97 Fun Facts About Coral Reefs

What primary mineral compound forms the rigid structure of most coral reefs?

Coral polyps, the tiny animals that build reefs, extract calcium and carbonate ions from seawater to secrete calcium carbonate, forming their hard exoskeletons. These skeletons accumulate over time to create the massive structures of coral reefs.

To which phylum of animals do corals belong?

Corals are invertebrates classified under the phylum Cnidaria, a group that also includes jellyfish and sea anemones.

What is the name of the microscopic algae that live symbiotically within coral tissues?

Corals have a crucial symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the coral polyps and provide them with vital nutrients through photosynthesis.

Which is the largest coral reef system in the world?

The Great Barrier Reef is the world's largest coral reef system, stretching roughly 2,300 km along Australia's north-east coast and covering about 344,000 square km.

What is it called when stressed corals expel their symbiotic algae and lose their colour?

Coral bleaching is the loss of symbiotic algae and their pigments; the coral turns white and, if conditions do not improve, often starves.

Which broad ocean region holds the most diverse coral reefs, including the Coral Triangle?

The Indo-Pacific region, particularly the area known as the Coral Triangle, is widely recognized as having the highest biodiversity of coral reefs globally, home to the most species of corals.

About what percentage of marine species depend on coral reefs?

About 25% of marine species, including more than 4,000 fish species, rely on coral reefs at some stage of life, earning reefs the nickname rainforests of the sea.

Atmospheric carbon dioxide dissolving into seawater directly causes which major threat to coral reefs?

When excess carbon dioxide from the atmosphere dissolves into seawater, it forms carbonic acid.

Coral reefs cover roughly what share of the world's ocean floor?

About 0.1%: coral reefs cover roughly 284,000 square km of the 361 million square km ocean, yet rank among the most diverse ecosystems on Earth, hence the nickname rainforests of the sea.

Charles Darwin conceived his theory that atolls form around sinking volcanic islands during the voyage of which ship?

HMS Beagle carried Darwin on its 1831–36 voyage, during which he reasoned that reefs keep growing upward as volcanic islands slowly subside, leaving ring-shaped atolls around lagoons.

In Darwin's model, what is the first reef stage, forming right against a volcanic island?

Fringing reefs grow directly from the shore of a young volcanic island; as the island subsides a lagoon opens and the reef becomes a barrier reef, then a ring-shaped atoll once the island vanishes.

The Great Barrier Reef, which formed on a continental shelf rather than round a volcano, is roughly how old?

About 20,000 years: the living Great Barrier Reef began growing on an older limestone platform around 20,000 years ago, as sea levels rose after the last glacial maximum.

The Great Barrier Reef lies off the coast of which Australian state?

Queensland: the reef runs about 2,300 km along the state's coast from the tip of Cape York south to near Bundaberg.

Which explorer's ship ran aground on the Great Barrier Reef in June 1770, needing seven weeks of repairs?

James Cook's HMS Endeavour struck a shoal near what is now Cooktown on 11 June 1770 while he charted Australia's east coast; the ship was beached for repairs for seven weeks.

The Coral Triangle holds more than what share of the world's shallow-water reef-building coral species?

76%: Veron's 2009 survey counted 605 of the world's 798 reef-building coral species in the Coral Triangle, which spans Indonesia, Malaysia, Papua New Guinea, the Philippines, Solomon Islands and Timor-Leste.

The Belize Barrier Reef is a 300-km section of which larger 900-km system, the world's second largest?

The Mesoamerican Barrier Reef System runs about 900 km from Cancún to the Bay Islands of Honduras; Darwin called the Belize section 'the most remarkable reef in the West Indies'.

Which explorer's 1971 expedition to Belize's Great Blue Hole made it famous?

Jacques Cousteau brought the Calypso to the Great Blue Hole in 1971 and ranked it among the world's top dive sites; the sinkhole is about 318 m across and 124 m deep.

How does the crown-of-thorns starfish feed on living coral?

