Biodiversity in the open sea: from the smallest to the largest, what lives in the open sea?

This maritime area represents more than 50% of the oceans and plunges from the surface down to the deepest abyss. The "water column" between the surface and the seabed remains one of the most poorly understood regions of the ocean.

This immense space seems deserted, but marine life is everywhere, from 0 to 11,000 metres down into the Mariana Trench. As soon as an opportunity arises, biodiversity develops and a whole network settles in, even if it is temporary.

A shoal of tiny fry, clouds of shrimp, large fish such as tuna, swordfish, or sharks, marine mammals such as whales are just some of the inhabitants of the open sea.

The definition of biodiversity according to the Office Français de la Biodiversité refers to "all living beings and the ecosystems in which they live. This term also includes the interactions of species with each other and with their environment."

• Less than 20% of the oceans have been explored,
• 72% of the planet is taken up by the oceans, with the high seas accounting for 50%.
• The oceans represent more than 90% of the habitable volume for the living world.

Out of all the species described by scientists, only 13% are marine species, i.e. 240,000 species. In effect, there is still little knowledge of the deep ocean zones, which are difficult to access, and many micro-organisms have not yet been discovered.

Scientists estimate the number of marine species to be between 500,000 and 10 million. New species are discovered every year and if we consider the microbial world, the number of species would approach ten billion.

There is still a lot to be done to identify the diversity of the species in the high seas. Each one can be the source of new molecules, genes and precious substances for humanity, thanks to biotechnologies that are opening up many horizons in a wide range of fields such as health, medicines and cosmetics.

From the smallest to the largest, living beings in the marine world have a role and an importance in marine biodiversity and in the healthy functioning of the planet and contribute to the services provided by the ocean.

Life on Earth would not be possible without the ocean, and the basis of marine life is plankton.

• Plankton are all the living things that drift in the open water and are carried by the sea currents: tiny algae, animal larvae, small crustaceans and jellyfish.
• Only 20-30% of planktonic organisms have been identified and almost nothing is known about marine bacteria and viruses. It is estimated that there are between 10 and 100 billion micro-organisms in a single litre of seawater!
• 95% of the marine living mass is made up of micro-organisms such as plankton.
• When you breathe, half the oxygen that enters your lungs comes from the ocean. Phytoplankton algae release oxygen and fix CO2 through the process of photosynthesis.
• Plankton is the basis of most marine food chains: phytoplankton can be grazed by zooplankton (microscopic marine animals) which are then eaten by fish or planktivorous marine animals such as whale sharks or basking sharks, or even whales.
• Phytoplankton fix CO2 through the process of photosynthesis. Phytoplankton can be grazed by animal plankton or die. Once dead, it falls to the ocean floor where some of the carbon it contains is trapped in marine sediments and remains stored in mineral form. Thus, the deep sea is a CO2 "sink" and plankton helps to regulate the climate.

The 2015 oceanographic campaign by Tara dedicated to plankton showed the abundance of viruses, bacteria and unicellular organisms that exist in the ocean: more than 100,000 species of unicellular microalgae and nearly 150 million genes were discovered.

Living conditions on the high seas depend on many parameters such as water movement, light, temperature and pressure.

Marine life thrives in many places: oceanic upwellings that stir up nutrients trigger "blooms" of life, a flourish of phytoplankton; the "sardine run" during the migration of millions of pilchards from California to South Africa attracts all sizes of predators. Even a cluster of seaweed and a piece of floating wreckage form pockets of life in the open sea.

Small species of pelagic fish move from one food oasis to another. They follow the plankton and attract larger predatory fish.

The inhabitants of the open sea comprise a vast array of animals such as jellyfish, squid, fish, sharks, marine mammals and sea turtles. While some fish swim around alone, such as sunfish or sailfish, others live in pairs or small groups, such as the common dolphinfish, but many live in shoals (tuna, sardines, mackerel, etc.).

They swim from one region to another to feed or breed. Some seabirds such as the albatross also live out at sea, feeding on fish and only coming ashore to breed.

The leatherback turtle Dermochelys coriacea is the largest of the sea turtles and of all turtles. It is also a globetrotter that crosses the oceans to lay its eggs on the same beaches. It can dive down to more than 1,000 metres below the surface and stay underwater for 80 minutes without breathing.

It feeds on jellyfish and is a victim of plastic pollution in the oceans. It can choke to death if it mistakes a plastic bag for a jellyfish. Urban development along the coast is a threat to turtle reproduction: nesting sites are thus diminished and eggs are sometimes destroyed.

The ocean sunfish Mola mola is a strange fish. It looks like a large white disc and floats near the surface. Sunfish can weigh up to 1 metric ton, which makes them the heaviest bony fish in the world. It is a voracious consumer of jellyfish and is a victim of plastic bags, which it mistakes for jellyfish, and of dragnets.

The lion's mane jellyfish Cyanea capillata is the largest of jellyfish. Jellyfish are the delicacy of sea turtles and sunfish, and are proliferating due to rising ocean temperatures and the disappearance of some of their predators.

The Indo-Pacific sailfish Istiophorus platypterus is the fastest of all fish and is a migratory fish, a fierce hunter of shoals of fish. It is a by-catch of fishing nets and a target for sport fishing.

The bottlenose dolphin Tursiops truncatus also called the dauphin souffleur (blower dolphin) in French – is one of the best-known dolphin species. Like other cetaceans, the dolphin is affected by noise pollution from ship traffic, which disrupts its echolocation.

The whale shark Rhincodon typus is the largest fish in the world. This placid shark is harmless to humans. Fishing, like accidental catches, are a threat to the whale shark, as are pollution and collisions with ships.

The Rice's whale Balaenoptera edeni is a pelagic baleen whale. The threats to this cetacean are bycatch and throttling on fishing nets, and sometimes collisions with ships too . These marine mammals are sensitive to noise pollution from maritime traffic, which disrupts communication between individuals.

The sardine Sardinops sagax also called the South American pilchard is a migratory fish that forms huge shoals several kilometres long. They are prey to swordfish, dolphins and seabirds. The effects of global warming could lead to variations in population distribution areas, i.e. sardines could migrate to cooler waters, which would lead to changes in the food chains.