By Victoria Gill
Science and nature reporter, BBC News
Deep diversity: The team captured thousands of images of life in the trench
Biologists have discovered a new species of fish in one of the world's deepest ocean trenches, previously thought to be entirely devoid of fish.
They captured images of the creature, a type of snailfish, in the Peru-Chile trench in the south-east Pacific Ocean.
The trench is more than 8,000m deep; the fish were found at 7,000m.
This is the fifth deep trench the team has investigated and they found it to harbour the greatest diversity of species of any they have explored.
The snailfish species was discovered at a depth of 7,000m
Dr Alan Jamieson, the University of Aberdeen marine biologist who led the study, said that he and his team also captured images of a group of cusk-eels in what he described as a "feeding frenzy".
"The eels were at 6,000m and we've never seen anything at that particular depth before," he told the BBC.
"I'd put money on [the cusk-eels] being a new species too, but that's difficult to confirm from a few photographs. We really need to bring a specimen to the surface."
During a three-week expedition, the team used a lander containing a deep-sea camera. This took 6,000 images inside the trench, between 4,500m and 8,000m (15,000 - 26,000 feet).
As well as the snailfish and cusk-eels, the team also captured images of several crustaceans - scavengers that feed on the remains of larger animals.
"When a [marine animal] dies on the surface, its body falls to the bottom," explained Dr Jamieson. The current carries the odour of the remains and this brings in the scavengers.
The scientists were surprised to see a "feeding frenzy" at 6,000m
This meant that the scientists were able to draw in the scavengers by baiting their lander with fish from the local fish market.
"We found ourselves in a fish market in Ecuador at midnight, negotiating over the price of tuna carcasses," recalled Dr Jamieson.
The snailfish make use of the feeding frenzy by eating the scavengers. They are suction feeders that detect their prey with sensors on their heads. These pick up any disturbances in the water.
"When they sense movement, they suck in all the water in front of them in the hope that there are crustaceans in that water," said Dr Jamieson.
The team has been investigating extreme depths across the globe for three years. Their findings to date have included capturing the world's deepest fish on camera for the first time.
The next stage of their project is to re-visit the Japan trench, which is in the northern part of the Pacific. They plan to lower fish traps to capture specimens of some of these strange creatures, which they can study in order to reveal exactly how they are adapted for such extreme depths.
The expedition is the seventh to take place as part of Hadeep - a collaborative research project between the University of Aberdeen's Oceanlab and the University of Tokyo's Ocean Research Institute, with support from New Zealand's National Institute of Water and Atmospheric research institute (NIWA).