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The vast majority have not been identified - and cataloguing them all could take more than 1,000 years.
The number comes from studying relationships between the branches and leaves of the "family tree of life".
If this is correct, then only 14% of the world's species have yet been identified - and only 9% of those in the oceans.
"The rest are primarily going to be smaller organisms, and a large proportion of them will be dwelling in places that are hard to reach or hard to sample, like the deep oceans," said Dr Tittensor.
"I think it's definitely a creative and innovative approach, but like every other method there are potential biases and I think it's probably a conservative figure," he told BBC News.