Scientists know more about the moons of Jupiter than they do about what lives on the bottom of the ocean, the head of the Australian Institute of Marine Science (AIMS) says.

In the first biodiversity study of its kind, researchers from the north Queensland-based AIMS, US-based Scripps Institution of Oceanography and the Pacific Islands Fisheries Science Centre will carry out a four-year census of the world's coral reefs.

The Australian study, one of 17 being carried out by the global network of researchers, will try to discover how many different species of plants and animals exist on coral reefs.

AIMS chief executive Ian Poiner described reefs as "the rainforests of the sea" because of their extraordinary biodiversity.

But little is known about them compared to their terrestrial counterpart.

"The oceans are the largest habitat on the earth," Dr Poiner said.

"And, in terms of what lives on the bottom ... we know more about the moon or we know more about some of the moons of Jupiter than we know about the ocean."

The study will assess various areas, including The Great Barrier Reef, the Caribbean, the Hawaiian islands and the Indian Ocean.

Dr Poiner said scientists knew about corals, fish and molluscs but were missing key information about most of the animals that lived in and around coral reefs.

"We might have a bit of information about this reef here or this reef there, but in terms of being able to say what is the biodiversity and how many different species exist in the world's coral reefs, we can't answer that question," he said.

Current estimates range from a million species to nine million species.

"But those estimates come from pretty wild extrapolations from rainforest data and also some extrapolations from measuring artificial reefs and aquariums, so they're pretty flimsy," Dr Poiner said.

He said scientists aimed to figure out how to maintain ocean biodiversity, particularly in the Caribbean, where humans have had an impact on 80 per cent of its reefs.

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