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97 free Coral Reefs trivia questions with answers — nature quiz, new questions added Aug 2026.
10 free coral reefs trivia questions with answers. Dive deep into the colorful world of coral reefs with our engaging trivia quiz! Test your knowledge about these vital ocean ecosystems, their inhabitants, and why they matter to our planet. Whether you're a marine biology enthusiast or just curious about the underwater world, this quiz will challenge what you know.
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Q 01What primary mineral compound forms the rigid structure of most coral reefs?
Calcium carbonate
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.
Q 02To which phylum of animals do corals belong?
Cnidaria
Corals are invertebrates classified under the phylum Cnidaria, a group that also includes jellyfish and sea anemones.
Q 03What is the name of the microscopic algae that live symbiotically within coral tissues?
Zooxanthellae
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.
Q 04Which is the largest coral reef system in the world?
Great Barrier Reef
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.
Q 05What is it called when stressed corals expel their symbiotic algae and lose their colour?
Coral bleaching
Coral bleaching is the loss of symbiotic algae and their pigments; the coral turns white and, if conditions do not improve, often starves.
Q 06Which broad ocean region holds the most diverse coral reefs, including the Coral Triangle?
Indo-Pacific
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.
Q 07About what percentage of marine species depend on coral reefs?
25%
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.
Q 08Atmospheric carbon dioxide dissolving into seawater directly causes which major threat to coral reefs?
Ocean acidification
When excess carbon dioxide from the atmosphere dissolves into seawater, it forms carbonic acid.
Q 09Coral reefs cover roughly what share of the world's ocean floor?
About 0.1%
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.
Q 10Charles Darwin conceived his theory that atolls form around sinking volcanic islands during the voyage of which ship?
HMS Beagle
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.
Q 11In Darwin's model, what is the first reef stage, forming right against a volcanic island?
Fringing
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.
Q 12The Great Barrier Reef, which formed on a continental shelf rather than round a volcano, is roughly how old?
About 20,000 years
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.
Q 13The Great Barrier Reef lies off the coast of which Australian state?
Queensland
Q 21Which body of water is the world's northernmost tropical sea, with reefs among the healthiest anywhere?
The Red Sea
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.
Q 22How many nuclear devices did the United States detonate at Bikini Atoll between 1946 and 1958?
23
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.
Q 23Australia's Ningaloo Reef is famous for seasonal visits from which giant fish?
Whale sharks
Queensland: the reef runs about 2,300 km along the state's coast from the tip of Cape York south to near Bundaberg.
Q 14Which explorer's ship ran aground on the Great Barrier Reef in June 1770, needing seven weeks of repairs?
James Cook
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.
Q 15The Coral Triangle holds more than what share of the world's shallow-water reef-building coral species?
76%
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.
Q 16The Belize Barrier Reef is a 300-km section of which larger 900-km system, the world's second largest?
Mesoamerican
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'.
Q 17Which explorer's 1971 expedition to Belize's Great Blue Hole made it famous?
Jacques Cousteau
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.
Q 18How does the crown-of-thorns starfish feed on living coral?
By everting its stomach over the 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.
Q 19Which reef fish grinds up coral, digests the edible parts and excretes the rest as sand that builds beaches?
Parrotfish
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.
Q 20In a clownfish group, what happens when the dominant female disappears?
The breeding male changes sex
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.
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.
Q 24Raja Ampat, home to about 75% of the world's known hard coral species, is in which country?
Indonesia
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.
Q 25Tubbataha Reefs Natural Park, a World Heritage atoll reef in the Sulu Sea, belongs to which country?
Philippines
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.
Q 26The continental United States' only living barrier reef lies a few miles seaward of which island chain?
The Florida Keys
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.
Q 27Which country's continental shelf holds the world's largest known cold-water coral reef?
Norway
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.
Q 28Fire coral, which painfully stings divers, is not a true coral but is closest to what?
Hydra
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.
Q 29The largest brain corals can live for roughly how long?
900 years
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.
Q 30Which disease drove the Caribbean staghorn coral's collapse and its critically endangered listing?
White-band disease
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.