By everting its stomach over the coral: the starfish pushes its stomach out through its mouth, digests the living tissue externally, and leaves a white scar of bare skeleton.

Which reef fish grinds up coral, digests the edible parts and excretes the rest as sand that builds beaches?

Parrotfish excrete the ground-up rock as sand, helping create small islands and sandy beaches; some species also spin a mucus cocoon at night, which may warn them of prowling moray eels.

In a clownfish group, what happens when the dominant female disappears?

The breeding male changes sex into the new female, and the largest non-breeding juvenile matures into the breeding male; clownfish are protandrous hermaphrodites in a strict size-based hierarchy.

Which body of water is the world's northernmost tropical sea, with reefs among the healthiest anywhere?

The Red Sea, reaching nearly 30°N at the Gulf of Aqaba, is one of the warmest and saltiest seas because of extreme evaporation, yet hosts over 200 coral species.

How many nuclear devices did the United States detonate at Bikini Atoll between 1946 and 1958?

23 nuclear devices were detonated at Bikini between 1946 and 1958, including the 15-megaton Castle Bravo shot of 1954; the islanders were evacuated in 1946 and the atoll became a World Heritage Site in 2010.

Australia's Ningaloo Reef is famous for seasonal visits from which giant fish?

Whale sharks gather at Ningaloo from March to August, drawn by plankton blooms after the mass coral spawning; the 260-km reef fringes the Western Australian coast unusually close to shore.

Raja Ampat, home to about 75% of the world's known hard coral species, is in which country?

Indonesia: Raja Ampat, off the Bird's Head Peninsula of West Papua, has about 1,500 islands at the heart of the Coral Triangle and more than 1,400 reef fish species.

Tubbataha Reefs Natural Park, a World Heritage atoll reef in the Sulu Sea, belongs to which country?

Philippines: Tubbataha lies in the Sulu Sea about 150 km south-east of Puerto Princesa on Palawan, and its two atolls sit atop the extinct volcanic Cagayan Ridge.

The continental United States' only living barrier reef lies a few miles seaward of which island chain?

The Florida Keys sit just inside the Florida Reef, which runs about 270 km along the 20-metre depth contour; its densest sections, off Key Largo and Elliott Key, are protected by John Pennekamp Coral Reef State Park.

Which country's continental shelf holds the world's largest known cold-water coral reef?

Norway: the Røst Reef off the Lofoten Islands, about 40 km long, is the largest known cold-water coral reef, built by Lophelia pertusa at 300–400 m depth without symbiotic algae.

Fire coral, which painfully stings divers, is not a true coral but is closest to what?

Hydra and fire coral are both hydrozoans (class Hydrozoa): fire coral's hair-like stinging polyps protrude through pores in its skeleton, and contact brings burning pain and sometimes hives.

The largest brain corals can live for roughly how long?

900 years: the largest brain corals, growing slowly into domes up to 1.8 m across, can live for roughly nine centuries; by day they wrap their tentacles over the grooves for protection.

Which disease drove the Caribbean staghorn coral's collapse and its critically endangered listing?

White-band disease, which strips living tissue from the branches in a spreading white band, devastated Caribbean staghorn coral after 1980 and led the IUCN to list the species as critically endangered in 2008.

Elkhorn coral, once dominant on Caribbean reefs, declined by roughly what percentage from 1980 to 2006?

97%: elkhorn coral, whose antler-like branches once formed the framework of shallow Caribbean reefs, collapsed after 1980 from disease, hurricanes and bleaching, and was listed as threatened under the US Endangered Species Act in 2006.

The symbiotic 'zooxanthellae' inside coral tissue turned out, once cultured, to be what kind of organism?

Dinoflagellates, it turned out, once swimming gymnodinioid cells were cultured from coral; the genus Symbiodinium is now among the most abundant eukaryotic microbes on reefs.

The word 'atoll' comes from Dhivehi, the language of which island nation?

Maldives: 'atoll' entered English from the Dhivehi word atholhu; the Maldives itself is a chain of coral atolls in the Indian Ocean.

