# Coral

Corals are sea animals, related to jellyfish. A coral is a colony of tiny polyps, and many corals build a hard limestone skeleton. Over thousands of years, these skeletons pile up into coral reefs!

- **Category:** [Invertebrate](https://www.snappit.app/explore/categories/invertebrate)
- **Rarity:** Common
- **Danger:** 1/5 · Very low
- **Also known as:** Reef Coral, Sea Coral

![Coral](https://res.cloudinary.com/drieglelr/image/upload/v1771424729/objects/5394.jpg)

## Appearance

Corals grow as branching, table-shaped or boulder-shaped colonies, like underwater gardens. A healthy reef coral is usually golden-brown, because of the tiny algae living inside it, and its polyps open their tentacles at night. Some corals have bright colors of their own and glow under blue light. A bleached coral turns white.

## Taxonomy

- **Kingdom:** Animalia
- **Phylum:** Cnidaria
- **Class:** Anthozoa

## Coral, up close

Corals are sea animals related to jellyfish and sea anemones; reef-building stony corals (order Scleractinia) live as colonies of tiny polyps that build limestone skeletons and keep microscopic algae in their tissues, which make most of their food. Over thousands of years these skeletons pile up into coral reefs, the largest structures built by living things on Earth. Soft corals are related animals that do not build reefs.

Also called: Stony coral, Hard coral, Reef-building coral, Scleractinian coral.

Tiny animals with algae living inside them that together build the biggest structures made by living things. Spin the reef, tap a polyp, then test yourself.

### Special abilities

- **Tiny Builders** (up to 20 cm of branch a year (staghorn coral)): Polyps lay down a stony skeleton of calcium carbonate, layer by layer. Branching corals race ahead; boulder-shaped corals grow slowly but can last for centuries. [10][4][14][20]
  - The skeleton is aragonite, a form of calcium carbonate. A 2017 study found that the coral itself steers how the crystals form, helped by special proteins.
  - Healthy staghorn coral can add up to about 20 cm of branch a year; massive corals grow about 0.3–2 cm.
  - It can take up to 10,000 years for a coral reef to form, and 100,000 to 30 million years for big barrier reefs and atolls.
  - Growth in a year: Staghorn coral (branching) up to ≈20 cm; Massive corals 0.3–2 cm; Pillar coral ≈0.8 cm
- **Solar-Powered Partners** (up to 90% of the algae’s food goes to the coral): Microscopic algae live inside the coral’s tissues. They use sunlight to make food and pass most of it to their host; in return they get shelter and the coral’s waste carbon dioxide. [6][21][22][23]
  - As much as 90% of what the algae make by photosynthesis is transferred to the coral.
  - This partnership is why reef-building corals can thrive in clear tropical water that is poor in nutrients.
  - Because the algae need light, most reef-building corals live where sunlight reaches, mostly shallower than about 30 m.
  - About half of all stony coral species, more than 700, have no algae at all and live in dark or cold water.
  - Food made by the algae (100%): Up to 90% goes to the coral
- **Night Shift** (15–35% of daily needs from catching food): After dark, most polyps stretch out their tentacles and snatch drifting plankton. The algae make most of the food, but hunting brings nutrients that sunlight can’t. [1][21][20]
  - In healthy corals, catching food meets 15–35% of daily energy needs; in bleached corals it can be up to 100%.
  - Feeding also supplies nitrogen and phosphorus that photosynthesis can’t provide.
  - Not every coral waits for night: pillar coral keeps its tentacles out during the day.
  - Daily energy from catching food: Healthy coral 15–35%; Bleached coral up to 100%
- **Stinging Tentacles** (2 kinds of tentacle: hunting and fighting): Each tentacle is packed with stinging cells that deliver toxins to catch prey. Some corals also grow special sweeper tentacles to sting neighbours that crowd them. [1][24][25]
  - Stinging cells (nematocysts) sit in the tentacles and outer tissue, the same kind of cell a jellyfish uses.
  - In galaxy coral, hunting tentacles and sweeper tentacles carry different stinging cells and different venoms.
  - Hunting tentacles pack more nerve-attacking toxins for catching prey; sweeper tentacles are used to fight neighbouring corals for space.
  - Fire coral, which stings swimmers, is not a true coral but a relative of jellyfish.
- **Mass Spawning** (105 species spawning on the same nights): Each year, a few nights after a full moon, many kinds of coral release their eggs and sperm at the same time. Eggs are fertilised in the open water and grow into drifting larvae. [26][4][27]
  - On the Great Barrier Reef in 1983, scientists predicted the nights and watched 105 species spawn, at reefs up to 500 km apart.
  - There it happens in late spring, between the full moon and the last quarter. Mass spawning was first seen there in 1981 and 1982.
  - In the Caribbean, staghorn coral spawns once a year after the full moon in late summer.
  - Water temperature and day length may set the season; the moon sets the night, and sunset the moment of release.
- **Bleaching** (+1 °C above the usual summer high can start it): When the water stays too warm, corals lose the algae that feed and colour them, and turn white. A bleached coral is alive but starving: it can recover if the water cools in time, or die. [28][29][12][30][31]
  - Scientists measure heat stress in “degree heating weeks”. About 4 weeks at 1 °C above the usual warmest month (4 °C-weeks) brings significant bleaching; 8 °C-weeks brings severe bleaching and deaths.
  - On the Great Barrier Reef in 2016, corals began to die once heat stress passed 3–4 °C-weeks.
  - From January 2023 to September 2025, bleaching-level heat reached about 84% of the world’s reef area, the biggest event on record.
  - Severe bleaching now returns about every 6 years on average, too often for reefs to fully recover.
  - World reef area hit by bleaching-level heat: 2023–2025 event 84%; 2014–2017 event 68%
- **Neon Glow** (2–3 weeks after mild heat, some turn neon; still debated): Many corals make fluorescent proteins, cousins of the jellyfish’s green fluorescent protein. Under blue or ultraviolet light they shine green, yellow or red. [32][33][9][8][34]
  - In 1999 scientists found six of these glowing proteins in reef corals that cannot make their own light.
  - Why corals glow is still debated. Ideas include managing light for the algae, attracting free-living algae and luring plankton prey.
  - After mild or short heat stress, some bleached corals turn neon within 2–3 weeks (“colourful bleaching”). The pigments may shield the tissue while algae move back in.
  - This glow is fluorescence, which needs light shining in. Making light (bioluminescence) is found in other anthozoans, mainly soft corals, most of them in the deep sea.

