An expedition to one of the deepest places on earth has discovered a species of huge crustacean that looks like a 28cm prawn.

Scientists scouring the waters of the Kermadec Trench, north of New Zealand, found the "supergiant" amphipods at a depth of 7km.

The team, made up of scientists from the National Institute of Water & Atmospheric Research (NIWA) and the University of Aberdeen in the UK, made the discovery when they deployed a camera in an attempt to recover specimens of deep-sea snailfish.

Voyage leader Alan Jamieson, from the University of Aberdeen, recalled: "At the moment the traps came on deck we were elated at the sight of the snailfish as we have been after these fish for years. However, seconds later I stopped and thought 'what on earth is that?' whilst catching a glimpse of an amphipod far bigger than I ever thought possible."

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The specimens are the biggest amphipods caught and found at the deepest level.

"The surprising thing here is that we have already been to this deep trench twice and never come across these animals before. In fact a few days after the discovery we deployed all the equipment again on the same site and we failed to photograph or capture a single supergiant; they were there for a day and gone the next," Dr Jamieson said.

Ashley Rowden, from NIWA, said for such a large and conspicuous animal to go unnoticed for so long showed how little we know about life in New Zealand's deepest habitat.

The team will now try to determine why, of the hundreds of species of deep-sea amphipods, these ones have evolved to be so large.