New Zealand scientists have photographed the deepest fish in the Southern Hemisphere - over 7km down.

The scientists with colleagues from Britain and Japan caught on camera a dozen snailfish - scientifically called Notoliparis kermadecensis - at a depth of 7561 metres while "fishing" in the Kermadec Trench, northeast of New Zealand.

Some species of snailfish live in shallow water and even rock pools, but these live only in trenches and on the abyssal plain of the ocean floor, below 6000 metres.

They survive in total darkness, near freezing temperatures and immense water pressure - the Kermadec fish were swimming at pressure of about 8000 tonnes per square metre, equivalent to that of 1600 elephants standing on the roof of a Mini car.

They feed on tiny shrimp-like creatures that scavenge the carcasses of dead fish and detritus reaching the ocean floor, and gathered sociably around a bait impaled below a camera dropped over the side of the research vessel Kaharoa.

Dr Ashley Rowden, of National Institute of Water and Atmospheric Research (Niwa), said the fish were strikingly similar to the deepest fish ever caught on film - another snailfish known as Pseudoliparis amblystomopsis - at 7700m in the Japan Trench in 2008.

Researchers said the two trench-dwelling species must share a common ancestry, but were isolated geographically by thousands of kilometres.

They could only survive at the immense pressure found at these depths and had no apparent means of travelling between the trenches.

A specimen of the Kermadec fish has only been caught once, in 1952 by the Danish Galathea expedition, and in 2007 it was photographed at a shallower depth of 6900m.

The voyage leader, Dr Alan Jamieson from Aberdeen University in Scotland said: "It's fascinating to find these large aggregations of fish which are so similar yet incredibly isolated from one another.

"Why these delicate fish have chosen to inhabit such immense depths is still a mystery."

The Kermadec Trench expedition was the sixth by the HADEEP project, a joint project between Aberdeen's Oceanlab and the Tokyo University's Ocean Research Institute. It was funded by the Nippon Foundation and supported by Niwa.