10 facts about coral: a marine organism essential to ocean biodiversity
Coral reefs are essential to the health of the Ocean and to human societies. Yet, they are strongly threatened by climate change and pollution. For two years, the schooner Tara has examined these ecosystems to better understand their decline. By rethinking the way we study them, the Tara Pacific expedition brings a new perspective on reefs that have fascinated us for hundreds of years.
1. Coral: animal, plant, or mineral?
What is coral?
Le corail intrigue depuis toujours : est-il un animal, un végétal, un minéral ? En réalité, il est un peu des trois. En effet, un corail est une colonie d’animaux fixés sur leur squelette calcaire, abritant dans ses tissus des micro-algues qui contribuent à leur donner ces couleurs éclatantes.
Ces animaux, appelés polypes, sont de minuscules anémones dotées d’un corps et de tentacules entourant leur “bouche”. Les polypes bâtissent à leur base un squelette calcaire qui constitue la charpente du récif.
Ensemble, ces polypes forment une colonie qui donne l’impression d’un organisme unique. C’est cet assemblage collectif qui permet la construction des récifs coralliens, parfois visibles depuis l’espace tant ils s’étendent sur de vastes zones.
Le corail est donc à la fois un animal, associé à des algues microscopiques et bâti sur une structure minérale. C’est cette triple identité qui le rend unique dans l’écosystème marin.
2. Coral–Algae symbiosis: a unique model
The vital role of zooxanthellae
Polyps cannot feed themselves sufficiently through what they capture with their tentacles. Over time, they evolved to form a symbiotic relationship with microalgae called zooxanthellae, which live inside their tissues. This relationship is essential:
- Zooxanthellae benefit from a protected habitat, light, and CO₂.
- Polyps use the oxygen and nutrients (carbohydrates) produced by photosynthesis.
Without zooxanthellae, corals could neither survive properly nor display their brilliant colors.
Why do corals mostly live in tropical areas?
This symbiosis explains why most corals thrive in clear tropical waters, where sunlight penetrates easily. It makes coral a remarkable model of interspecies cooperation that sustains marine life.
Tropical waters are extremely nutrient-poor (oligotrophic). The coral–zooxanthellae symbiosis gives coral a key evolutionary advantage in these environments.
3. Coral Reefs: pillars of marine biodiversity
Extraordinary biodiversity on just 0.2% of the Ocean
Though coral reefs cover only 0.2% of the ocean’s surface, they harbor more than 30% of known marine biodiversity. Colorful fish, mollusks, crustaceans, sea turtles, and many species of sharks all depend on reefs.
One square kilometer of reef can host as much macroscopic biodiversity as all of mainland France.
Reefs as fish nurseries
The crevices of reefs provide shelter for countless species, protecting them from predators during early stages of development. Reefs thus act as nurseries for many oceanic fish populations.
The “Island Mass Effect”
Reef ecosystems also depend on a phenomenon known as the “Island Mass Effect.” Around islands and atolls, waters are enriched with plankton biomass thanks to runoff carrying nutrients from land and the influence of ocean currents deflected by islands. These nutrient inputs feed phytoplankton and, in turn, the entire food chain.
This effect explains both the abundance and the global distribution of coral reefs.
4. Coral protects coasts and communities
The role of coral goes beyond biodiversity. These millennia-old structures form natural barriers that protect shorelines. Their complex formations act as shields against waves and erosion. According to the IPCC’s Sixth Assessment Report, some reefs absorb up to 97% of wave energy.
In island regions, reefs are often the first line of defense against cyclones. Without them, many coastal communities would face devastating damage. Coral is therefore critical to human safety as well as ecological balance.
5. Coral: a vital resource for millions of people
The unique richness of coral reefs is intrinsically tied to the livelihoods of many human societies. Worldwide, more than 500 million people directly depend on reef survival. Tourism, fishing, coastal protection, and even medical research benefit from coral ecosystems.
In 2016, the French Initiative for Coral Reefs (IFRECOR) estimated that coral ecosystems in French waters provide services worth €1.3 billion annually—nearly 2% of the country’s GDP. France’s overseas territories host about 10% of the world’s coral reefs.
