96 Fun Facts About Coral Reefs

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

Calcium carbonate: polyps draw dissolved calcium from seawater and lay it down as rock beneath them, so a reef is mostly old skeleton under a thin living skin.

To which phylum of animals do corals belong?

Cnidaria: corals share the phylum with jellyfish and sea anemones, and like them fire stinging cells from their tentacles to catch passing prey, mostly at night.

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

Zooxanthellae live inside the coral's own cells and hand over sugars made by photosynthesis, supplying up to 90% of the energy the coral needs.

Which animated film's young hero is taken from the Great Barrier Reef to a fish tank in a Sydney dentist's office?

Finding Nemo sends his father Marlin south to Sydney on the East Australian Current, a real flow of warm water that sweeps down the coast from the reef.

What term describes heat-stressed corals expelling the symbiotic algae that live in their tissues?

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 holds roughly ten times as many reef-building coral species as the western Atlantic, and in 2009 six nations pledged to manage its richest corner together.

About what percentage of marine species depend on coral reefs?

About 25% of marine species, including more than 4,000 kinds of fish, depend on coral reefs at some stage of life, even though reefs cover only a sliver of the ocean floor.

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

Ocean acidification: dissolved carbon dioxide forms carbonic acid, which uses up the carbonate corals need, so their skeletons become harder to build and maintain.

Which Pacific climate pattern drove the 1998 ocean heatwave that killed about 16% of the world's coral reefs?

El Niño spread unusually warm water across the tropics in 1997–98, and overheated corals cast out the algae that feed them in the first worldwide event of its kind on record.

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

HMS Beagle was still off South America when Darwin thought the theory out, before he had seen a true coral reef; he tested it on the reefs he met later in the voyage.

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.

On the Great Barrier Reef, corals spawn together once a year, in the nights after which event?

A full moon in spring or early summer is the cue: colonies release millions of egg and sperm bundles on the same nights, so that bundles from different corals can meet in open water.

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

Queensland's rivers wash farm run-off onto the reef in tropical floods, one of its main threats, because the reef hugs some 2,300 km of the state's coast.

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

James Cook's crew heaved the ship's guns overboard to float Endeavour off the coral, patched the hole with a sail, then beached her near today's Cooktown.

Which nation, independent only since 2002, is the smallest of the six Coral Triangle countries?

Timor-Leste chose independence in a UN-supervised referendum in 1999; its waters lie in a region that holds 76% of the world's reef-building coral species.

The Belize Barrier Reef is a section of which larger reef system, shared by four countries?

The Mesoamerican Barrier Reef runs from Mexico's Yucatán to Honduras; Belize's stretch came off UNESCO's danger list in 2018 after the country curbed offshore oil activity.

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

Jacques Cousteau's team brought up stalactites from its drowned caves, showing the hole formed on dry land long ago, before the ocean rose and flooded it.

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 that builds beaches and islands; many also sleep in a mucus cocoon, which experiments show keeps blood-sucking parasites off.

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 coral-fringed body of water is the world's northernmost tropical sea?

The Red Sea is unusually salty because desert heat evaporates more water than rain and rivers return, yet about 200 kinds of soft and hard coral thrive in it.

Castle Bravo, the largest US nuclear test, was detonated in 1954 at which Pacific coral atoll?

Bikini Atoll's Bravo blast was more than twice as strong as predicted, and its fallout sickened islanders on Rongelap and the crew of a Japanese fishing boat.

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

Whale sharks, the largest fish alive, gather at Ningaloo from March to August to filter plankton, and are so docile that visitors snorkel alongside them.

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

Indonesia: Raja Ampat sits where the Indian and Pacific oceans meet, so currents carry its coral and fish larvae outwards to reseed reefs across the region.

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

Philippines: when the US Navy minesweeper Guardian ran aground there in 2013, it was cut up and removed in pieces, and Washington paid about $2 million for the damage.

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

The Florida Keys are themselves old reef: the upper and middle islands are the exposed top of a fossil coral reef, with today's living reef growing just offshore.

Røst Reef, one of the largest cold-water coral reefs ever found, lies off which country's Lofoten Islands?

Norway: the reef grows in cold darkness 300 to 400 m down, so its Lophelia corals catch drifting food instead of relying on sunlit algae as tropical reef builders do.