### Body parts

- **Polyps** (1–3 mm across, in most stony corals) [1][2][3][4]
  - Most corals are colonies of hundreds to hundreds of thousands of tiny animals called polyps.
  - Most stony coral polyps are just 1–3 mm across.
  - Every polyp in a colony is an exact copy of the others: a clone.
  - A thin sheet of living tissue joins the polyps, so the colony works together.
- **Mouth** (1 opening for food in and waste out) [1][2]
  - A polyp’s stomach opens at one end only, so food goes in and waste comes out through the same mouth.
  - Thread-like filaments inside the stomach digest the food.
  - Many corals also trap fine particles in sheets and strands of mucus, then draw them into the mouth.
- **Tentacles** (Night when most polyps feed) [1][5]
  - A ring of tentacles around the mouth catches food, fends off attackers and clears away debris.
  - They carry stinging cells (nematocysts), the same kind of cell that makes a jellyfish sting.
  - Prey ranges from tiny zooplankton to small fish, depending on the size of the polyp.
  - Stony corals are hexacorals: their bodies are usually built in sixes. Soft coral polyps always have 8 tentacles.
- **Algae inside** (up to 90% of the algae’s food goes to the coral) [6][7][8][9]
  - Microscopic algae (family Symbiodiniaceae, also called zooxanthellae) live inside the tissues of most reef-building corals.
  - As much as 90% of the food the algae make by photosynthesis is passed to the coral.
  - They give corals their golden-brown colour. Most corals pick up their algae from the seawater rather than inheriting them.
  - The algae family began to diversify about 160 million years ago, just as modern-style stony corals were spreading.
- **Skeleton** (240 million years old: the oldest stony coral fossils) [3][10][5]
  - Each polyp builds a limestone cup of calcium carbonate, in the form of the mineral aragonite, and sits inside it.
  - Every so often the polyp lifts off its base and lays a new floor above the old one, so the colony grows.
  - Stony corals are the only anthozoans with a solid skeleton like this. Their oldest fossils are about 240 million years old.
  - When disturbed, a polyp pulls back into its cup until almost nothing shows.
- **Branching coral** (up to 20 cm of branch a year (staghorn)) [4][11][12]
  - Branching corals such as staghorn coral are among the fastest-growing corals: up to about 20 cm (8 inches) of branch a year when healthy.
  - Their thickets shelter fish and help protect shores from waves.
  - Broken pieces can reattach and grow into new colonies, which is how coral nurseries work.
  - Branching and table corals died in huge numbers when a heatwave hit the Great Barrier Reef in 2016.
- **Brain coral** (0.3–2 cm a year (massive corals)) [13][14][15][16]
  - Brain corals are named for the maze of ridges and grooves on their rounded surface.
  - Like other ball- or boulder-shaped (massive) corals, they grow slowly, about 0.3–2 cm a year, and waves rarely damage them.
  - A boulder-shaped Porites coral on the Great Barrier Reef, 5.3 m tall, is estimated to be 421–438 years old.
  - Two Atlantic brain corals, the grooved and the symmetrical brain coral, are listed as Critically Endangered.
- **Table coral** [17][12][16]
  - Table corals grow broad, flat tops that catch more light for their algae and more plankton for their tentacles.
  - Fast-growing table and branching corals suffered a catastrophic die-off in the 2016 heatwave on the Great Barrier Reef.
  - The Indo-Pacific table coral Acropora hyacinthus is listed as Endangered.
- **Reef life** (4,000+ fish species depend on reefs) [18][19][4]
  - An estimated 25% of all marine life, including over 4,000 fish species, depends on coral reefs at some point in life.
  - Scientists estimate about 830,000 species of many-celled life live on reefs, counting ones not yet discovered.
  - Thickets of branching staghorn coral are important homes for fish and other reef animals.

### Stats

| Stat | Value | Notes |
|---|---|---|
| Polyp size | 1–3 mm | across, for most stony coral polyps [2][3] |
| Species | ≈1,300 | described stony coral species (a 2007 count), plus about 3,000 soft corals [5] |
| Growth | up to 20 cm | of branch a year (staghorn coral); massive corals 0.3–2 cm; Up to ≈10× faster [4][14] |
| Algae food | up to 90% | of what the algae make is passed to the coral [6] |
| Reef area | 348,361 km² | of shallow coral reefs worldwide, mapped from satellites; ≈0.1% of the ocean’s surface [37][38] |
| Reef life | ≈25% | of all marine life depends on reefs at some point [18][39] |
| Great Barrier Reef | 3,863 reefs | stretching more than 2,000 km along Queensland [12][40] |
| Bleaching 2023–25 | 84% | of the world’s reef area hit by bleaching-level heat [30] |
| Threatened | 44% | of 892 reef-building coral species (IUCN, 2024) [41] |
| Old age | 400+ yrs | estimated for a 5.3 m tall Porites colony on the Great Barrier Reef [15] |

