An international team of researchers led by Jim Thomas discovered the new species of sea slugs (nudibranchs), feather stars (crinoids) and amphipods (genus Leucothoe).
There was more variety of these indicator species found than there is in the entire length of Australia's 2,574-kilometres Great Barrier Reef, researchers said.
"In the Madang Lagoon, we went a half mile out off the leading edge of the active Australian Plate and were in 6,000 metres of water," said Thomas, a researcher at Nova Southeastern University's National Coral Reef Institute in Hollywood, Florida.
"This was an astonishing discovery. We returned to our labs and began to formally assess our collections. We had no idea this lagoon's bounty was so profound," Thomas said.
The international team Thomas led included researchers from and the Scripps Institute of Oceanography in San Diego, the California Academy of Sciences and the National Botanical Gardens of Ireland.