Though such estimates are utilitarian, they highlight the immense value of coral reefs. For many human societies, understanding and conserving reefs is an urgent priority.
6. The coral life cycle: slow and millennial
Coral growth is extraordinarily slow. Depending on species and conditions, corals grow only a few millimeters to a few centimeters per year.
This means that today’s massive reefs are the result of thousands—or even millions—of years of accumulation. Destroying a reef in days is equivalent to erasing centuries of biological construction.
This slow pace makes reefs highly vulnerable: their ability to regenerate is limited, especially given the speed of climate change and human pressures.
As global temperatures rise, so does sea level, caused by thermal expansion. Corals that evolved at optimal depths now find themselves submerged deeper, farther from the sunlight necessary for zooxanthellae photosynthesis. The question is whether coral can grow fast enough to keep pace with rising seas.
7. Coral under pressure from human activities
On top of global changes, reefs are exposed to numerous human-induced pressures:
- Industrial, chemical, pesticide, and plastic waste
- Agricultural runoff causing algal “blooms”
- Overfishing and destructive practices
- Coastal urbanization and concrete development
- Mass tourism
Pollutants can be directly toxic to corals or promote competitors such as algae. Agricultural fertilizers, for instance, trigger algal blooms that suffocate reefs.
Other threats include destructive fishing practices, coral mining, coastal development, and the aquarium trade. Combined, these pressures make coral reefs among the most fragile ecosystems on Earth.
Greenhouse gas emissions add further stress. Rising CO₂ levels increase ocean acidity as carbonic acid dissolves in seawater. Since coral skeletons are made of calcium carbonate, acidity weakens and dissolves them.
With warming oceans, marine heatwaves, and acidification combined, reefs face extreme pressure. In some regions, 50% of coral has already disappeared in the past 30 years. According to the IPCC, if global warming reaches +2°C, more than 99% of warm-water reefs could vanish by the end of the century.
8. Coral Bleaching: a symptom of climate change
The greenhouse effect warms the ocean’s surface, threatening coral survival. Coral’s delicate symbiosis is extremely sensitive: even a 1°C rise in sea temperature can kill a reef in just days.
When stressed by heat, zooxanthellae detach from their coral hosts. Without their symbiotic algae—coral’s main food source and source of color—reefs turn white, a process known as bleaching.
If high temperatures persist for 2–3 weeks, the separation becomes irreversible and the polyps die.
From the Great Barrier Reef in Australia to reefs in the Indian Ocean, bleaching events are multiplying worldwide, showing how corals are a powerful indicator of climate change.
9. Coral: a climate change sentinel
Climate archives in coral skeletons
Coral skeletons preserve records of past variations in ocean temperature, salinity, and acidity.
By extracting cores from massive corals, scientists can reconstruct past climates with remarkable precision—much like ice cores or tree rings.
Indicators for understanding the ocean
Corals provide reliable records of climate changes over the past 50–100 years, including temperature, salinity, and pH. These data help scientists model future environmental changes.
10. The role of research and scientific expeditions
Coral heritage is vital, both ecologically and economically.
Recognizing this, the Tara Ocean Foundation launched a unique expedition: Tara Pacific (2016–2018). For two and a half years, scientists and sailors explored 32 Pacific reef sites to study:
- how coral ecosystems function,
- their resilience to climate change,
- and the links between marine biodiversity and human health.
Thousands of samples—from coral to marine microorganisms—were collected to analyze biodiversity, genomics, and reef resilience. This expedition created a crucial database, now accessible to the international scientific community.
Such research is essential to anticipate the future of reefs and guide conservation strategies.
Protecting coral is protecting marine biodiversity
Coral is far more than a marine organism: it is the architect of vital ecosystems, a coastal shield, and a pillar of global biodiversity. Fragile and endangered, it is also a sensitive indicator of climate change.
The Pacific hosts 40% of the world’s coral reefs and shows significant biodiversity gradients. Southeast Asia, in particular, shelters the “Coral Triangle,” one of the most biodiverse reef regions on the planet.
Despite decades of research, we have only scratched the surface of coral’s complexity. That is why in 2025, the Tara Ocean Foundation will set sail again to study this vital ecosystem.
Protecting coral means protecting biodiversity, the balance of the ocean—and humanity’s future.
