Scientists dived to depths of between 700m right down to 3,600m for more than 300 hours using UK's deepest diving remotely operated vehicle, Isis.
Researchers surveyed flat plains, cliff faces and slopes of the giant mountain range that divides the Atlantic Ocean into two halves, east and west.
The team found a little known group of animals close to the missing link in evolution between backboned and invertebrate animals.They have no eyes, no obvious sense organs or brain but there is a head end, tail end and indication of a backbone.
The team also discovered sea cucumbers, or holothurians on steep slopes, small ledges and rock faces of the underwater mountain range.
"We were surprised how species, elsewhere considered rare, were found in abundance on the Mid Atlantic Ridge and we were finding new species up to the last minute of the last dive in the voyage," said Dr Andrey Gebruk, Shirshov Institute, Moscow.