A rich and surprisingly diverse array of marine animals has been discovered living in total darkness in the deepest parts of the Atlantic where no sunlight ever penetrates. They range from a giant octopus-like creature with eight legs and fins that flap like an elephant's ears to tiny crustaceans that shine like gold-encrusted jewels.

Marine biologists have been astonished by the range of animals they have found during an underwater expedition that that took them down 5,000m (three miles), where they have now identified 17,650 deep-sea species.

One of the most surprising animals was a rare specimen of a primitive creature called a cirrate, or finned octopod, commonly called a "Dumbo" because they swim by flapping a pair of ear-like fins, rather like the Disney cartoon character. But the particular species the biologists found is now called "Jumbo Dumbo", because it grows up to 2m long and weighs about 6kg, the largest specimen of the type ever discovered, records the Census of Marine Life, the umbrella organisation overseeing the global survey of the oceans.

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The Dumbos collected by the scientists were found between 1,000m to 3,000m (0.6 to 1.9 miles) down on the mid-Atlantic ridge, a vast chain of underwater mountains that stretch north-south along the seabed. The scientists also took samples of mud from the seabed and found that it contained a surprisingly rich collection of fauna. Most of these animals are only a few millimetres in size, and they live on the sediments that fall from above, ranging in size from dead plankton to the residue of the carcasses of massive whales.

"To survive in the deep, animals must find and exploit meagre or novel resources, and their great diversity in the deep reflects how many ways they are to adapt," said Robert Carney of Louisiana State University, a co-leader of the deep-sea project. "Some scientists have likened the deep mud's biodiversity to that of tropical forests. In college, I was taught that high biodiversity is a function of habitat diversity, many nooks and crannies. But it is hard to imagine anything as monotonous, nook-less and cranny-less as deep-sea mud," Dr Carney said.

"There is both a great lack of information about the 'abyss' and substantial misinformation. Many species live there. But the abyss has long been viewed as a desert. Worse, it was viewed as a wasteland where few to no environmental impacts could be of any concern. The abyss is vast and best yet, hidden from sight."

The scientists used autonomous, unmanned submarines to explore the deepest reaches of the ocean floor, extending down several miles. At between 1.25 and 1.5 miles, the scientists found a bizarre, elongated orange fish-like animal called Neocyema, only the fifth specimen of the species to be caught. Another slow-growing fish living in complete darkness, called a rat-tail, was found a similar depths feeding on crustaceans. The scientists also collected about 680 specimens of microscopic animals called copepods, which live in the plankton, but they were able to identify only seven of them. The rest appeared to be new to science, they said, including one that shone like a jewel with a golden sheen when lit.

"The distribution of species in the deep sea is full of mysteries," said Dr David Billett of the National Oceanography Centre in Southampton. "In addition to the boundaries caused by underwater topography, ridges and seamounts, there are unseen, and as yet unexplained walls and barriers that determine supplies of food and define the provinces of species in the deep sea. The abyssal fauna is so rich in species diversity and so poorly described that collecting a known species is an anomaly. Describing for the first time all the different species in any coffee cup-sized sample of deep-sea sediment is a daunting challenge."

Mireille Consalvey, of the New Zealand National Institute of Water and Atmospheric Research, said the conditions of the expedition were difficult, with many scientists struggling with seasickness amid high winds and 30-foot swells. "It can be a tough environment down there," she added. "I recall the abject fear when our video-imaging system snagged for 40 minutes on a rockface: the slow, scary process of recovering it, and the shared worry that our valuable recording equipment would arrive at the surface battered and bent. Thankfully, the recorder survived the ordeal better than many of us and yielded brilliant new footage of this remote realm."