An individual coral polyp is typically how large in diameter?

A few millimetres: most polyps are 1–3 mm across, each a sac-like animal with tentacles around a central mouth that secretes an exoskeleton at its base; colonies grow by asexual budding.

Sea anemones, close relatives of coral, are named after what?

A flowering plant: sea anemones take their name from the anemone, a terrestrial flower of the buttercup family, because their ring of tentacles resembles its petals; unlike jellyfish they have no medusa stage.

Which US refuge in the Northern Line Islands hosted Pan Am flying-boat stopovers in 1937–38?

Kingman Reef, with just 3 hectares of dry land, served as a Pan American flying-boat stopover on the 1937–38 route to New Zealand and became a National Wildlife Refuge in 2001.

A 2012 study found the Great Barrier Reef had lost over what fraction of its coral since 1985?

Half: the 2012 study by De'ath and colleagues found coral cover fell from 28% to 13.8% between 1985 and 2012, blaming cyclones, crown-of-thorns outbreaks and bleaching; a 2020 study confirmed a similar loss for 1995–2017.

The Great Barrier Reef was inscribed as a UNESCO World Heritage Site in which year?

Queensland later named it a state icon in 2006, and it has long been central to Aboriginal and Torres Strait Islander cultures.

The Coral Triangle covers about what share of the planet's ocean area?

1.6%: the Coral Triangle covers about 5.7 million square km of ocean around Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor-Leste and Solomon Islands.

Coral reefs first appeared roughly 485 million years ago at the dawn of which geological period?

Ordovician: the period opened about 485 million years ago, and its early coral reefs displaced the microbial and sponge reefs that had dominated the Cambrian seas.

Coral reefs cover an area of ocean roughly equal to half of which country?

France: reefs cover about 284,000 square km, roughly half of metropolitan France's 551,000 square km, yet shelter a large share of all marine species.

What is the tiny free-swimming larva of a coral called?

A planula is released usually at night, often three to six days after a full moon, and settles within days.

Charles Darwin's The Structure and Distribution of Coral Reefs was published in which year?

He noted that the largest, strongest corals grew where the surf was most violent, part of what is now called Darwin's paradox.

The entire Maldives is made up of how many atolls?

26 natural atolls make up the Maldives, grouped into 20 administrative atolls and holding about 1,190 coral islands along a 90,000-square-km chain.

Biorock reef-restoration structures grow mineral coatings on steel using what?

The current makes dissolved minerals precipitate as aragonite, the same mineral natural reefs are built from, and corals quickly colonise it.

Which sea urchin's 1983 Caribbean mass die-off let algae overgrow the reefs?

The long-spined sea urchin Diadema antillarum, a key grazer, lost about 95% of its Caribbean population to a 1983–84 disease, and algae overgrew reefs no longer kept clear.

The Great Barrier Reef is made up of over how many individual reefs?

2,900: the Great Barrier Reef comprises over 2,900 individual reefs and about 900 islands, stretching more than 2,300 km along the Coral Sea.

How often does the Great Barrier Reef Marine Park Act 1975 require an Outlook Report on the reef?

Every five years: section 54 of the Act requires the Outlook Report, first produced in 2009 and followed in 2014, 2019 and 2024, covering the reef's health, pressures and likely future.

The world's most southerly barrier coral reef, at 31°30'S, rings which Australian outpost?

Lord Howe Island, a crescent-shaped volcanic remnant 600 km east of the New South Wales coast, has a sheltered western lagoon inside the reef; the island group was World Heritage-listed in 1982.

Apo Reef, the largest coral reef in the Philippines, lies west of which island?

Mindoro: Apo Reef lies in the Mindoro Strait off the island's west coast, a roughly triangular 34-square-km reef split by a deep channel into two atoll-like halves.

Which government created the vast Chagos Marine Protected Area in April 2010?

The United Kingdom declared the 640,000-square-km reserve on 1 April 2010; it was controversial because the Chagossians had been expelled in the 1960s–70s to make way for the US base on Diego Garcia.

