Scientists went 4km underwater - and found hundreds of creatures we didn't know we had

At the same time, the study found that the testing of industrial metal mining techniques is already markedly reducing biodiversity on the seabed, although less than expected.

The results of the international project are published in the journal Nature Ecology & Evolution.

Metals for the "green transition" - and the risk to the deep seabed

Demand for so-called critical metals is growing rapidly: they are needed for batteries, electronics and green technologies. Significant reserves of these metals are concentrated in the form of polymetallic nodules in the deep ocean plains. More and more countries and companies want to mine them at the bottom of the seas, but until now there has been little data on what exactly lives in these places and how mining would affect the ecosystem.

"Critical metals are essential for the green transition and they are in short supply. Many of them are found in large quantities on the deep sea floor, but until now no one has shown how they can be mined and what the ecological effect would be," says marine biologist Thomas Dahlgren of Gothenburg University, one of the leaders of the work.

160 days at sea and five years of analysis

The study was conducted under the rules of the International Seabed Authority (ISA), which regulates mining in international waters and requires baseline assessments of ecosystems and the impact of industrial activities.

Over five years, the scientists:

• surveyed the seabed in the Clarion-Clipperton zone between Mexico and Hawaii,

• spent a total of 160 days at sea,

• tested the effects of a prototype deep-sea harvester that collected metal nodules.

Following the passage of the machine:

• the number of individual animals decreased by 37 per cent,

• species diversity by 32%.

"Our data will be important to the International Seabed Authority, which decides in which form to allow industrial mining in international waters," notes Dahlgren.

A world where sediment is growing by a thousandth of a millimetre a year

The part of the seabed under study is at a depth of about 4 kilometres, where no sunlight reaches. It is an extremely nutrient-poor environment: the sediment layer grows by just one thousandth of a millimetre per year.

By comparison:

• you can find up to 20,000 animals in a soil sample from the bottom of the North Sea,

• in a similar volume of soil from the deep sea floor - about the same number of species, but only about 200 animals.

The scientists collected 4,350 animals larger than 0.3 mm living in and on the surface of the sediment.

• A total of 788 species were identified.

• The predominant species were polychaete worms (polychaetes), crustaceans and molluscs (snails, bivalves).

New species from microscopic worms to coral

The University of Gothenburg team was responsible for identifying the polychaetes.

"I have been working in the Clarion-Clipperton area for more than 13 years, and this is the most extensive study that has been carried out here. Most species have never been described before, so molecular (DNA) data was critical to assess the biodiversity and ecology of these deep-sea communities," says Dahlgren.

Among the findings:

• tiny marine polychaete worms 1-2 mm long,

• sea spiders (relatives of land spiders, but a separate group),

• a new species of solitary coral growing on polymetallic nodules called Deltocyathus zoemetallicus.

Scientists have also noticed that the composition of communities on the bottom changes naturally over time, probably due to fluctuations in the amount of organic "snow" - food particles deposited from the upper layers of the ocean.

What we don't know at all yet

It remains unclear how widespread the species discovered are across the deep Pacific Ocean floor.

"What's important now is to try to assess the risk of biodiversity loss due to mining. To do this, we need to survey the part of Clarion-Clipperton (about 30 per cent of the area) that is formally protected. At the moment we have little or no idea what lives there," emphasises the paper's senior author, Adrian Glover of London's Natural History Museum.