"We have to recognize that it is completely impossible to draw up a list of all the terrestrial animals. And by Hercules, in the ocean, so vast it were, nothing exists which is unknown for us and, really marvellous fact, the things that nature has hidden in the depths are to us more familiar". And Pline the old, the author of this assertion, to draw up the complete and final "list of marine fauna "; it includes 176 species! from Natural Stories, Pline the old (23 front. J.C. -75).

However, the recent estimates predict more than 10 million of species in the deep ocean. We are far from the 176 species about which Pline spoke!

What are the different landscapes of the abyss ?

The deep sea is the largest environment on the planet, the least well known and one of the least studied. It contains extremely large, continuous habitats such as the millions of km² of abyssal plains and the 65,000 km long mid-oceanic ridge system. At the same time, it encloses relatively small localised geological features such as hydrothermal vents and fluid seepages which support unique microbiological and faunal communities.

The deep-sea ecosystems comprise a complex patchwork of distinctive, specialised habitats that are driven by different sources of energy (photosynthetic versus chemosynthetic).

An unknown and threatened environment...

Our knowledge of deep-sea ecosystems is at a very early stage, where exploration and experimentation still play a major role.

Because of the increasing pressure of human activities in deep waters (offshore exploitation, wrecks, waste disposal, fish trawling), the deep-sea habitat is increasingly the focus of international interests. For example, the OSPAR Convention has included a number of deep-sea habitats (i.e. deep-water corals, seamounts) in a list of endangered habitats. Similarly, deep-sea ecosystems are included in issues dealt by the United Nations Convention on the Law of the Sea as vulnerable habitats requiring special protection. The  value of creating Marine Protected Areas (MPAs) for the conservation and management of ecosystems with a high, valuable, sensitive or rare biodiversity that are potentially threatened is well established.

Why are we studying deep ocean habitats ?

What little we know about deep-sea ecosystems supports the hypothesis that more species occur in the deep sea than anywhere else on Earth.

The use of deep-sea submersibles during the last decades brought new insights into deep-sea environments with the discovery of unusual biologically rich areas on continental and plate margins. These particuliar ecosystems are related to the emission of reduced fluids (cold seeps, hydrothermal vents), peculiar topographic structures (seamounts, deep corals) or to massive organic inputs (whale carcasses, sunken woods).

Their understanding is crucial to understanding carbon cycling in the oceans as well as the functioning of the global biosphere.
 

How do we access to ocean depths ?

Deep-sea research is very expensive and depends heavily on technological developments in the same way as the exploration of space. The main physical constraints to overcome are the pressure and the seawater :