### Photos

- [Healthy corals on Flynn Reef, Great Barrier Reef](https://commons.wikimedia.org/wiki/File:Coral_Outcrop_Flynn_Reef.jpg) (photo: Toby Hudson, CC BY-SA 3.0)
- [Table corals (Acropora hyacinthus) in American Samoa](https://commons.wikimedia.org/wiki/File:Fagatele_Bay_Corals_(29032296958).jpg) (photo: Wendy Cover / NOAA, Public domain)
- [Boulder brain coral (Colpophyllia natans)](https://commons.wikimedia.org/wiki/File:Colpophyllia_natans_(Boulder_Brain_Coral)_entire_colony.jpg) (photo: Nick Hobgood, CC BY-SA 3.0)
- [Galaxy coral polyps, Red Sea](https://commons.wikimedia.org/wiki/File:Coral_(Galaxea_fascicularis)%2C_mar_Rojo%2C_Egipto%2C_2023-04-18%2C_DD_128.jpg) (photo: Diego Delso, CC BY-SA 4.0)
- [Polyps releasing egg and sperm bundles, Flower Garden Banks, Gulf of Mexico](https://commons.wikimedia.org/wiki/File:Coral_Spawning_(38797556434).jpg) (photo: G.P. Schmahl / NOAA, Public domain)
- [Bleached staghorn coral on the Great Barrier Reef](https://commons.wikimedia.org/wiki/File:Bleached_Staghorn_Coral.jpg) (photo: Matt Kieffer, CC BY-SA 2.0)
- [A coral’s fluorescent proteins glowing green, photographed in a public aquarium](https://commons.wikimedia.org/wiki/File:Fluorescent_coral_in_aquarium.jpg) (photo: Vincent Niochet, CC BY-SA 4.0)
- [Dead brain coral overgrown by algae, Florida Keys](https://commons.wikimedia.org/wiki/File:Dead_Brain_Coral_Pickles_Reef_20230713.jpg) (photo: Jstuby, CC0)
- [Part of the Great Barrier Reef seen from the International Space Station in 2022](https://commons.wikimedia.org/wiki/File:Earth_observation_(iss068e004262).jpeg) (photo: NASA Johnson Space Center, Public domain)
- [A soft coral (probably Sarcophyton), Maldives: soft corals don’t build reefs](https://commons.wikimedia.org/wiki/File:Maldives_soft_coral.jpg) (photo: Tchami, CC BY-SA 2.0)

### Questions

- **Is coral an animal or a plant?** Coral is an animal. Each coral is a colony of tiny polyps, related to jellyfish and sea anemones. Most reef-building corals have plant-like algae living in their tissues, which make most of their food. [18][1][6]
- **Is coral alive or a rock?** Both, in a way. The hard part is a limestone skeleton, but a thin layer of living polyps covers it and keeps building it. When the polyps die, algae grow over the bare skeleton. [42][2][35]
- **What is coral bleaching?** When water stays too warm, corals lose the algae that feed and colour them and turn white. About 4 weeks at 1 °C above the usual summer high is enough for significant bleaching. Bleached corals are alive and can recover if the heat eases, but many die. [28][29]
- **How fast does coral grow?** It depends on the shape. Healthy staghorn coral can grow up to about 20 cm a year, while boulder-shaped massive corals grow about 0.3–2 cm a year. Building a whole reef can take up to 10,000 years. [4][14]
- **Can you see the Great Barrier Reef from space?** Yes. Astronauts on the International Space Station, about 420 km up, photograph it. But it isn’t the only living thing visible from orbit: huge plankton blooms have been photographed too. [43][44]
- **Can you touch coral?** No. Touching, standing on or breaking coral damages the living polyps, and fire coral, a jellyfish relative that looks like coral, stings. Float above the reef and use mooring buoys instead of anchors. [58][25]
- **Are coral reefs dying?** Many reefs are in trouble. In 2024 the IUCN found 44% of reef-building coral species threatened, mainly by climate change, and from 2023 to 2025 bleaching-level heat reached about 84% of the world’s reef area. Cutting emissions, protecting reefs and keeping water clean all help. [41][30][60]
- **What do corals eat?** Most of their food comes from the algae living inside them. At night the polyps also catch plankton with stinging tentacles, which meets 15–35% of a healthy coral’s daily needs. [6][1][21]

### Coral myths and facts

Tap Myth or Fact for each claim, then check the answer.

- "Coral is a plant." **Myth!** Corals are animals, related to jellyfish and sea anemones. They do have plant-like algae living inside them, which is part of why people get confused. [18][42][6]
- "Coral is just a rock." **Myth!** The hard part is a limestone skeleton, but living polyps build it and cover it in a thin layer of tissue. A reef is rock made by animals. [42][2][3]
- "You can see the Great Barrier Reef from space." **Fact!** Astronauts on the International Space Station, about 420 km up, photograph it: it stretches more than 2,000 km. But it isn’t the only living thing visible from orbit. Astronauts have also photographed what look like huge plankton blooms in the same sea. [43][40][44]
- "Coral reefs cover less than 1% of the ocean but support about a quarter of sea life." **Fact!** Shallow reefs cover about 348,000 km², roughly 0.1% of the ocean’s surface. About 25% of marine life is estimated to depend on them; the exact share is hard to pin down. [37][38][18][39]
- "A bleached coral is dead." **Myth!** Bleached corals are alive but starving. If the heat eases soon enough they can recover; if it lasts, many die. [28][42]
- "All corals live in warm, shallow water." **Myth!** Over half of all known coral species live in deep, dark water at 4–12 °C, without algae, and some build reefs down there. [45]
- "Fire coral is a kind of coral." **Myth!** Fire corals help build reefs and look like coral, but they are hydrozoans, more closely related to jellyfish. They sting if touched. [25]
- "Sunscreen is the main reason coral reefs are dying." **Myth!** Climate change is the main threat. Some chemical sunscreen filters can harm corals, so mineral sunscreen or a sun shirt is a kinder choice, but heat stress is the big problem. [41][46]

### Coral records

Superlatives and record holders, each checked against its source.

