How Many Species Live in the Ocean? And How Many Haven’t We Discovered Yet?

The ocean covers around 70% of our planet and contains some of the most diverse ecosystems on Earth.

From enormous blue whales to microscopic plankton, coral reefs bursting with colour to animals living kilometres beneath the surface, marine life comes in an extraordinary variety of shapes and sizes.

So you might think we have a pretty good idea of how many species live down there.

We don’t.

In fact, scientists are still discovering thousands of marine species, and a huge part of ocean biodiversity may still be unknown to science.

So, how many species actually live in the ocean?

How Many Marine Species Do We Know About?

Today, the World Register of Marine Species lists more than 250,000 accepted marine species.

That already sounds like a lot.

But describing a species is not as simple as spotting an unfamiliar fish and giving it a name.

Scientists need to examine specimens, compare them with known species, study their characteristics and, increasingly, use genetic analysis to determine whether they really are looking at something new.

And the list is constantly growing.

On average, around 2,000 new marine species are described every year.

Yet the species we know about may represent only a fraction of what is actually out there.

So, How Many Marine Species Could There Be?

This is where things get complicated.

Nobody knows exactly how many species live in the ocean, and scientists have produced different estimates because counting life across an environment as enormous and inaccessible as the sea is extremely difficult.

One widely cited scientific study estimated that there could be around 2.2 million species of marine eukaryotes, organisms whose cells contain a nucleus, which includes animals, plants, fungi and many other forms of life.

If that estimate is anywhere close to reality, an enormous number of marine species are still waiting to be formally described.

And that estimate does not tell the whole story. Once we start considering the immense diversity of bacteria and other microorganisms, defining and counting marine species becomes even more complicated.

Why Haven’t We Found Them All?

The obvious answer is that the ocean is enormous.

But size is only part of the problem.

The ocean has an average depth of around 3,682 metres, and much of it is dark, cold and under enormous pressure.

Exploring these environments requires specialised vessels, remotely operated vehicles, submersibles and other sophisticated technology.

Then there are habitats that are particularly difficult to study.

Deep-sea trenches, underwater caves, sediments and hydrothermal vents can all contain highly specialised communities that we are only beginning to understand.

And not every undiscovered species is hiding somewhere in the abyss.

Some are simply tiny.

Others look almost identical to species we already know and are only revealed to be different through genetic analysis.

Sometimes, discovering a new species means travelling thousands of metres beneath the ocean surface.

Sometimes, it means looking more closely at something that has been in front of us all along.

Where Are New Species Being Found?

The deep sea is an obvious place to look, and expeditions regularly encounter animals that are new to science.

But new species are also being described from coral reefs, mangroves, seagrass beds and other comparatively familiar environments.

Even groups of animals that humans have studied for centuries continue to surprise us.

New species of fish, crustaceans, molluscs, corals, worms and many other marine organisms are described every year.

It is one of the things that makes marine biology so exciting:

Our understanding of ocean life is still changing.

What About the Really Tiny Things?

When we imagine an undiscovered marine species, it is tempting to picture some enormous, bizarre deep-sea creature with giant teeth and its own built-in flashlight.

The reality is often much smaller.

A huge amount of marine biodiversity consists of tiny animals and microorganisms that are easily overlooked.

Plankton, worms, crustaceans and organisms living inside sediments or associated with other animals can be incredibly diverse.

And once we start including microorganisms, the seemingly simple question of "how many species live in the ocean?" becomes even harder to answer.

There is an entire world of life in the ocean that we cannot even see with the naked eye.

Why Does Knowing How Many Species Exist Matter?

Finding and identifying species is about much more than satisfying our curiosity.

Every species forms part of an ecosystem.

Some provide food for other animals. Some recycle nutrients. Some create habitats. Others keep populations in balance or form relationships with completely different organisms.

Understanding which species exist helps scientists understand how marine ecosystems function — and how those ecosystems respond to threats such as climate change, pollution, habitat destruction and overfishing.

After all, it is difficult to understand and protect an ecosystem if we don't even know everything that lives there.

An Ocean Still Full of Discoveries

We sometimes talk about the ocean as though it is somewhere we have already explored.

But in many ways, we are still getting acquainted.

More than 250,000 marine species have already been recognised, and scientists continue to describe around 2,000 more every year.

Yet potentially far more remain unnamed.

Some may live in places humans have rarely visited. Others may be hiding among coral reefs, beneath the sand or even inside another organism.

So the next time you go diving, snorkelling or simply peer into a rock pool, remember:

There is still an extraordinary amount left to discover.

The ocean isn't just full of life.

It's full of life we haven't even met yet. 🌊