On which day of 1777 did James Cook reach the world's largest atoll by land area?

Christmas Eve: Cook's ships reached Kiritimati on 24 December 1777 and he named it Christmas Island; the atoll has the largest land area of any atoll, about 388 square km.

Aldabra, the world's second-largest coral atoll, hosts 100,000 of which animal?

Giant tortoises: Aldabra's population of about 100,000 Aldabra giant tortoises is the world's largest; a 1960s scientists' protest known as the 'Aldabra Affair' helped block a planned military airfield.

Kwajalein Atoll, ringing one of the world's largest lagoons, is used by the US mainly for what?

Missile tests: the atoll hosts the Ronald Reagan Ballistic Missile Defense Test Site, where ICBMs fired from California splash down in its 2,174-square-km lagoon.

Røst Reef, the world's largest known deep-water coral reef, was discovered in which year?

It lies about 100 kilometres west of Røstlandet in the Lofoten Islands, at depths of 300 to 400 metres, and is now closed to bottom-touching fishing gear.

The cold-water coral Lophelia has been found growing on which North Sea structures?

Oil rigs: Lophelia pertusa was spotted on the Brent Spar oil platform that Greenpeace campaigned to stop being dumped at sea in 1995, and a later survey found it on 13 of 14 North Sea platforms examined.

The 2017 reef documentary Chasing Coral was released worldwide on which streaming service?

Netflix released Chasing Coral worldwide on 14 July 2017 after it won the Audience Award for U.S. Documentary at Sundance; director Jeff Orlowski had made Chasing Ice five years earlier.

Which navigator gave the Great Barrier Reef its name after mapping it in 1802?

Matthew Flinders, who charted Australia's east coast aboard HMS Investigator in 1802, wrote of 'barrier reefs' for any reef blocking the swell and of the whole chain as the Great Barrier Reefs.

The Shen Neng 1, which hit the Great Barrier Reef's Douglas Shoal in 2010, carried what?

Coal: the Chinese bulk carrier was hauling about 65,000 tonnes of coal when it ran aground on 3 April 2010, leaking up to four tonnes of fuel oil and gouging a 3-km scar into the shoal.

In 2018 Australia's government handed a AUD$443 million grant to which organisation?

The Great Barrier Reef Foundation, a small charity, received the AUD$443.3 million grant in April 2018 without a tender, for water quality, crown-of-thorns starfish control and reef restoration science.

The Bramble Cay melomys, the Great Barrier Reef's only endemic mammal, was what?

A rodent: the melomys was a small mosaic-tailed rat last seen in 2009 on Bramble Cay, the reef's northernmost island; its 2016 declaration of extinction was called the first mammal loss driven by human-caused climate change.

Heron Island and its reef research station open which Arthur C. Clarke novel?

The island's real station, run by a Brisbane university since 1970, is one of the world's principal coral reef research facilities.

Lady Elliot Island, the Great Barrier Reef's southernmost cay, was mined in the 1860s for what?

Guano, the dried seabird droppings prized as fertiliser, was dug from the island from 1863 to 1873, stripping it of vegetation; the island was named in 1816 after the ship Lady Elliot that found it.

Kure Atoll, the northernmost atoll on Earth, belongs to which island chain?

Northwestern Hawaiian Islands: Kure, at 28°25'N, is the chain's westernmost island and the northernmost atoll on Earth; most of the world's roughly 440 atolls lie in the Pacific and Indian oceans.

Which island at 32°18'N is sometimes called the northernmost atoll, though really a pseudo-atoll?

Bermuda: its ring of reefs resembles an atoll, but it is classed as a pseudo-atoll because the reefs did not form by Darwin's subsidence process; the Gulf Stream keeps its waters warm enough for coral at 32°N.

Stony coral tissue loss disease, first seen off Miami in 2014, kills a coral within what time?

Weeks or months: stony coral tissue loss disease kills even large colonies within months and small ones in weeks, attacking at least 22 reef-building species across Florida and, by 2019, the wider Caribbean.