- Biggest structure built by living things: Great Barrier Reef, Australia, 2,000+ km (3,863 reefs along the coast of Queensland) [40][12]
- Largest known coral colony (reported): A shoulder-blade coral (Pavona clavus), Great Barrier Reef, 111 m long (about 3,970 m², found by citizen scientists in 2026 · preliminary measurements) [47][48][49]
- Mega coral of the Solomon Islands: Pavona clavus colony found by National Geographic Pristine Seas, 34 × 32 m (about 5 m tall and roughly 300 years old · announced as the world’s largest in 2024) [49][48]
- Widest Porites coral measured: Muga dhambi (“big coral”), Goolboodi (Orpheus Island), 10.4 m (wide and 5.3 m tall · estimated 421–438 years old) [15]
- Oldest coral dated (2009 study): A deep-sea black coral (Leiopathes), 4,265 yrs (a black coral, not a stony coral · deep-sea corals like it grow only 4–35 µm a year) [50]
- First forecast mass spawning: 105 coral species, Great Barrier Reef, 1983 (after mass spawning was first seen in 1981–82, scientists forecast the nights and watched 105 species spawn at reefs up to 500 km apart) [26]
- Longest coral larval life recorded (2008 study): Five reef-building species (lab study), 244 days (maximum larval lifespans of 195–244 days, long enough to drift between distant reefs) [51]

### Life of a coral

From a drifting egg to a colony that can live for centuries.

- **Spawning night**: Eggs and sperm set free. A few nights after a full moon, a colony releases its eggs and sperm into the water, at the same time as its neighbours. Most stony corals that make eggs and sperm spawn like this. [26][27]
- **≈2 hours**: First divisions. Fertilised eggs start dividing about 2 hours after spawning. [52]
- **1.5–2 days**: A swimming larva. The embryo becomes a swimming larva called a planula. In one study, all the larvae checked were moving by 36 hours and fully mobile by 48 hours. [52]
- **4–7 days**: Ready to settle. In the lab, larvae of spawning corals settled 4–7 days after fertilisation. Some swim for weeks, and in the lab larvae have survived up to 244 days. [52][27][51]
- **Settling**: Choosing a home. The larva sinks to the reef and picks a spot. Chemicals from crusty coralline algae can trigger it to turn into its first polyp, which starts building a limestone cup. [53][27]
- **Early life**: Picking up algae. Most corals take in their algae from the water around them rather than getting them from their parents. [9]
- **≈3 years**: A small colony. Polyps bud off clones and the colony spreads. In a restoration trial in the Philippines, young colonies averaged about 8 cm across after 35 months. [27][54]
- **Grown up**: Ready to spawn. Staghorn coral can breed once it is about 17 cm (7 inches) tall. Broken branches can also grow into new colonies. [4]
- **Centuries**: A long life. Because polyps keep budding, colonies can live for hundreds of years. One Porites on the Great Barrier Reef is estimated at 421–438 years old. [42][4][15]

### Why coral reefs matter

Coral reefs take up a tiny share of the ocean but do a huge amount of work.

- **≈0.1%** of the ocean’s surface. Shallow coral reefs cover about 348,000 km², according to satellite maps published in 2024. [37][38]
- **≈25%** of marine life depends on reefs. At some point in their lives, including more than 4,000 fish species. [18]
- **830,000** species estimated on reefs. An expert estimate of all many-celled species living on coral reefs, known and not yet discovered (fungi not counted). [19]
- **97%** of wave energy stopped. Reefs cut wave energy by 97% on average, protecting coasts where 100 million or more people may benefit. [55]

These are estimates: reef area depends on how a reef is defined, and many reef species are still undiscovered.

### Are corals endangered?

Many are. Marine heatwaves are now the biggest danger, on top of pollution, disease and physical damage.

