A “riot of species” has been uncovered in the world’s oceans by the most comprehensive survey ever conducted of life in the seas, according to scientists.

The Census of Marine Life found around 6,000 potential new species found by the project and the overall estimate of known marine species increasing from 230,000 to nearly 250,000.

Some of the “most beautiful and wonderful” species found in the decade of discovery included a Jurassic shrimp thought to have become extinct 50 million years ago and a crab named the Yeti crab.

Discoveries included the revelation that North Atlantic tuna on the eastern US seaboard were the same fish as those off the coast of Spain or in the Mediterranean as they migrated across the ocean.

The census also showed life was found in the most in-hospitable places, and was much more connected than previously thought, through genetic relationships between creatures, the movement of species around the oceans and the “snow” of food falling from the upper layers of the sea into the deep.

In some cases, populations had plummeted by 90 per cent from historical baselines, and fish such as swordfish which were being caught now were also much smaller than in the past.

And creatures at the bottom of the food chain known as phytoplankton, near the surface had declined globally, analysis of observations from ocean-going vessels since 1899 showed.

According to the research, numbers of some species have declined within a human generation.

It also revealed that people began catching marine creatures a long time ago and on a much broader scale than previously thought.

The project has also generated a website, iobis.org, on which anyone can see the distribution of a species in the ocean from a giant database of names and “addresses” of marine creatures.