Months later, the scientists are still not entirely sure if the animal was a fish. "It was a type of animal that didn't match the characteristics of any we know of," said Michael Vecchione, a deep-sea expert and part of the expedition.
Yesterday's preliminary report on the census of marine life was an occasion in the underwater world, where scientists have toiled relatively alone and overlooked, deprived of the glories and glamour heaped on space explorers.
It was hoped the billion dollar marine project might alter that. During the past three years, more than 300 scientists from 53 countries have identified three new species of fish each week. They now believe there are 15,304 species of fish in the seas, and as many as 210,000 other varieties of marine life.
By the time the census is complete in 2010, the scientists believe they may be cataloguing as many as 25,000 newly discovered species in the ocean.
Sadly, the specimen discovered by Mr Vecchione's crew disintegrated when it came to the surface. However, as the submersible bumped along the craggy ocean floor plumbing depths almost three miles below the Atlantic surface, the scientists encountered other unknown species. They included a 30cm-long (1ft), webbed octopus that looked a little like a cartoon ghost, and a "lizard" skittering along the ocean floor.
That voyage along the underwater mountain ridge in the Atlantic, and explorations off the coast of Alaska in the northern Pacific and elsewhere, are intended to overcome "the lingering obscurity of the sea world". Until now, relatively little has been documented on marine life, aside from the 200 or so relatively large species fished commercially. Prohibitive costs prevented further exploration, especially in the deep seas, until the advent of submersibles and remotely operated underwater vehicles.
The marine biologists assembled yesterday at Washington's Smithsonian Institute argued that without a complete picture of the numbers and varieties of marine life, it would be impossible to realise the full impact of climate change, or environmental damage, such as pollution or overfishing, on marine habitats and feeding systems.
To that end, scientists in British Columbia, Canada, have fitted miniature electronic tags to young salmon to chart their perilous journey from river to sea in an attempt to determine what is depleting their numbers.
Other researchers have fitted more than 2,000 yellow and blue fin tuna with acoustic devices to map their feeding areas and migration paths. The devices, which are linked to satellites, also gather information about deep-ocean terrain, including extinct undersea volcanos.
Meanwhile, geneticists are using new DNA sequencing techniques to catalogue microbes less than a millimetre long. By the time the project is finished, they hope to have established a marine life "base line" from which they will be able to predict change.
"Many parts of the ocean have never been explored," said Ron O'Dor, the chief scientist for the census. "We estimated that no more than one tenth of 1% of the ocean has been sampled biologically - or even less than that."