(CNN) -- A ten-year study of sea life has revealed just what lives beneath the waves.

An inventory published today in the journal "PLoS ONE" has found that the distribution and diversity of marine life varies greatly across regions.

The richest waters for marine life are around Australia and Japan; each feature almost 33,000 known species. The average number of species in a region is 10,750 across the 25 areas studied.

On average, crustaceans is the biggest group populating the seas, making up around one-fifth of sea life, followed by mollusks (17 percent) and fish (12 percent). Algae and plant life and one-cell organisms make up 10 percent respectively.

Rather than trying to answer the impossible question "how many fish in the sea?" it has taken only a snapshot of marine life, recording the different species.

Dr. Patricia Miloslavich, a co-senior scientist on the project, said that correlating the reports, many from local sources, was like "bringing order to chaos."

"This previously scattered information is now all reviewed, analyzed and presented in a collection of papers at an open access journal," she said.

Despite the input from 360 scientists over ten years, authors of the inventory, a precursor to the full census to be released in October, believe that most ocean species remain a mystery.

For every marine species that is known, the census scientists estimate that at least four have yet to be discovered. In a few groups, like fish, scientists believe more than 70 percent of species have been recorded, but for most other groups they say it is likely that less than one-third are known.

The report's scientists believe that the tropics, deep seas and southern hemisphere hold the most undiscovered marine species.

"At the end of the Census of Marine Life, most ocean organisms still remain nameless and their numbers unknown," said biologist Dr. Nancy Knowlton of the Smithsonian Institution, leader of the Census' coral reef project.

"This is not an admission of failure. The ocean is simply so vast that, after 10 years of hard work, we still have only snapshots, though sometimes detailed, of what the sea contains. But it is an important and impressive start."

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Many species live in more than one region, with the Manylight Viperfish winning the title of the most cosmopolitan fish -- it has been recorded in more than one-quarter of the world's marine waters.

The more enclosed seas -- the Mediterranean, Gulf of Mexico, China's coastal waters, Baltic, and Caribbean -- have the most threatened biodiversity, according to the report. The Mediterranean also has the dubious title of being home to more invasive species than any other, most of which arrived via the Suez Canal.

The relatively isolated regions Australia, New Zealand, Antarctica and South Africa have the most species endemic to those areas.

The impact of coastal urbanization, over-fishing and habitat loss are cited by the report as the main threats to marine species.

The hope of the scientists involved is that the census will boost the discovery and management of marine biodiversity.

"We must increase our knowledge of unknown biodiversity more quickly, lest much of it is lost without even being discovered," said Miloslavich.

"International sharing of data, expertise and resources is the most cost-effective way of achieving this."

In a press statement Dr. Knowlton added: "The sea today is in trouble. Its citizens have no vote in any national or international body, but they are suffering and need to be heard. Much has changed just in the few decades that I have spent on and under the sea, but it remains a wondrous and enriching place, and with care it can become even more so."