The Andros Barrier Reef, edging the Tongue of the Ocean, lies in which country?

The Bahamas: the Andros Barrier Reef runs about 190 km along the eastern edge of Andros Island, dropping into the 2,000-m-deep Tongue of the Ocean trench.

Which 1992 hurricane is blamed for freeing six lionfish from a Florida aquarium?

Hurricane Andrew, which struck south Florida in August 1992, is said to have smashed a Biscayne Bay aquarium; the story is shaky, since a lionfish had been recorded off Dania Beach in 1985.

Roatan Marine Park in Honduras has tried training which animals to eat invasive lionfish?

Sharks: from 2011 Roatan Marine Park divers speared lionfish and fed them to Caribbean reef sharks, hoping the sharks would learn to hunt them; about 93% of the Atlantic invaders are red lionfish.

Publication of the Amazon Reef's discovery, at the mouth of the Amazon, came in which year?

2016: the Moura team described the Amazon Reef in Science Advances in April 2016, a coral, sponge and rhodolith system of about 9,500 square km, comparable to Cyprus, running some 1,000 km off Brazil and French Guiana.

Precious red coral, prized for jewellery, is found mainly in which sea?

Mediterranean Sea: Corallium rubrum grows on rocky seabeds from 10 to 300 m deep, and the coast of Alghero in Sardinia is nicknamed the Coral Riviera for its harvest.

Black corals are recognised by skeletons made of which material?

Also called thorn corals, they live at almost every ocean depth except brackish water, and their only modern use is jewellery.

Each soft coral polyp carries how many feather-like tentacles?

That eightfold symmetry is the 'Octo' in Octocorallia, and each tentacle is fringed with tiny side branches called pinnules.

What gives the skeleton of blue coral, Heliopora coerulea, its distinctive colour?

Iron salts in the skeleton give blue coral its colour; it is the only living octocoral to build a massive aragonite skeleton, which makes it easy to recognise in fossil outcrops.

Giant clams of tropical reefs can reach roughly what maximum weight?

200 kg: Tridacna gigas can exceed 200 kg and 120 cm across; the largest known specimen measured 137 cm, and Antonio Pigafetta described them on Magellan's voyage in 1521.

Which reef organisms make up 70 to 95 percent of the hawksbill turtle's diet?

Its sharp curved beak is built for prising them out of crevices, and its shell was once the main source of decorative tortoiseshell.

Which reef fish was reported in 2019 to pass the mirror self-recognition test?

Bluestreak cleaner wrasse: in a 2019 study, wrasses given a coloured throat mark tried to scrape it off after seeing their reflection, the classic mark test of self-recognition used on apes and dolphins.

The humphead wrasse, the largest of all wrasses, is also known by which emperor's name?

Napoleon wrasse is the humphead's other name, from the hump on adult males' heads that recalls the emperor's bicorne hat; males can reach 229 cm and 191 kg, though the average fish is about 60 cm.

Triton's trumpet, a natural predator of the crown-of-thorns starfish, is what kind of animal?

A sea snail: the giant triton Charonia tritonis, a gastropod with a shell up to half a metre long, tears the starfish apart with its file-like radula; titan triggerfish and white-spotted puffers also eat them.

The first known crown-of-thorns outbreak on the Great Barrier Reef came in which decade?

1960s: the first documented outbreak hit Green Island off Cairns in 1962; a later 2000 outbreak helped strip 66% of live coral cover from sampled reefs, though the starfish has lived on the reef for thousands of years.

A microatoll, a coral colony dead on top but alive at the rim, is a natural recorder of what?

Sea level: a microatoll's flat dead top marks the lowest low-tide level, so its growth rings record past sea level; scientists also use them to date earthquakes that suddenly lifted or dropped a coastline.

Finger-like Porites corals are often bored into by which colourful reef animal?

Christmas tree worms, Spirobranchus giganteus, colonise Porites so often that aquarium shops sell the colonies as 'Christmas tree worm rocks'; Porites skeletons also record past sea temperatures in their oxygen isotopes.