- **IUCN Red List:** Varies by species. Corals don’t have one single Red List status: each species is assessed on its own. In 2024 the IUCN reassessed 892 reef-building coral species and found 44% threatened with extinction, up from about one third in 2008. Endangered is the most common threatened category among stony corals on the Red List (244 of 891 species). [41][16]
- IUCN Red List: reef-building corals, Nov 2024
- **Threat: Marine heatwaves** (84%): Climate change is the main threat. During the fourth global bleaching event (2023–2025), bleaching-level heat reached about 84% of the world’s reef area. Severe bleaching now returns about every 6 years. [41][30][31]
- **Threat: Ocean acidification** (+30%): The ocean absorbs carbon dioxide, which makes seawater more acidic: about 30% more since the Industrial Revolution. That makes it harder for corals to build their skeletons. [56]
- **Threat: Crown-of-thorns starfish** (42%): Outbreaks of this coral-eating starfish caused 42% of the coral loss on the Great Barrier Reef from 1985 to 2012. Storms caused 48% and bleaching 10%. [57]
- **Threat: Pollution and runoff**: Sediment smothers corals, and fertiliser runoff feeds algae that can take over. Oil near the surface can damage eggs and sperm during spawning. [35]
- **Threat: Disease**: About 40 coral diseases affect more than 200 hard coral species. Stony coral tissue loss disease, first reported off Florida in 2014, has almost wiped out pillar coral there. [36][20]
- **Threat: Physical damage**: Anchors, boat groundings, tangled litter and fishing gear, and people standing on reefs all break corals. [58]
- **Helping: Cutting emissions** (1.5 °C): Scientists project a further 70–90% loss of coral reefs at 1.5 °C of global warming and more than 99% at 2 °C. Cutting greenhouse gases is the main way to save corals. [59][41]
- **Helping: Protected areas**: No-take zones on the Great Barrier Reef, expanded in 2004, seem to have fewer crown-of-thorns outbreaks and more coral. Protection alone can’t stop extreme heat, though. [60][61]
- **Helping: Cleaner water**: Better water quality could help reduce starfish outbreaks. Without them, coral cover on the Great Barrier Reef could grow about 0.89% a year even with storms and bleaching. [57]
- **Helping: Restoration** (60–70%): Coral nurseries grow fragments and replant them, and some teams sow coral larvae. Projects report 60–70% survival but usually cover only about 100 m², and scientists stress it is no replacement for climate action. [62][11][54]
- **Helping: Reef-safe visits**: Float above the reef, never stand on, touch or break coral, use mooring buoys instead of anchors, and choose mineral sunscreen or a sun shirt. [58][46]
- **Staghorn coral (Acropora cervicornis)** (CR): A fast-growing Caribbean branching coral that helped build reefs there for 5,000 years. Hit by heat, disease, pollution and hurricanes. [41][4]
- **Elkhorn coral (Acropora palmata)** (CR): Staghorn’s partner reef-builder in the Caribbean, facing the same threats. [41]
- **Grooved brain coral (Diploria labyrinthiformis)** (CR): An Atlantic brain coral. Atlantic corals are especially threatened by yearly severe bleaching, pollution and disease. [16][41]
- **Pillar coral (Dendrogyra cylindrus)** (CR): Naturally rare. Stony coral tissue loss disease has almost removed it from Florida. [16][20]
- **Table coral (Acropora hyacinthus)** (EN): An Indo-Pacific table coral like the one in the model. [16]
- **Lobe coral (Porites lobata)** (LC): A slow-growing boulder coral of the Indo-Pacific. [16][13]
- **White coral (Desmophyllum pertusum)** (VU): A cold-water reef-builder of the deep sea, threatened by bottom trawling and other deep-sea activities. [41]

### Stony coral vs soft coral vs sea anemone

All three are anthozoans: animals that live their whole lives as polyps. Only stony corals build reefs.

| | Stony coral | Soft coral | Sea anemone |
|---|---|---|---|
| Group [5] | Order Scleractinia (hexacorals) | Subclass Octocorallia | Order Actiniaria (hexacorals) |
| Species [5] | ≈1,300 | ≈3,000 | ≈1,200 |
| Skeleton [5][3] | Hard limestone cup under every polyp | Tiny limestone spikes (sclerites) in soft tissue | None: soft-bodied |
| Body plan [5] | Usually in sixes | Always 8 tentacles, usually feathery | Usually in sixes; tentacles not feathery |
| Lives as [1][45][5] | Mostly colonies; some live alone | Colonies (one known solitary species) | Single polyps |
| Builds reefs [1][3] | Yes: the main reef builders | No | No |

Fire coral looks like a stony coral and helps build reefs, but it is a hydrozoan, more closely related to jellyfish.

### Reading a coral’s health

- Tentacles out: Feeding: most polyps open up at night to catch plankton. [1]
- Polyps pulled into their cups: Disturbed or stressed: the polyp retracts until almost nothing shows. [3]
- Golden-brown colour: Healthy: the algae inside are doing their job. [4][8]
- Pale or white: Bleached: the algae have gone. The coral is alive but starving. [28]
- Sudden neon colours after a heatwave: Colourful bleaching: stressed, but the glowing pigments may help it recover. [8]
- A spreading band of bare white skeleton: Tissue loss, often from a disease such as stony coral tissue loss disease. [36]
- Fuzzy algae over the skeleton: Dead coral: algae have taken over the bare skeleton. [35]

### Health

- **Healthy**: Golden-brown tissue over the skeleton. At night the polyps open their tentacles. (Algae inside make most of its food) [6][8][1]
- **Glowing**: Neon green, yellow or red: the coral’s own fluorescent proteins shining under blue light. Some heat-stressed corals turn extra bright. (Fluorescence, not bioluminescence) [32][8][34]
- **Bleached**: White or very pale: the algae have left, so the white skeleton shows through the tissue. (Alive but starving; it can recover if the water cools) [28][8]
- **Dead**: Bare skeleton covered in a fuzzy turf of algae. No living polyps are left. (Algae take over the space where corals lived) [35]

### Coral quiz

1. Is coral an animal, a plant or a rock? **An animal.** Corals are animals related to jellyfish and sea anemones. The rock is the skeleton they build. [18][42]
2. What lives inside reef-building corals and makes most of their food? **Tiny algae.** Microscopic algae pass up to 90% of the food they make to the coral. [6]
3. When do most coral polyps feed? **At night.** Most polyps stretch out their tentacles after dark to catch plankton. [1]
4. What do corals use to catch food? **Stinging cells in their tentacles.** Stinging cells called nematocysts, like a jellyfish’s, catch prey from zooplankton to small fish. [1]
5. How much can healthy staghorn coral grow in a year? **Up to about 20 cm.** Staghorn coral is one of the fastest-growing corals: up to about 20 cm (8 inches) of branch a year. [4]
6. What is coral bleaching? **Coral turning white after losing its algae.** Heat stress makes corals lose their algae, so the white skeleton shows through. [28]
7. Is a bleached coral dead? **No: it is starving but can recover if the water cools.** Bleached corals are still alive. They can recover if the heat eases in time. [28]
8. How many coral species spawned on predicted nights on the Great Barrier Reef in 1983? **105.** Scientists forecast the nights and watched 105 species spawn at reefs up to 500 km apart. [26]
9. What share of reef-building coral species did the IUCN find threatened in 2024? **44%.** 44% of 892 reef-building coral species are threatened, up from about one third in 2008. [41]
10. About how much of the ocean’s surface do shallow coral reefs cover? **About 0.1%.** About 348,000 km², roughly 0.1% of the ocean, yet about a quarter of marine life depends on reefs. [37][38][18]
11. Fire coral is most closely related to… **Jellyfish.** Fire corals are hydrozoans, not true corals, and they sting if touched. [25]
12. You are snorkelling over a reef. What should you do? **Float above it and never touch the coral.** Touching, standing on or breaking coral damages the living polyps. [58]

