A coral polyp most often measures less than 1.5 cm in diameter, according to the Smithsonian. Accumulated generation after generation, these tiny animals have nevertheless built the Great Barrier Reef, which is about 2,300 km long according to the Australian Marine Park Authority.
We have revised this list because the previous version included unverifiable figures, and each fact below now refers to one of the two sources cited at the bottom of the page.
Detailed anatomy? Our complete guide on coral covers that. Here, we keep the ten most surprising facts, and only those that can be verified.
Is coral an animal, a plant, or a rock?
Coral is an animal. It belongs to the cnidarians, like jellyfish, and shares with sea anemones a simple structure called a polyp. This small sac equipped with a mouth and a crown of tentacles lives alone or in colonies, and in hard corals it produces a calcium carbonate skeleton, hence the confusion with rock.
1. A cousin of jellyfish
The kinship with jellyfish is evident at the tip of the tentacles.
These tentacles carry stinging cells, nematocysts, which capture small organisms that come too close. The same is true for jellyfish. But contact remains a bad idea, both for the reef and your skin, and we explain why in our article that explains why you should never touch marine life. A single touch on a reef can damage living tissues.
2. A polyp smaller than a fingernail
The Smithsonian gives less than 1.5 cm in diameter for an ordinary polyp. Some mushroom corals are an exception, as the same museum describes them as over 12 cm wide for a single polyp. A hard coral colony gathers polyps connected by their tissues, and it is their limestone deposited layer after layer, year after year, that eventually shapes the reefs you see in photos. Soft corals, on the other hand, do not build reefs. They remain flexible.
Why do corals need light and warm water?
Reef-building corals host microalgae, zooxanthellae, in their tissues. These algae perform photosynthesis and give part of their production to the coral. They therefore need light, which confines most reefs to clear, shallow waters, between 22 and 29 °C according to the Smithsonian.
3. Colors largely come from the algae
The greens, browns, and reds of a reef largely come from the zooxanthellae. The skeleton itself is white. When stressed coral expels its algae, its tissues become transparent and the skeleton shows through. This is bleaching. A bleached coral is not yet dead, but it has lost a source of food and can die if the stress persists.
The previous version of this article stated that algae cover about 90% of the coral’s needs.
We removed this percentage due to lack of confirmation in our two sources.
4. One degree too much is enough
According to the Smithsonian, water that exceeds the usual seasonal maximum by about 1 °C can already cause bleaching. Thus, the Great Barrier Reef experienced massive episodes in 2002, 2016, 2017, 2020, 2022, and 2024. One degree, on a seasonal average, seems small from the beach, yet it is enough to push entire colonies into a state of stress they cannot endure for long. Our dossier on threats facing corals covers other causes. Remember this one-degree threshold.
How fast does coral grow and how long does it live?
Slowly, for most. The fastest species gain more than 15 cm per year, but the majority grow less than 2.5 cm per year, according to the Smithsonian. Some deep colonies are over 4,000 years old. A reef you fly over in a few minutes thus represents centuries of growth.
5. Less than 2.5 cm per year for the majority
This pace changes everything.
A branch broken by a fin or an anchor does not regrow in one season. For a coral that gains one or two centimeters per year, it takes years to regain lost size, provided the water remains favorable and nothing breaks the colony in the meantime. Restoration programs rely precisely on fragmentation. In short, patience.
6. Colonies over 4,000 years old, down to 6,000 m deep
Not all corals live near the surface. The Smithsonian notes deep corals down to about 6,000 m, without algae or light. Its collection includes about 4,820 coral species, nearly 65% of which live in deep water. And it is precisely deep colonies that hold age records, over 4,000 years. The Smithsonian cites a deep black coral, Leiopathes annosa, among these elders. It is also among the longest-living marine animals.
The old article mentioned “116 new reefs” discovered in 2020 between 200 and 1,200 m, a figure we found in no reliable source and have removed.
How do corals reproduce?
Corals have two modes. Sexual reproduction involves releasing eggs and sperm, or incubating larvae within the polyp. Asexual reproduction occurs by budding new polyps or by fragmentation, when a piece of colony breaks off and starts elsewhere. Depending on the species, a coral is male, female, or both.
7. A mass spawning, often only one night per year
In species that release gametes into the water, spawning often occurs once a year. In some regions, all individuals of the same species spawn simultaneously, and this mass spawning generally happens at night, increasing the chances of eggs and sperm meeting. However, not all species operate this way. Brooding species keep their larvae and often reproduce several times a year, following the lunar cycle, according to the Smithsonian.
A spawning date announced well in advance remains an estimate to be cross-checked with local data for the year.
8. A larva settles in a few hours or a few weeks
After fertilization, the larva drifts in the plankton. The Smithsonian indicates it can settle in a few hours, or after a few days or even weeks depending on the species, before building its first skeleton on a hard substrate. This window determines the distance traveled. In restoration, fragments obtained by fragmentation can be implanted in 6 to 12 months. For a reef, that’s short.
What place do reefs occupy in the ocean?
A tiny place. According to the Smithsonian, reefs cover less than 2% of the ocean floor. Yet perhaps a quarter of marine species depend on them for food or shelter. The previous version of this article mentioned 0.2% of surface and 30% of species, figures we have corrected.
9. Less than 2% of the floor, a quarter of the species
This imbalance explains the attention given to reefs. The Smithsonian remains cautious and writes “perhaps,” which we deliberately keep. The total number of marine species remains unknown, so the exact proportion cannot be decided yet. This ecosystem concentrates a large part of known marine life on a very small surface. And every square meter lost weighs more than elsewhere.
10. The Great Barrier Reef in verified figures
The figures in the old article no longer matched official data, so here are those published by the Australian Marine Park Authority (GBRMPA), in their updated version of January 2026.
| Reference | Old version | GBRMPA (2026) |
|---|---|---|
| Length | 2,340 km | about 2,300 km |
| Area | not indicated | 344,400 km² |
| Coral reefs | not indicated | about 3,000 |
| Hard corals | more than 400 species | 450 |
| Soft corals and sea feathers | not indicated | at least 1,000 species |
| Fish | 1,500 species | 1,625 |
| Mollusks | 4,000 species | more than 6,000 |
For context and reef health, read our article on the Great Barrier Reef in Australia. An honest note to finish. The Smithsonian gives 2,600 km length, GBRMPA about 2,300 km, and the difference is probably due to measurement methods (we keep the official Australian figure, for lack of a better one). If you ever dive on a reef, these figures translate into a simple rule: no hands on the reef.
FAQ
Is coral a plant, an animal, or a mineral?
Coral is an animal from the cnidarian family, like jellyfish and anemones. In hard corals, each polyp produces a calcium carbonate skeleton. It is this mineral skeleton that causes confusion with rock.
How do corals feed?
Reef-building corals get part of their food from zooxanthellae, algae that live in their tissues and perform photosynthesis. They also capture small organisms with the stinging cells on their tentacles.
Is a bleached coral dead?
Not necessarily. Bleaching means the coral has expelled its algae due to stress, often from water that is too warm. It can recover if conditions return to normal, but it dies if the stress lasts too long.
How fast does coral grow?
The fastest species gain more than 15 cm per year, according to the Smithsonian. Most corals grow less than 2.5 cm per year, so a broken colony takes years to recover.
Sources
- Corals and Coral Reefs (Smithsonian Ocean, 2025)
- Reef facts (Great Barrier Reef Marine Park Authority, 2026)