Extinct Permian rugosans are commonly nicknamed what, after their skeleton shape?

Horn corals: the solitary rugosans grew curved, conical skeletons like a cow's horn; 'Rugosa' means wrinkled in Latin, after their ridged outer walls, and some species reached nearly a metre long.

Rudists, box- and cone-shaped bivalves, were major reef builders in which period?

Cretaceous: rudists dominated warm Tethyan reefs of the Late Cretaceous, one valve growing as a spike-like cone and the other as a flat lid; the group vanished in the end-Cretaceous extinction that took the dinosaurs.

The word 'cay', a low sandy island on a reef, comes from which people's word for island?

Lucayan, the Taíno people of the Bahamas, called an island 'cairi'; Spanish turned it into 'cayo' before it reached English, where Americans spell and say it 'key'.

Reefs in which body of water survive winters of 13 °C and summers of 38 °C?

Persian Gulf corals endure extreme swings, from 13 °C in winter to 38 °C in summer, whereas most reefs prefer a steady 26–27 °C and few exist where water drops below 18 °C.

Who used a microscope to show coral had animal cell membranes, not plant ones?

William Herschel, the astronomer who discovered Uranus in 1781, examined coral under a microscope and found the thin cell membranes typical of animal tissue, confirming coral was no plant.

In 1896–98 the Royal Society drilled boreholes to test atoll formation on which atoll?

Funafuti: the Royal Society's three expeditions of 1896–98 bored 340 m into the atoll, the 1897 effort led by Sydney professor Edgeworth David; the atoll became Tuvalu's capital at independence in 1978.

Rangiroa, the largest atoll of the Tuamotus, has a name meaning what in Tuamotuan?

Vast sky, from Tuamotuan ra'i roa, describes its enormous lagoon of about 1,400 square km, 355 km north-east of Tahiti, which is nonetheless smaller than those of Kwajalein and Huvadhu.

The name of Wakatobi National Park is a portmanteau built from the names of what?

Four islands: Wangi-wangi, Kaledupa, Tomia and Binongko supply the syllables WA-KA-TO-BI; the park in Southeast Sulawesi is home to many sea-going Bajau fishers.

Marovo Lagoon, the world's largest double-barrier enclosed lagoon, lies in which country?

Solomon Islands: the 700-square-km lagoon lies off New Georgia in Western Province, its outer islands forming a double barrier reef along the island's north-east coast.

New Caledonia's barrier reef shelters the third-largest population of which endangered mammal?

Dugong: the lagoons hold one of the world's largest dugong populations, usually ranked third; the 1,500-km barrier reef encircles Grande Terre and was World Heritage-listed in 2008.

Which Dutch ship, wrecked on the Houtman Abrolhos in 1629, was followed by a mutineers' massacre?

Batavia, a Dutch East India Company ship, struck Morning Reef on 4 June 1629; while commander Pelsaert sought help, Jeronimus Cornelisz's mutineers murdered about 125 survivors on the islands.

In 2009 Palau created the world's first what?

Shark sanctuary: in September 2009 Palau banned commercial shark fishing across its 600,000-square-km exclusive economic zone, the first nation to do so; its Rock Islands became a World Heritage Site in 2012.

In 2017 Palau became the first nation to stamp an environmental pledge into what?

Visitors' passports: from December 2017 the Palau Pledge, a promise to act responsibly written for Palau's children, was stamped into arriving tourists' passports for them to sign.

Which US president first proclaimed Papahānaumokuākea a marine national monument?

George W. Bush proclaimed the monument on 15 June 2006, then the largest marine reserve on Earth; Barack Obama more than quadrupled it in 2016, pushing the boundary to the edge of the exclusive economic zone.

The Coral Beach Nature Reserve, Earth's northernmost shallow coral reef, is in which resort city?

Eilat: the reserve on the Gulf of Aqaba, about 1.2 km of shoreline, holds Israel's only coral reefs; the city also has the Coral World Underwater Observatory, opened in 1975.