### Sources (checked 2026-10-16)

1. [NOAA National Ocean Service. Corals Tutorial: What are corals? (anatomy).](https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html)
2. [NOAA National Ocean Service. Corals Tutorial: Polyps up close (anatomy diagram).](https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral01a.html)
3. [NOAA National Ocean Service. Corals Tutorial: How do coral reefs grow? (hard and soft corals).](https://oceanservice.noaa.gov/education/tutorial_corals/coral03_growth.html)
4. [NOAA Fisheries. Staghorn coral (Acropora cervicornis). Species directory.](https://www.fisheries.noaa.gov/species/staghorn-coral)
5. [Daly M, Brugler MR, Cartwright P, et al. (2007). The phylum Cnidaria: a review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668:127–182.](https://doi.org/10.11646/zootaxa.1668.1.11)
6. [NOAA National Ocean Service. Corals Tutorial: Zooxanthellae… what?](https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html)
7. [LaJeunesse TC, Parkinson JE, Gabrielson PW, Jeong HJ, Reimer JD, Voolstra CR, Santos SR (2018). Systematic revision of Symbiodiniaceae highlights the antiquity and diversity of coral endosymbionts. Current Biology 28(16):2570–2580.e6.](https://doi.org/10.1016/j.cub.2018.07.008)
8. [Bollati E, D’Angelo C, Alderdice R, Pratchett M, Ziegler M, Wiedenmann J (2020). Optical feedback loop involving dinoflagellate symbiont and scleractinian host drives colorful coral bleaching. Current Biology 30(13):2433–2445.e3.](https://doi.org/10.1016/j.cub.2020.04.055)
9. [Aihara Y, Maruyama S, Baird AH, Iguchi A, Takahashi S, Minagawa J (2019). Green fluorescence from cnidarian hosts attracts symbiotic algae. PNAS 116(6):2118–2123.](https://doi.org/10.1073/pnas.1812257116)
10. [Von Euw S, Zhang Q, Manichev V, et al. (2017). Biological control of aragonite formation in stony corals. Science 356(6341):933–938.](https://doi.org/10.1126/science.aam6371)
11. [NOAA National Ocean Service. Corals Tutorial: Restoring corals.](https://oceanservice.noaa.gov/education/tutorial_corals/coral15_restoring-corals.html)
12. [Hughes TP, Kerry JT, Baird AH, et al. (2018). Global warming transforms coral reef assemblages. Nature 556:492–496.](https://doi.org/10.1038/s41586-018-0041-2)
13. [NOAA National Ocean Service. Corals Tutorial: Massive corals.](https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral03h.html)
14. [NOAA National Ocean Service. Corals Tutorial: Types of coral reefs.](https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.html)
15. [Smith A, Cook N, Cook K, et al. (2021). Field measurements of a massive Porites coral at Goolboodi (Orpheus Island), Great Barrier Reef. Scientific Reports 11:15334.](https://doi.org/10.1038/s41598-021-94818-w)
16. [IUCN (2026). The IUCN Red List of Threatened Species, version 2026-1, accessed via the GBIF checklist (dataset 19491596-35ae-4a91-9a98-85cf505f1bd3).](https://doi.org/10.15468/0qnb58)
17. [NOAA National Ocean Service. Corals Tutorial: Table corals.](https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral03d.html)
18. [US Environmental Protection Agency. Basic information about coral reefs.](https://www.epa.gov/coral-reefs/basic-information-about-coral-reefs)
19. [Fisher R, O’Leary RA, Low-Choy S, et al. (2015). Species richness on coral reefs and the pursuit of convergent global estimates. Current Biology 25(4):500–505.](https://doi.org/10.1016/j.cub.2014.12.022)
20. [NOAA Fisheries. Pillar coral (Dendrogyra cylindrus). Species directory.](https://www.fisheries.noaa.gov/species/pillar-coral)
21. [Houlbrèque F, Ferrier-Pagès C (2009). Heterotrophy in tropical scleractinian corals. Biological Reviews 84(1):1–17.](https://doi.org/10.1111/j.1469-185X.2008.00058.x)
22. [NOAA National Ocean Service. Corals Tutorial: Where are reef-building corals found?](https://oceanservice.noaa.gov/education/tutorial_corals/coral05_distribution.html)
23. [Vuleta S, Nakagawa S, Ainsworth TD (2024). The global significance of Scleractinian corals without photoendosymbiosis. Scientific Reports 14:10161.](https://doi.org/10.1038/s41598-024-60794-0)
24. [Yosef O, Popovits Y, Malik A, Ofek-Lalzer M, Mass T, Sher D (2020). A tentacle for every occasion: comparing the hunting tentacles and sweeper tentacles, used for territorial competition, in the coral Galaxea fascicularis. BMC Genomics 21:548.](https://doi.org/10.1186/s12864-020-06952-w)
25. [Kropp LM, Parsley CB, Burnett OL (2018). Millepora species (fire coral) sting: a case report and review of recommended management. Wilderness & Environmental Medicine 29(4):521–526.](https://doi.org/10.1016/j.wem.2018.06.012)
26. [Babcock RC, Bull GD, Harrison PL, Heyward AJ, Oliver JK, Wallace CC, Willis BL (1986). Synchronous spawnings of 105 scleractinian coral species on the Great Barrier Reef. Marine Biology 90(3):379–394.](https://doi.org/10.1007/BF00428562)
27. [NOAA National Ocean Service. Corals Tutorial: How do corals reproduce?](https://oceanservice.noaa.gov/education/tutorial_corals/coral06_reproduction.html)
28. [NOAA National Ocean Service. What is coral bleaching? Ocean Facts.](https://oceanservice.noaa.gov/facts/coral_bleach.html)
29. [NOAA Coral Reef Watch. 5km methodology: Degree Heating Weeks.](https://coralreefwatch.noaa.gov/product/5km/methodology.php)
30. [NOAA Coral Reef Watch. Current global bleaching: status update (updated 2 Jun 2026).](https://coralreefwatch.noaa.gov/satellite/research/coral_bleaching_report.php)
31. [Hughes TP, Anderson KD, Connolly SR, et al. (2018). Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science 359(6371):80–83.](https://doi.org/10.1126/science.aan8048)
