When we talk about ocean health, we're not referring to an abstract idea. It can be measured. It can be observed. And, in many cases, it can be "read" through certain species that act as true thermometers of the ecosystem.
In ecology, there's a key concept: marine indicator species. These are animals (or even microscopic organisms) whose presence, abundance, or behavior tells us whether an ecosystem is functioning correctly.
If these species thrive, the ecosystem is usually doing well. If they disappear or decline, something important is failing.
Today, we analyze five key species that help us understand the true state of our marine biodiversity.
1. Coral reefs: the nervous system of the tropical ocean
Although they occupy less than 1% of the ocean floor, coral reefs are home to approximately 25% of all known marine species. They are true underwater cities.
The problem is that they are extremely sensitive to temperature. When the water warms up more than normal for prolonged periods, corals bleach. This happens because they expel the algae that provide them with energy and color.
If the thermal stress continues, they die.
When a reef collapses, it's not just the coral that disappears. Fish, invertebrates, predators, shelters, and entire food chains disappear.
That's why reefs are one of the clearest indicators of the impact of climate change on the ocean.
2. Sharks: invisible regulators of balance
Sharks have been on the planet for over 400 million years. They have survived 5 mass extinctions. But now they face a different threat: human pressure.
They are apex predators. This means that, due to their position at the top of the marine food chain, they regulate the populations of many other species. Essentially, if sharks disappear, the entire trophic chain of that ecosystem collapses.
Why? Broadly speaking, by eliminating sharks, we are reducing the natural predators of intermediate species in the trophic chain, such as groupers or tuna, leading to an overgrowth of these intermediate species. In turn, this causes an excessive reduction in the species below them. This domino effect ends up unbalancing and degrading reefs and seagrass beds.
An ocean with sharks is a structured ocean.
An ocean without sharks is synonymous with an unbalanced ocean, much more fragile to other factors such as overfishing, rising water temperatures, and acidification due to increased CO2 in the Earth's atmosphere.
The problem is already very serious because the populations of most shark species have disappeared by up to 90% and many of them are in serious danger of extinction.
In recent decades, we have been causing these majestic animals to disappear through overfishing, bycatch (accidental fishing), the trade of their fins, and the pollution of marine ecosystems at a totally unsustainable rate. It is estimated that every year we kill between 70 and 150 million sharks.
If we do not implement urgent and radical measures, in a few years we could witness the total extinction of these key animals for the health of the oceans.
3. Sea turtles: sentinels of pollution and overfishing
Sea turtles travel thousands of kilometers throughout their lives. They pass through open waters, coastal areas, and beaches. They are oceanic travelers.
Therefore, their status reflects multiple threats at the same time: plastic pollution, accidental fishing, habitat destruction, rising sea levels.
When we find turtles affected by ghost nets or with plastic in their digestive system, we are seeing the direct impact of our waste. In fact, the latest research from the Marine Animal Conservation Center (CRAM) reveals that 9 out of 10 turtles already have plastic in their digestive system.
They are silent sentinels of human pressure on the sea.
According to the IUCN, 6 out of the 7 species of sea turtles are classified as vulnerable, endangered, or critically endangered.
4. Seahorses: the thermometer of the Mediterranean
If we look closer to home, to the Mediterranean, there is one particularly revealing species: seahorses.
They live in seagrass beds and shallow coastal areas. They are fragile, discreet, and very sensitive to environmental degradation.
The problem is that they live in the areas most affected by human activity: coastal urbanization, pollution, boat anchoring, accidental fishing.
In many areas of the Mediterranean, seahorses have significantly declined. They don't usually make headlines, but their disappearance is a clear sign that seagrass beds (fundamental ecosystems for carbon capture and biodiversity) are under pressure.
If the seahorse disappears, the problem is not just the animal. It's the entire habitat.
5. Phytoplankton: the invisible lung
It has no eyes. It has no recognizable shape. And yet, it produces approximately half of the oxygen we breathe.
Phytoplankton is the base of the marine food chain. It feeds zooplankton, which feeds small fish, which feed large fish... and so on.
In addition, it absorbs carbon dioxide and plays a key role in climate regulation.
When temperatures, water acidity, or ocean currents change, phytoplankton also changes. And when it changes, everything else changes.
It's invisible. But without it, the ocean doesn't function.
Protecting these species is protecting everything else
Marine indicator species help us understand something fundamental: the ocean does not function in isolated parts. It is a network.
When an apex predator disappears, the trophic balance is altered.
When seagrass beds degrade, coastal species lose shelter.
When corals bleach, thousands of organisms are left homeless.
When phytoplankton changes, the base of the entire food chain changes.
Ocean health is not just an environmental issue. It is climatic, economic, and social.
A final thought
Observing these species is not just a scientific matter. It's a way of understanding where we stand.
If we want living oceans in 20, 50, or 100 years, we need informed decisions today: responsible consumption, support for conservation policies, and a real reduction in waste.
If this article has provided you with useful information, please share it. The more people understand how the ocean works, the better chance we have of protecting it.
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