Up to two thirds of the plant and animal species in the world's oceans may be undiscovered, according to the largest study of the oceans' biodiversity ever conducted.

The new estimate, which suggests that there may be as many as 1 million species of non-bacterial life in the world's waterways, is based on research by 270 experts from around the world. The estimate is considered to be the most accurate yet, and is far lower than some previous estimates.

"Ten years ago, we thought there were at least 10 million species in the ocean, now we think it's less than 1 million," says Ward Appeltans, a marine biologist with the United Nations Educational Scientific and Cultural Organization (UNESCO). "It means that eventually, we might be able to describe most of the unknown species. If you consider fish, we estimate there are 5,000 species still undescribed. We're discovering 150 new species of fish every year — 30 years at that rate, and it's mission accomplished."

Appeltans says that previous estimates were done by a fewer number of scientists — this one took experts in specific classes of organisms and had each expert give their best estimate for the number of species remaining undiscovered in their area of expertise.

"It's never been done with such a large group of experts sitting together and coming up with a number — it's always been a small group or a single person," Appeltans says. "We asked them each individually how many species they thought were in their group of expertise only. When you pull that data together, you come up with somewhere between 700,000 and 1 million."

Besides fish, the study suggests there are as many as eight undiscovered whale and dolphin species, 10 undescribed marine reptiles, and thousands of sponges, crustaceans, algae, plants, and other species still to be found.

So far, about 226,000 species have been described by scientists, with another 65,000 species waiting to be described in specimen collections. It can take years for scientists to accurately describe a new species, because they have to be compared to previously discovered species in order to determine if a new type of creature has been found.

Appeltans says interest in discovering unknown species has been renewed over the past decade. During that time, 780 new crabs, 29 lobsters, 286 shrimps, four sea snakes, three whales, and three dolphins have been found.

"The rate of new discoveries in the ocean is still increasing — on land, the rate of discovery isn't increasing anymore," he says. "There are more and more people involved with describing new species, using new techniques and going to new habitats and places."

Despite an increasing rate of species discovery, as much as 95 percent of the world's oceans remain unexplored.

"When you go to the deep sea, every time you take a sample, you'll find a new species," he says. "But we think the deep sea is less diverse than we thought previously. Most of the diversity is in the tropics and the coastlines and the small islands of the Pacific, where little exploration has been done."

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