a must see: New species discovered.

The most comprehensive survey ever undertaken of the world's oceans has uncovered around 6,000 potential new species.

Census of Marine Life

The first Census of Marine Life, which has taken a decade to complete, estimates the number of known species to increased from 230,000 to nearly 250,000.

But it also suggests many species, from turtles to seabirds and sharks, are in decline thanks to human activity.

The £413m research project was established in 2000 with the aim of answering three questions:

- What lived in the oceans?

- What does live in the oceans?

- What will live in the oceans?

The ensuing survey involved more than 2,700 researchers from 80 nations, who spent a total of 9,000 days at sea on more than 540 expeditions.

Census of Marine Life

Various forms of new technology were employed, such as tagging fish to see where they were swimming, fitting seals with monitors to record data as they dived, and using acoustic systems to measure fish populations as large as Manhattan Island.

Dr Ian Poiner, chairman of the project's scientific steering committee said that, from the Poles to tropical waters and the deep sea, there was an abundance of life.

But even after a decade of work researchers cannot be sure of the exact number of species in the world's oceans.

It is thought there could be at least a million species in total.

"This co-operative international 21st Century voyage has systematically defined for the first time both the known and the vast unknown, unexplored ocean," Dr Poiner told the BBC.

"All surface life depends on life inside and beneath the oceans," he added.

"Sea life provides half of our oxygen, a lot of our food and regulates the climate. While much remains unknown, including at least 750,000 undiscovered species and their roles, we are better acquainted now with our fellow travellers and their vast habitat."

Some of the "most beautiful and wonderful" species found during the survey include a Jurassic shrimp thought to have become extinct 50 million years ago and a crab named the Yeti crab.

Census of Marine Life

The census also included genetically sequencing tiny microbes to tell them apart.

Dr Poiner said there could be as many as one billion different types in the ocean.

It is hoped the Census of Marine Life will help set a baseline for measuring changes that humanity and nature will cause in the future.