Marine biodiversity: how many species actually live in the ocean?



By reading this word you probably picture luxuriant tropical forests abundant with birds, ferns, insects, mammals and streaming water. Or on a more negative note, you may think of species extinction given the overwhelming quantity of news on this topic. In any case, I doubt the ocean comes first in your representation of biodiversity. And that is not your fault! Media coverage of the marine world is almost nonexistent and it is natural to instinctively think of our surroundings (there are no whales, corals or fish in your garden as far as I know). We will see that the ocean is more luxurious than we may think and so far pretty unknown! 1/3 to 2/3 of marine species have not been discovered yet. But before that, let’s start with a quick definition…


What’s hidden behind the word “biodiversity”?

It is a concept involving multiple dimensions (terrestrial and marine systems as well as ecological complexes of which they are apart) that is central to ecology and conservation. To make it simple, when scientists refer to biodiversity, they quite simply mention the number of species existing. For the marine world, it is called marine biodiversity. Therefore biodiversity refers to biological diversity at 3 levels: genetic diversity within species, species diversity between them and on a bigger spectrum, ecological diversity.


Why give any interest to biodiversity?

After all, what could we gain from knowing the number of species living on the planet?

Knowing if there are 10 or 100 sea star species living by our sides will not change our lives. But being aware of the number of living beings inhabiting this planet reminds us that humans are only one little part of this massive ecosystem called Earth. And this is a good reminder don’t you think? In biology, it is important to study how many species there are on Earth to better understand the structure, functioning and evolution of biodiversity at a local and global scale. It is in fact a reference measure for species conservation study.


Our current knowledge of marine biodiversity

That’s all very interesting but, what is the knowledge of marine biodiversity nowadays?

Of 1.6 million species described by scientists, around 220 000 belong to the marine world. Yes, it only represents 12,5% of recorded species. The depth — the deepest trench is 11 000 m deep compared to 9 000 m for the highest summit on Earth; the complete darkness at 200 m and above all human inability to breathe underwater make the ocean an extremely difficult environment to study and explore.


In fact, we would have explored only 5% of the ocean until today! It is only around 1940 that oceanographers became interested in the seabed and that scuba diving turned out to be autonomous as we know it. This explains the gap with terrestrial biodiversity knowledge.


The Ocean: an Aladdin’s Cave

But be reassured, as I was telling you above, 1/3 to 2/3 of marine species should be unknown!

According to recent scientific studies, the marine world would host around 2.2 million species and Earth itself 8.7 million (without taking into account bacteria and viruses!). Some estimations even assess 100 million existing marine species. We are far far away from knowing everything!


The different estimations are due to the diverse scientific methods used. Scientists can estimate from expert opinions, extrapolations or from one geographic area.

In those estimations, scientists also have to take into account what we call cryptic species: no morphological difference is visible but they are genetically distinct. They would add tens of thousands of species, particularly in species groups with few visible diagnostic characteristics such as corals or marine worms. But identifying cryptic species is a true challenge: you must sequence the DNA of every single species in question.

The least known marine group species would be sponges, copepods and isopods, nematodes, marine fungi, gastropods and much more.


If we do not know the exact number of species inhabiting the ocean, we at least know that marine biodiversity is far more extended than terrestrial biodiversity. Indeed, the marine world hosts 34 phyla — groups in which living beings share the same ancestor. By comparison, land only hosts 15 phyla and 90% of terrestrial species belong to the Arthropod phylum — spiders, insects or crustaceans.


What can we expect in the years to come?

More and more species are discovered every year thanks to technology and genetic progress allowing better studies of the ocean floor, but also thanks to sampling methods diversification and an increasing number of scientists.

More species have been discovered in the past 10 years (20 000) than during the last decades. And the number of scientists describing new species is increasing faster than the number of new species being described. From now on, we can even study and sample geographic areas previously undersampled or not approachable such as marine caves, hydrothermal vents or the deep sea.


In the Galapagos Islands, the use of submersibles and deep-diving resulted in the discovery of 30 new fish species. It is enormous when we know that this region has already been heavily studied! Marine biodiversity will never cease to amaze us. We are far away from discovering all its secrets. In 1991, we thought there were less than 200 000 marine species. 30 years later we exceeded this estimation!


Anyhow, whether it is 2.2 or 10 million species navigating within the ocean, it is a world as important as the terrestrial world and maybe even more diverse and richer! With its surface area and volume, I highly doubt the opposite. Especially as everything debuted in the ocean more than 1.5 billion years ago.


This mysterious universe fascinates me since I can remember hence my choice of specializing in marine biology. Without the ocean, life on Earth would be nonexistent. Therefore I desire to share my knowledge and information on this universe that I cherish. I profoundly believe that the more people read news about the ocean, the more awareness will be raised and the more people will feel concerned for its conservation. Let’s protect the ocean and its species as much as we can from where we are. And where better than to start by following the wonderful Blutopia’s advice?


A very close-up picture of an animal we call a nudibranch. It’s only about 2 cm long and is a sea slug.


This article was written in collaboration with Blutopia, a French association whose aim is to share ways how to protect the ocean.


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