At least one new species, a tiny crustacean called a seed shrimp, is likely new to science, researchers said.
Another exciting find was a "spiral poo worm," an animal first identified in 2005 that deposits spiral-shaped feces, some of which have been found in the fossil record dating back hundreds of millions of years.
"We found lots of these primitive species," said expedition leader Monty Priede, director of the Oceanlab research center at Britain's University of Aberdeen.
The expedition brought together an international team of 31 scientists coordinated by the Norway-based MAR-ECO project and the global Census of Marine Life program initiative.
"It was like going to a new country," Priede said.
Using the latest technology, including remotely operated underwater vehicles, the researchers were able to observe creatures living between depths of 2,600 and 11,500 feet (800 and 3,500 meters).
Until now this region of the ocean had scarcely been explored because of its remoteness and depth.
But the latest findings show that the Mid-Atlantic Ridge is teeming with life. Many species found in abundance there had only recently been discovered and were thought to be very rare. "The Mid-Atlantic Ridge is roughly equivalent in size to the European Alps and is one of the largest areas of habitat available in the ocean," Priede said.
Compared to long, thin sections of the ocean floor that lie closer to continents, the Mid-Atlantic Ridge is much larger and more varied, he said.
The ridge is thought to have a major effect on ocean currents, which influences the productivity and biodiversity of the ocean.
And as is the case with mountain ranges on land, some species prefer one side to the other, Priede said.
"We see different species living on the American and European sides of the ridge," Priede said.
The team brought back thousands of specimens for analysis and left behind six automatic observing stations to provide a continuous feed of measurements and photos over the next two years.
Further voyages planned for 2008 and 2009 will retrieve this equipment and collect more samples, Preide said.