An expedition to one of the deepest parts of the ocean has discovered a "supergiant" species.

The huge crustacean was discovered more than four miles (7km) deep in waters north of New Zealand by scientists from the University of Aberdeen.

The creature is a type of amphipod, commonly found in the deep sea, which are usually 0.8in-1.2in (2cm-3cm) long. The new specimen measured 11in (28cm).

Using submergence cameras and a large trap designed by the university's Oceanlab, the team was able to explore up to depths of six miles (9.9km).

The team was hoping to find specimens of deep-sea snailfish which have been photographed before but have not been seen since the 1950s.

Expedition leader Alan Jamieson said: "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.

"It's a bit like finding a foot-long cockroach."

Seven specimens were caught in the trap and up to nine were photographed gathering around the camera system.

Scientists say the term "supergiant" was coined by US researchers when they found some large amphipods in the early 1980s.

The supergiant amphipod has not been reported since and has faded into the realms of rare and mysterious deep-sea creatures.

Dr Jamieson said: "The surprising thing 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 didn't photograph or capture a single supergiant. They were there for a day and gone the next."

The researchers said the newly found amphipods are the biggest whole specimen of supergiant caught, and have never been seen so deep in the sea.

New Zealand's National Institute of Water and Atmospheric Research also took part in the expedition.

Ashley Rowden, from the Wellington-based institute, said: "It just goes to show that the more you look, the more you find.

Dr Rowden added: "For such a large and conspicuous animal to go unnoticed for so long is just testament to how little we know about life in New Zealand's most deep and unique habitat."

Researchers will try to establish whether the new samples are the same species as those found by the US scientists near Hawaii in the 1980s.