32. [Matz MV, Fradkov AF, Labas YA, et al. (1999). Fluorescent proteins from nonbioluminescent Anthozoa species. Nature Biotechnology 17(10):969–973.](https://doi.org/10.1038/13657)
33. [Ben-Zvi O, Lindemann Y, Eyal G, Loya Y (2022). Coral fluorescence: a prey-lure in deep habitats. Communications Biology 5:537.](https://doi.org/10.1038/s42003-022-03460-3)
34. [DeLeo DM, Bessho-Uehara M, Haddock SHD, McFadden CS, Quattrini AM (2024). Evolution of bioluminescence in Anthozoa with emphasis on Octocorallia. Proceedings of the Royal Society B 291:20232626.](https://doi.org/10.1098/rspb.2023.2626)
35. [NOAA National Ocean Service. Corals Tutorial: Pollution.](https://oceanservice.noaa.gov/education/tutorial_corals/coral09_pollution.html)
36. [NOAA National Ocean Service. Corals Tutorial: Diseases.](https://oceanservice.noaa.gov/education/tutorial_corals/coral10_disease.html)
37. [Lyons MB, Murray NJ, Kennedy EV, et al. (2024). New global area estimates for coral reefs from high-resolution mapping. Cell Reports Sustainability 1(2):100015.](https://doi.org/10.1016/j.crsus.2024.100015)
38. [Charette MA, Smith WHF (2010). The volume of Earth’s ocean. Oceanography 23(2):112–114.](https://doi.org/10.5670/oceanog.2010.51)
39. [Plaisance L, Caley MJ, Brainard RE, Knowlton N (2011). The diversity of coral reefs: what are we missing? PLOS ONE 6(10):e25026.](https://doi.org/10.1371/journal.pone.0025026)
40. [NASA Goddard Space Flight Center / USGS Landsat 7 (1999, posted 2017). Great Barrier Reef. NASA Image and Video Library.](https://images.nasa.gov/details/GSFC_20171208_Archive_e001931)
41. [IUCN (2024, 13 Nov). Over 40% of coral species face extinction – IUCN Red List. Press release, Baku (COP29).](https://iucn.org/press-release/202411/over-40-coral-species-face-extinction-iucn-red-list)
42. [NOAA National Ocean Service. Are corals animals or plants? Ocean Facts.](https://oceanservice.noaa.gov/facts/coral.html)
43. [NASA Johnson Space Center (2022). iss068e004262: Earth observation of the Great Barrier Reef from the International Space Station. NASA Image and Video Library.](https://images.nasa.gov/details/iss068e004262)
44. [NASA Johnson Space Center (1990). STS032-520-014: Earth observation of the western Coral Sea and the Great Barrier Reef. NASA Image and Video Library.](https://images.nasa.gov/details/S32-520-014)
45. [NOAA Ocean Exploration. Do all corals live in warm water?](https://oceanexplorer.noaa.gov/ocean-fact/coral-water/)
46. [NOAA National Ocean Service. Skincare chemicals and marine life.](https://oceanservice.noaa.gov/news/sunscreen-corals.html)
47. [Hutchins R (2026, 26 Feb). Citizen scientists find largest known coral on Great Barrier Reef. Oceanographic.](https://oceanographicmagazine.com/news/citizen-scientists-find-largest-known-coral-on-great-barrier-reef/)
48. [Crisp S, van Hulten D, Southworth L, et al. (2026). Size and ecology of a giant Pavona clavus coral colony in the Kingdom of Tonga. Ecology and Evolution 16(7):e73940.](https://doi.org/10.1002/ece3.73940)
49. [National Geographic (2024, 14 Nov). Scientists discover world’s largest coral—so big it can be seen from space.](https://www.nationalgeographic.com/environment/article/world-largest-coral-colony-discovery)
50. [Roark EB, Guilderson TP, Dunbar RB, Fallon SJ, Mucciarone DA (2009). Extreme longevity in proteinaceous deep-sea corals. PNAS 106(13):5204–5208.](https://doi.org/10.1073/pnas.0810875106)
51. [Graham EM, Baird AH, Connolly SR (2008). Survival dynamics of scleractinian coral larvae and implications for dispersal. Coral Reefs 27(3):529–539.](https://doi.org/10.1007/s00338-008-0361-z)
52. [Babcock RC, Heyward AJ (1986). Larval development of certain gamete-spawning scleractinian corals. Coral Reefs 5(3):111–116.](https://doi.org/10.1007/BF00298178)
53. [Heyward AJ, Negri AP (1999). Natural inducers for coral larval metamorphosis. Coral Reefs 18(3):273–279.](https://doi.org/10.1007/s003380050193)
54. [dela Cruz DW, Harrison PL (2020). Enhancing coral recruitment through assisted mass settlement of cultured coral larvae. PLOS ONE 15(11):e0242847.](https://doi.org/10.1371/journal.pone.0242847)
55. [Ferrario F, Beck MW, Storlazzi CD, Micheli F, Shepard CC, Airoldi L (2014). The effectiveness of coral reefs for coastal hazard risk reduction and adaptation. Nature Communications 5:3794.](https://doi.org/10.1038/ncomms4794)
56. [US Environmental Protection Agency. Threats to coral reefs.](https://www.epa.gov/coral-reefs/threats-coral-reefs)
57. [De’ath G, Fabricius KE, Sweatman H, Puotinen M (2012). The 27-year decline of coral cover on the Great Barrier Reef and its causes. PNAS 109(44):17995–17999.](https://doi.org/10.1073/pnas.1208909109)
58. [NOAA National Ocean Service. Corals Tutorial: What are the other threats to corals?](https://oceanservice.noaa.gov/education/tutorial_corals/coral12_other_threats.html)
59. [IPCC (2018). Summary for Policymakers. In: Global Warming of 1.5 °C. Special Report.](https://www.ipcc.ch/sr15/chapter/spm/)
60. [McCook LJ, Ayling T, Cappo M, et al. (2010). Adaptive management of the Great Barrier Reef: a globally significant demonstration of the benefits of networks of marine reserves. PNAS 107(43):18278–18285.](https://doi.org/10.1073/pnas.0909335107)
61. [Hughes TP, Kerry JT, Álvarez-Noriega M, et al. (2017). Global warming and recurrent mass bleaching of corals. Nature 543:373–377.](https://doi.org/10.1038/nature21707)
62. [Boström-Einarsson L, Babcock RC, Bayraktarov E, et al. (2020). Coral restoration – a systematic review of current methods, successes, failures and future directions. PLOS ONE 15(1):e0226631.](https://doi.org/10.1371/journal.pone.0226631)