Fire coral, whose sting burns divers, is really a closer relative of which animal than of true corals?

Hydra and fire coral are both hydrozoans: fire coral's hair-fine stinging polyps reach out through pores in its skeleton, and a brush against them brings burning pain that can last for days.

Which reef coral forms rounded boulders covered in a maze of winding grooves?

Brain coral feeds mainly at night, stretching its tentacles out to catch drifting animals and pulling them back in by day.

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

White-band disease strips living tissue from the branches in a band that creeps from base to tip; it destroyed most Caribbean staghorn coral and left the species critically endangered.

In 2018 Hawaii passed a law to protect its coral reefs by banning the sale of some kinds of which product?

Sunscreen containing oxybenzone or octinoxate was barred from sale because the chemicals can build up in coral tissue, damage its DNA and deform its young.

Coral's symbiotic algae belong to which group of single-celled organisms, better known for causing red tides?

Dinoflagellates of the genus Symbiodinium swim freely in open water, but once inside a coral cell they settle into a round, tail-less form and feed their host by photosynthesis.

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

Maldives: English borrowed the Dhivehi word atholhu, and the country is itself a chain of 26 atolls so low-lying that its cabinet met underwater in 2009 to protest rising seas.

An individual coral polyp is typically how large in diameter?

A few millimetres: each polyp is a sac-like animal with a ring of tentacles round its mouth, and it buds off copies of itself until a colony holds many thousands.

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

A flowering plant: sea anemones are named after the anemone, a land flower, for their bright, flower-like looks; unlike jellyfish, they never have a free-swimming medusa stage.

Amelia Earhart vanished in 1937 while trying to find which tiny coral island in the central Pacific?

Howland Island had airstrips built for her stopover, and the Coast Guard cutter Itasca waited offshore to guide her in by radio, but her plane never arrived.

A 2012 study found the single biggest cause of the Great Barrier Reef's coral loss since 1985 was what?

Tropical cyclones caused 48% of the loss, crown-of-thorns starfish 42% and bleaching only 10%, the study found, as coral cover fell from 28% to 13.8%.

Which gold port founded in 1876 became a tourist gateway to the Great Barrier Reef?

Cairns began by shipping gold from inland fields, and the opening of its international airport in 1984 turned it into a base for reef tourism.

Which reef crustacean throws a punch so fast that the surrounding water flashes into vapour bubbles?

Mantis shrimp clubs reach about 23 metres a second from a standing start, and when the bubbles collapse they strike the prey a second time and give off a flash of light.

Coral reefs first appeared in which geological period, the one that came straight after the Cambrian?

Ordovician seas, from about 485 million years ago, saw corals join and then displace the microbes that had built the reefs before them.

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 a tiny flattened larva that swims with beating hairs until it finds a hard surface, settles, and turns into the first polyp of a new colony.

Darwin's paradox asks how coral reefs teem with life in tropical waters that are poor in what?

Nutrients are so scarce in clear tropical water that a reef is likened to an oasis in a desert; it gets by because corals and their algae pass the same nutrients back and forth.

Which coastal habitat acts as a nursery for many young coral reef fish before they move out to the reef?

Mangrove forests give young fish a maze of roots to hide in, and a Caribbean study found some reef species more than twice as plentiful on reefs with mangroves nearby.

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

Low-voltage electric current makes minerals dissolved in seawater settle on the steel as a crust of limestone, giving transplanted corals a solid base to grow on.

The sudden 1983 die-off of which Caribbean grazers let seaweed overgrow the region's coral reefs?

Sea urchins of the long-spined species Diadema antillarum were struck again in 2022, and this time scientists traced the deaths to a single-celled parasite; the cause of the 1983 die-off was never found.

In a 2019 experiment, what did scientists play through underwater speakers to draw young fish to patches of dead coral?

Sounds of a healthy reef doubled the number of fish that settled and stayed on the dead coral rubble over six weeks, and raised the number of species by half.

Which reef mollusc spears fish with a venomous harpoon powerful enough to kill a human?

Cone snail venom is so potent that one of its components became ziconotide, a painkiller approved in 2004 for severe pain that other drugs cannot ease.

