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This image from the Census of Marine Life shows prorocentrum, an unusual plankton with two large valves, often marked with pores that can be used in the identification of species. ((Bob Andersen and D. J. Patterson, Census of Marine Life/Associated Press))

Scientists conducting a count of every living thing in the ocean are turning their attention to hard-to-see organisms such as tiny microbes, zooplankton, larvae and burrowers in the seabed.

While, individually, they're tiny, microscopic life forms are important parts of marine ecosystems and provide food for larger animals.

The Census of Marine Life, a 10-year, worldwide project, is in its final year and involves more than 2,000 scientists in more than 80 countries. The group is scheduled to give a report in London Oct. 4.

The final year of the project has been devoted primarily to counting marine microbes.

"Scientists are discovering and describing an astonishing new world of marine microbial diversity and abundance, distribution patterns and seasonal changes," said Mitch Sogin of the Marine Biological Laboratory in Woods Hole, Mass., who heads up the microbe portion of the census.

The microbe census has found more than 5,000 previously unknown forms of life, and researchers believe that there may be many times that number that haven't been found yet.

Scientists involved in the census expect the number of known zooplankton species, for example, to double from the current 7,000 when the census is completed.

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This image from the Census of Marine Life shows an acantharian, one of the four types of large amoebae that occur in marine open waters. ((Bob Andersen and D. J. Patterson, Census of Marine Life/Associated Press))

After studying samples taken from more than 1,000 sites, scientists concluded there may be as many as 100 times more microbe genera in the sea than they had thought. Indeed, a 2007 study in the English Channel alone yielded 7,000 new genera of microorganisms.

Genus is the category of life ranked between family and species. For example, the hominid family has four living genera: homo (humans), pan (chimpanzees), gorilla and pongo (orangutans).

One of the findings of the global study is a thick mat of bacteria in the ocean off South America that's bigger than some countries.

Marine biologist Victor Ariel Gallardo led the team that spotted the mat off the coast of Chile and Peru. It consists of layers of bacteria, each one big enough to be seen with the naked one, between two and seven centimetres long.

"The bacterial mat extends over the size of Greece," said Ian Poiner of the Australian Institute of Marine Sciences, who was involved in the census.

The "Goliath" bacteria live in an oxygen minimum zone of the ocean and rely on hydrogen sulphide for their metabolism instead. Such low-oxygen ecosystems are similar to those that occurred during the Proterozoic period between 2.5 billion and 650 million years ago.

"These things are ancient forms of life," Poiner said.

Deap sea not a lifeless desert

Much other fascinating research has come out of the marine census over the past decade.

For example, scientists using remotely operated deep-sea submarines have found that roundworms can flourish in the deepest part of the oceans. In some cases, researchers found more than 500,000 of the creatures in just one square metre of soft clay.

"Such findings make us look at the deep sea from a new perspective," said  researcher Pedro Martinez Arbizu of the German Centre for Marine Biodiversity Research. "Far from being a lifeless desert, the deep sea rivals such highly diverse ecosystems as tropical rainforests and coral reefs."

Researchers also found more than 16,000 species of seaworms, or loriciferans, the smallest known multi-cellular marine animals. Scientists call the creatures "girdle wearers" because of their characteristic hind shells that resemble a corset.

Improved DNA analysis is allowing biologists to make new discoveries about previously known species of animals, too. 

Tracey Sutton of the Virginia Institute of Marine Science and colleagues, led by the Smithsonian Institution's G. David Johnson, used genetics to show that three types of fish thought to be different are really one species.

They discovered that what were previously classified as three completely different families of fish — the Mirapinnidae (tapetails), Megalomycteridae (bignose fishes) and Cetomimidae (whalefishes) — are really all one species at different stages of life.

The tapetails are the fish larvae, which grow into either female bignose fish or male whalefish, in an extreme case of what biologists call sexual dimorphism.

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With files from The Associated Press, Australian Broadcasting Corporation.