## Measurements & details

- **Color Range:** white, pink, red, orange, blue, purple, yellow

## Traits

- [Colorful](https://www.snappit.app/explore/traits/colorful): Colorful describes organisms or objects displaying a wide range of bright and distinct colors.
- [Symbiotic](https://www.snappit.app/explore/traits/symbiotic): Describes a relationship between two different species where they live in close association, often benefiting one or both.
- [Keystone Species](https://www.snappit.app/explore/traits/keystone-species): Describes a species whose presence and role have a disproportionately large effect on its environment.
- [Miniature](https://www.snappit.app/explore/traits/miniature): Pertaining to species that are significantly smaller than typical or average for their kind.
- [Natural](https://www.snappit.app/explore/traits/natural): Natural refers to organisms, materials, or phenomena existing or produced by nature, not artificially created.
- [Marine](https://www.snappit.app/explore/traits/marine): Marine habitats encompass all saltwater environments of the Earth's oceans, supporting an immense diversity of aquatic life.
- [Threatened](https://www.snappit.app/explore/traits/threatened): Threatened status denotes a species likely to become endangered in the foreseeable future without effective conservation interventions.

## Collections

- [Minerals](https://www.snappit.app/explore/collections/minerals)
- [Minerals & Rocks](https://www.snappit.app/explore/collections/minerals-and-rocks)

## Safety

**Danger:** 1/5 · Very low

Don't touch, stand on, break or collect coral: it is alive, and fire coral, a jellyfish relative that looks like coral, stings. Float above the reef and tell an adult if you get stung.

## Frequently Asked Questions

### How to identify Coral?

Compare body symmetry, shells, segments, legs, tentacles and how the animal moves. Invertebrates span many unrelated groups, so start with body structure. Invertebrate Identifier: upload a photo and compare the visible details.

### What colors can Coral be?

white, pink, red, orange, blue, purple, yellow

## Where to spot

- [Albuquerque Aquarium](https://www.snappit.app/guides/places/albuquerque-aquarium) — New Mexico, US · Zoo
- [Belle Isle Aquarium](https://www.snappit.app/guides/places/belle-isle-aquarium) — US · Zoo
- [Long Island Aquarium and Exhibition Center](https://www.snappit.app/guides/places/long-island-aquarium-and-exhibition-center-new-york) — New York, US · Zoo
- [Oregon Coast Aquarium](https://www.snappit.app/guides/places/oregon-coast-aquarium) — Oregon, US · Aquarium
- [Seattle Aquarium](https://www.snappit.app/guides/places/seattle-aquarium-washington) — Washington, US · Zoo
- [Shedd Aquarium](https://www.snappit.app/guides/places/shedd-aquarium-illinois) — Illinois, US · Zoo

[View all places to see Coral](https://www.snappit.app/guides/where-to-see/coral)

## More Invertebrate

- [Crab Spider](https://www.snappit.app/explore/crab-spider) (*Mecaphesa asperata*)
- [Garden Spider](https://www.snappit.app/explore/garden-spider) (*Araneus diadematus*)
- [Brown Recluse Spider](https://www.snappit.app/explore/brown-recluse-spider) (*Loxosceles reclusa*)

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Source: [Coral: Characteristics, Facts & Places to See — Snappit](https://www.snappit.app/explore/coral)