The ocean is the largest living space in the world and covers at present 70.8% of the surface of the Earth. But we should really think of the ocean in terms of volume – around 1,370 million km3. Biodiversity cannot be likened to a simple list of species that inhabit a particular ecosystem. It is considerably more than a catalog or inventory, and in fact includes the entire set of relationships between living beings among themselves and with their environment. The physical consequences of osmotic flux (water and electrolytes) in the marine environment led living organisms to different strategies. The recognized species diversity in the ocean does not exceed 13% of all living species currently described – less than 250 000. This is very little, and may be explained for two reasons.
The first is that our knowledge, especially for deep zones and for microorganisms, various bacteria and protists is still only very partial, so we significantly underestimate oceanic diversity. New techniques, such as coupling between flow cytometry and molecular probes, are allowing us to discover extraordinary biological diversity. At present, widespread sequencing of the ocean water mass, "random genome sequencing" provides data that seems to be mostly unknown. The very recent Tara Oceans expedition's circumnavigation of the world's ocean provides us with valuable information on the abundance and variety of viruses, bacteria and mainly protists. For all prokaryotes and very small eukaryotes, molecular approaches (sequencing of 16S or 18S ribosomal RNA among others) bring surprising new information every day.