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The Cute Dumbo (Grimpoteuthis discoveryi) is a species of cirrate, a rare primitive finned octopod. ((Mike Vecchione))

The creatures living in the depths of the ocean are as weird and outlandish as the creations in a Dr. Seuss book: tentacled transparent sea cucumbers, primitive "dumbos" that flap ear-like fins, and tubeworms that feed on oil deposits.

A report released Sunday recorded 17,650 species living below 200 metres, the point where sunlight ceases. The findings were the latest update on a 10-year census of marine life.

"Parts of the deep sea that we assumed were homogeneous are actually quite complex," said Robert Carney, an oceanographer at Louisiana State University and a lead researcher on the deep-sea census.

Thousands of marine species eke out an existence in the pitch-black depths by feeding on snowlike decaying matter that cascades down — even sunken whale bones. Oil and methane also are an energy source for the bottom-dwellers, the report said.

The researchers have found about 5,600 hitherto unknown species, in addition to 230,000 marine species previously documented, and hope to add several thousand more by October 2010, when the current census will be done.

The scientists say they could announce that a million or more species remain unknown. On land, biologists have catalogued about 1.5 million plants and animals.

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The transparent sea cucumber, Enypniastes, lives 2,750 metres below the surface in the northern Gulf of Mexico. It creeps on its tentacles at about two centimetres per minute while sweeping sediment into its mouth. ((Larry Madin/Associated Press))

The researchers have found 5,722 species living in the extreme ocean depths — waters more than one kilometre deep.

"The deep sea was considered a desert until not so long ago; it's quite amazing to have documented close to 20,000 forms of life in a zone that was thought to be barren," said Jesse Ausubel with the Alfred P. Sloan Foundation, a sponsor of the census. "The deep sea is the least explored environment on Earth."

More than 40 new species of coral were documented on deep-sea mountains, along with cities of brittlestars and gardens of anemone. Nearly 500 new species ranging from single-celled creatures to large squid were charted in abyssal plains and basins.

Also of importance were 170 previously undocumented species that get their energy from chemicals spewing from ocean-bottom vents and seeps. Among them was a family of "yeti crabs," which have silky, hairlike filaments on the legs.

In the mid-Atlantic, researchers found 40 new deep species and 1,000 in all, said Odd Aksel Bergstad, an oceanographer with the University of Bergen in Norway who was reached by telephone in the Azores.

"It was a surprise to me to find such rich communities in the middle of the ocean," he said. "There were not even good maps for the area. Our understanding of the biodiversity there was very weak."

More than 2,000 scientists from 80 countries are working to catalogue the oceans' species.

Researching the abyss is costly and difficult because it involves deep-towed cameras, sonar and remotely operated vehicles that cost $50,000 a day to operate, Carney said.

Once the census is complete, the plan is to publish three books: a popular survey of sea life, a second book with chapters for each working group and a third focusing on biodiversity.