Almost a million animal and plant species may live in the oceans, a third of which are unknown to science, experts believe.
The estimate is based on the findings of a huge survey of marine life conducted by hundreds of scientists around the world.
So far, 226,000 species of organism have been identified, with a further 65,000 in specimen collections still to be described.
The World Register of Marine Species (Worms) excludes life forms such as bacteria and viruses whose genetic information is not enclosed in a cell nucleus.
By studying the rate of discovery and numbers of species found in sampled areas, the scientists made a prediction of how many species in total the oceans may contain.
They arrived at a figure of between 704,000 and 972,000. Lead scientist Dr Ward Appeltans, from the Intergovernmental Oceanographic Commission of Unesco (United Nations Educational, Scientific and Cultural Organisation), based in Belgium, said: "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 oceans."
A report on the research is published in the journal Current Biology.
The register, produced by 270 experts from 32 countries, is now 95% complete and still being updated.
Fewer species live in the oceans than on land, but they represent much older evolutionary lineages that are fundamental to understanding life on Earth, said Dr Appeltans.