The world's most southerly barrier coral reef, at 31°30'S, lies off which Australian island?

Lord Howe Island owes its coral to the East Australian Current, which carries warm water down from the Great Barrier Reef each summer and with it tropical species.

Which reef predator has a second set of jaws in its throat that shoots forward to drag prey down?

Moray eels cannot suck prey in as most fish do, so the throat jaws lunge into the mouth, grip the catch and haul it back to be swallowed.

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, but in 2015 an international tribunal ruled that it had breached the law of the sea by ignoring Mauritius's rights there.

On which day of 1777 did a British expedition first reach Kiritimati, the atoll with the world's largest land area?

Christmas Eve: the expedition's captain named the atoll Christmas Island, and Kiritimati is the same word in Gilbertese spelling, pronounced roughly 'Kirismas'.

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.

Which fishing method, which can smash slow-growing cold-water coral reefs, is restricted over Røst Reef?

Bottom trawling drags nets held open by heavy metal doors across the seabed, breaking coral thickets that grow only about a millimetre a year.

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 in July 2017 after it won an audience award at Sundance; the film follows a team building underwater time-lapse cameras to record reefs dying as it happens.

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

Matthew Flinders chose the name because the maze of reefs was a barrier to safe navigation for his ship Investigator; the term first appeared in his 1814 book.

The Chinese bulk carrier Shen Neng 1, which gouged the Great Barrier Reef in 2010, was hauling which Australian export?

Coal: the carrier had left Gladstone for China with 65,000 tonnes aboard when it ran aground at full speed, about 10 km outside the shipping lane, and leaked fuel oil onto the reef.

In 2018 Australia's government handed a AUD$443 million reef grant, with no tender, to which organisation?

The Great Barrier Reef Foundation, a non-profit rather than a government agency, received the money to spend on water quality, starfish control and reef restoration science.

The Bramble Cay melomys, a Great Barrier Reef rodent, was the first mammal wiped out by what?

Climate change, researchers concluded: the sea washed over its low island again and again, destroying the plants it lived among; last seen in 2009, the rat was reported extinct in 2016.

Which science-fiction author who settled in Sri Lanka wrote The Coast of Coral about the Great Barrier Reef?

Arthur C. Clarke set the first part of his novel The Deep Range on the reef's Heron Island, and in Sri Lanka he found the underwater ruins of an ancient temple and ran a dive school.

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

Guano, seabird droppings prized as fertiliser, was dug from the cay for a decade from 1863; the miners stripped it of its trees and topsoil, and replanting began only in the 1960s.

Kure Atoll, the northernmost atoll on Earth, lies just beyond Midway at the far end of which island chain?

Northwestern Hawaiian Islands: Kure sits near the 'Darwin Point', the latitude beyond which coral can no longer grow fast enough to keep a sinking island at the surface.

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

Bermuda: its ring of reefs looks like an atoll, but its high ground and mostly submerged reefs set it apart, and coral survives that far north only because the Gulf Stream warms the water.

Divers slow stony coral tissue loss disease, first seen off Florida in 2014, by smearing infected colonies with what?

An antibiotic paste containing amoxicillin can stop a lesion spreading, though new ones may still form; untreated, the disease kills a colony within weeks or months.

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

The Bahamas: the reef drops into the Tongue of the Ocean, a basin more than a kilometre deep where the US Navy tests submarines and their weapons from a base on Andros.

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: park officials hoped they would learn to hunt lionfish, which have very few predators in the Atlantic; about 93% of the invaders are red lionfish.

A reef system announced in 2016 grows beneath the muddy freshwater plume at the mouth of which river?

Amazon sediment was thought to rule out reefs along that coast, yet the system runs for about 1,000 km beneath some of the murkiest water in the world.

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

Mediterranean Sea: Romans hung branches of its red coral round children's necks to ward off harm, and the coast around Alghero in Sardinia is still known as the Coral Riviera.

Black corals are recognised by dark, flexible skeletons made of protein and which tough material?

Chitin and protein make the skeleton dark and bendable, yet a living black coral is rarely black: its tissue can be white, red, green, yellow or brown.

Coral skeleton has been tested by surgeons as a graft material to help rebuild which human tissue?

Bone: mineral taken from sea coral has been trialled as a graft material, a scaffold that the patient's own tissue can grow into and replace.

