Appeltans, a marine biologist at the Intergovernmental Oceanographic Commission of UNESCO in Oostende, Belgium. "We will not save the world with this result, but we may start understanding it better."
The new findings also open up the possibility that we may eventually be able to identify just about every creature living in the sea.
"It may not be mission impossible to describe all the marine species in the ocean," Appeltans said. "We are describing 2,000 new marine species every year. If we can keep that momentum, we can start knowing exactly what's living on our planet."
In previous attempts to guess numbers of ocean species, scientists often made extrapolations based on rates of previous discoveries or numbers of unknown species in sample collections. Those methods led to crude estimates that ranged from 300,000 to more than 10 million total species in the seas.
To come up with something more accurate, Appeltans worked with 120 of the world's leading experts on specific groups of marine organisms. Based on their intricate knowledge of taxonomy, the experts came up with educated guesses about numbers of known and unknown species in their own particular fields. The study also employed a statistical model to incorporate expert assessments with known information about changes in rates of discovery over time.
Overall, the study counted about 400,000 described species of marine species, though about 40 percent of those had been described multiple times and had been given more than one scientific name. That led to a corrected tally of about 226,000 known marine species, the team reports today in the journal Current Biology. The list includes about 200,000 animal species, 7,600 plants and more than 1,000 fungi.
Still yet to be found and described are between 482,000 and 741,000 species, the study found. Most of those are probably crustaceans, mollusks, phytoplankton and other small organisms. But the experts also predicted that science have yet to describe between two and eight species of whales and dolphins, along with 10 species of sea snakes and other reptiles.
Besides the logistical achievement of coordinating the work of 120 scientists, the new study is a major step toward solving a basic mystery.
"The question of how many species there are is such a fundamental one and it's a huge embarrassment that we don't have the answer," said Stuart Pimm, a conservation biologist at Duke University in Durham, North Carolina. "In a compelling way, this paper has come up with a fairly credible number for how much we know and don't know."
It's not the exact number that's particularly important, Pimm added. Instead, simply having a number at all is what matters. Once we have a complete catalog of species in the oceans, we can begin to figure out if we're putting marine conservation areas in the right places or if environmental efforts need to be directed elsewhere.
"We know we're losing biodiversity at a rate that is 1,000 times faster than we should be, and if we're going to stop that hemorrhaging of species, we have to know what the species are and most important, where they are," Pimm said. "This is a vital first step in making decisions about where to act."