What do corals use to catch food?

Stinging cells in their tentacles

Is a bleached coral dead?

No: it is starving but can recover if the water cools

You are snorkelling over a reef. What should you do?

Float above it and never touch the coral

Coral

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 known as Stony coral, Hard coral, Reef-building coral

Coral abilities

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.
- 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 corals0.3–2 cm
Pillar coral≈0.8 cm
Staghorn figure is branch growth of a healthy colony (8 inches ≈ 20 cm).
Sources [4, 10, 14, 20]
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.
- 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
The coral also catches plankton for nutrients the algae can’t supply.
Sources [6, 21, 22, 23]
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.
- 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 coral15–35%
Bleached coralup to 100%
Sources [1, 20, 21]
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.
- 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.
Sources [1, 24, 25]
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.
- 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.
Sources [4, 26, 27]
+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.
- 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 event84%
2014–2017 event68%
The first two global bleaching events were in 1998 and 2010.
Sources [12, 28, 29, 30, 31]
2–3 weeks after mild heat, some turn neonStill 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.
- 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.
Sources [8, 9, 32, 33, 34]

Tap a pin on the model or a part below.

1–3 mm across, in most stony corals
- 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.
Sources [1, 2, 3, 4]
1 opening for food in and waste out
- 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.
Sources [1, 2]
Night when most polyps feed
- 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.
Sources [1, 5]
up to 90% of the algae’s food goes to the coral
- 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.
Sources [6, 7, 8, 9]
240 million years old: the oldest stony coral fossils
- 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.
Sources [3, 5, 10]
up to 20 cm of branch a year (staghorn)
- 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.
Sources [4, 11, 12]
0.3–2 cm a year (massive corals)
- 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.
Sources [13, 14, 15, 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.
Sources [12, 16, 17]
4,000+ fish species depend on reefs
- 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.
Sources [4, 18, 19]

Real corals

Photos from Wikimedia Commons. Four of them can repaint the 3D model.

HealthyGolden-brown tissue over the skeleton. At night the polyps open their tentacles.Algae inside make most of its foodHealthy corals on Flynn Reef, Great Barrier Reef. Photo: Toby Hudson, CC BY-SA 3.0
Table corals (Acropora hyacinthus) in American SamoaPhoto: Wendy Cover / NOAA, Public domain
Boulder brain coral (Colpophyllia natans)Photo: Nick Hobgood, CC BY-SA 3.0
Galaxy coral polyps, Red SeaPhoto: Diego Delso, CC BY-SA 4.0
Polyps releasing egg and sperm bundles, Flower Garden Banks, Gulf of MexicoPhoto: G.P. Schmahl / NOAA, Public domain
BleachedWhite or very pale: the algae have left, so the white skeleton shows through the tissue.Alive but starving; it can recover if the water coolsBleached staghorn coral on the Great Barrier Reef. Photo: Matt Kieffer, CC BY-SA 2.0
GlowingNeon green, yellow or red: the coral’s own fluorescent proteins shining under blue light. Some heat-stressed corals turn extra bright.Fluorescence, not bioluminescenceA coral’s fluorescent proteins glowing green, photographed in a public aquarium. Photo: Vincent Niochet, CC BY-SA 4.0
DeadBare skeleton covered in a fuzzy turf of algae. No living polyps are left.Algae take over the space where corals livedDead brain coral overgrown by algae, Florida Keys. Photo: Jstuby, CC0
Part of the Great Barrier Reef seen from the International Space Station in 2022Photo: NASA Johnson Space Center, Public domain
A soft coral (probably Sarcophyton), Maldives: soft corals don’t build reefsPhoto: Tchami, CC BY-SA 2.0

Coral by the numbers

Measured by scientists and agencies, with comparisons you can picture.

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; Staghorn (max)20 cm/yrMassive coral (max)2 cm/yr; 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]

Coral myths and facts

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

0 of 8 answered

“Coral is a plant.”
“Coral is just a rock.”
“You can see the Great Barrier Reef from space.”
“Coral reefs cover less than 1% of the ocean but support about a quarter of sea life.”
“A bleached coral is dead.”
“All corals live in warm, shallow water.”
“Fire coral is a kind of coral.”
“Sunscreen is the main reason coral reefs are dying.”

Coral records

Superlatives and record holders, each checked against its source.

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

Threats

Marine heatwaves84%
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]
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]
Crown-of-thorns starfish42%
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]
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]
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]
Physical damage
Anchors, boat groundings, tangled litter and fishing gear, and people standing on reefs all break corals.⁠[58]

What’s helping

Cutting emissions1.5 °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]
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]
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]
Restoration60–70%
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]
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]

Some corals and their Red List status

Staghorn coral (Acropora cervicornis)Critically Endangered
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)Critically Endangered
Staghorn’s partner reef-builder in the Caribbean, facing the same threats.⁠[41]
Grooved brain coral (Diploria labyrinthiformis)Critically Endangered
An Atlantic brain coral. Atlantic corals are especially threatened by yearly severe bleaching, pollution and disease.⁠[16, 41]
Pillar coral (Dendrogyra cylindrus)Critically Endangered
Naturally rare. Stony coral tissue loss disease has almost removed it from Florida.⁠[16, 20]
Table coral (Acropora hyacinthus)Endangered
An Indo-Pacific table coral like the one in the model.⁠[16]
Lobe coral (Porites lobata)Least Concern
A slow-growing boulder coral of the Indo-Pacific.⁠[16, 13]
White coral (Desmophyllum pertusum)Vulnerable
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 vs soft coral vs sea anemone
	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.

Coral quiz

12 questions. Every answer is somewhere on this page.

Question 1 of 12Score 0

Is coral an animal, a plant or a rock?

Coral field notes

Diet, food web, safety and badges from the Snappit field guide.

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.

Traits

Collections

Classification

Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa

Also known as

Stony coral · Hard coral · Reef-building coral · Scleractinian coral · Reef Coral · Sea Coral

Sources for these coral facts

62 studies and references · checked Oct 16, 2026

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

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.

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.

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.

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.

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.

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.

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.

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.