Which reef-floor animal defends itself by shooting sticky threads out of its rear end to entangle attackers?

Sea cucumbers of some reef species fire out these sticky tubules to tangle up a predator; they also breathe through the same opening, pumping water in and out of internal 'trees'.

Which reef mollusc gets most of its food from algae living in its flesh?

Giant clams can weigh more than 200 kg; once one fastens itself to a spot on the reef it stays there for life, opening by day to sun the algae in its mantle.

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

Sponges growing in the reef's cracks and crevices are reached with the narrow, hawk-like beak that gives the turtle its name; its shell once supplied most of the world's 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 common name; males can reach 229 cm and 191 kg, and it eats even toxic prey such as the crown-of-thorns starfish.

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

A sea snail: the giant triton, whose shell can reach 60 cm, is one of the few animals that eat the starfish, and has been described as tearing it to pieces with its file-like radula.

Divers can kill a crown-of-thorns starfish with one injection of which kitchen staple?

Vinegar kills the starfish within two days, a 2015 Australian university study found, and trials on the Great Barrier Reef showed it does no harm to other reef life.

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

Sea level: a microatoll dies back wherever it is left exposed to air, so its shape tracks past sea level, and scientists read old ones to map the ruptures of great earthquakes.

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

Christmas tree worms settle on the coral as larvae and burrow in; at any threat they pull their spiral crowns back into the tube and plug the hole behind them.

Rugose corals, extinct since the Permian, left curved, cone-shaped fossils nicknamed what?

Horn corals laid down a fine growth line each day, and counting them showed that a year in the Devonian had about 400 days, each shorter than today's.

Rudists, odd cone-shaped clams, built reefs in which period before dying out alongside the dinosaurs?

Cretaceous rudists built reefs across the Tethys Ocean, and because their reefs are so porous the buried remains now make favoured traps for oil.

Which reef animal's barb killed 'Crocodile Hunter' Steve Irwin while he filmed on the Great Barrier Reef in 2006?

A stingray's barb pierced his chest at Batt Reef while he was working on the documentary Ocean's Deadliest; it is believed to be the only fatal stingray strike ever caught on video.

Reefs in which body of water are the world's hottest, in summer seas too warm for most corals?

Persian Gulf corals survive summer seas of about 35 °C, so scientists cross them with corals from cooler waters to see whether heat tolerance can be passed on.

Hawaii's state fish, the humuhumunukunukuāpuaʻa, belongs to which family of reef fish?

Triggerfish: the reef triggerfish was named state fish in 1985 under a law that lapsed after five years, and was only restored for good in 2006.

Funafuti, the capital atoll of which nation, was drilled in the 1890s to test Darwin's atoll theory?

Tuvalu's atoll was bored by Royal Society expeditions three years running, but the results were judged inconclusive; Darwin was proved right only in the 1950s, when deeper drilling on another Pacific atoll struck volcanic rock.

Rangiroa, the largest atoll of the Tuamotus, has a lagoon big enough to swallow which whole island?

Tahiti covers 1,044 square km against the lagoon's 1,446; Rangiroa's name means 'vast sky', and bottlenose dolphins gather to play in the tidal rush of its Tiputa Pass.

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.

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

Dugong: the animal lives almost wholly on seagrass, which ties it to shallow coastal meadows like those inside this 1,500-km reef.

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 the Pacific island nation of Palau declared its waters the world's first what?

Shark sanctuary: Palau banned commercial shark fishing throughout its waters, and several other island and coastal nations soon followed its lead.

In 2017 Palau began placing an environmental pledge, a world first, in what?

Visitors' passports: the Palau Pledge is stamped in on arrival, and every visitor must sign it in front of an immigration officer before being allowed into the country.

Which US president first proclaimed Papahānaumokuākea a marine national monument?

George W. Bush proclaimed the monument in 2006; Barack Obama enlarged it more than fourfold in 2016, pushing its boundary out to the edge of the exclusive economic zone.

Israel's only coral reef is protected in the Coral Beach Nature Reserve, off which resort city?

Eilat: the reserve covers about 1.2 km of shore, and its corals stand up to heat so well that scientists see the northern Gulf of Aqaba as a possible refuge for reefs.