Easy
Oceans & Marine Life
Tides, Depths & Reefs
The physics of the sea and the life that adapted to it.
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What is the deepest known point in the ocean?
Coral Reefs and the Shallow Sea
A coral reef is a limestone structure built by animals that farm algae inside their own tissue. These ten questions cover how that arrangement works and what happens when it breaks down.
About this topic
Reefs occupy a tiny fraction of the sea floor and hold a large share of its species, which is odd given that tropical surface water is famously short of nutrients. The trick is that reef corals do not really depend on the water around them: they house algae in their tissue and take most of their food directly from them. That partnership explains almost everything about where reefs grow — shallow, clear, warm and well lit — and why they are so sensitive when conditions shift. The questions below cover the building materials, the partnership itself, and a few of the animals whose day jobs shape the reef around them.
Questions & answers
Coral Reefs and the Shallow Sea: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1The single-celled algae that live inside coral tissue are known as what?
- A Foraminifera
- B Zooxanthellae Correct
- C Diatoms
- D Cyanobacteria
Answer: B. Zooxanthellae
Why: Zooxanthellae photosynthesise inside the coral and hand over most of the sugar they produce. Because they need light, reef-building corals are restricted to clear, shallow water.
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2What happens to a coral during a bleaching event?
- A It expels the algae living in its tissue, usually after a spell of unusually warm water Correct
- B Its skeleton dissolves and the colony collapses into rubble
- C A fungus spreads across the colony and kills it outright
- D Sediment settles over it and smothers the polyps
Answer: A. It expels the algae living in its tissue, usually after a spell of unusually warm water
Why: The white appearance is the bare limestone skeleton showing through tissue that has lost its colour along with its algae. A bleached coral is not dead and can take its algae back if conditions return to normal reasonably quickly.
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3What is a fringing reef?
- A A ring of reef enclosing a lagoon with no island in the middle
- B A reef separated from the land by a wide, deep lagoon
- C A reef growing on a submerged seamount far out at sea
- D A reef growing directly against the shoreline with little or no lagoon behind it Correct
Answer: D. A reef growing directly against the shoreline with little or no lagoon behind it
Why: Fringing reefs are the commonest type by a wide margin and the easiest to reach from shore. Sitting right against the land also makes them the most exposed to runoff, sediment and pollution.
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4Coral polyps build their skeletons out of which mineral?
- A Chitin
- B Calcium phosphate
- C Calcium carbonate Correct
- D Silica
Answer: C. Calcium carbonate
Why: Each polyp secretes a small carbonate cup beneath itself, and a reef is those cups piled up over thousands of years. It is the same mineral that makes up limestone and most seashells.
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5Much of the white sand on a tropical beach has passed through which animal?
- A The sea cucumber
- B The parrotfish Correct
- C The sea urchin
- D The hermit crab
Answer: B. The parrotfish
Why: Parrotfish bite chunks off the reef to reach the algae growing in it, grind the rock in a set of throat teeth, and pass the remains out as fine carbonate sand. A single large fish can produce a few hundred kilograms of it in a year.
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6How does a clownfish avoid being stung by the anemone it lives in?
- A A thick mucus coat that stops the anemone's stinging cells from firing Correct
- B It is immune to every kind of venom
- C It bites off the tentacles that touch it
- D It enters only at night, when the tentacles are withdrawn
Answer: A. A thick mucus coat that stops the anemone's stinging cells from firing
Why: The mucus keeps the anemone from recognising the fish as prey, and in return the fish gets a shelter almost nothing else will enter. All clownfish start out male, and when the resident female dies the dominant male changes sex to replace her.
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7Which reef animal eats living coral and periodically appears in destructive outbreaks?
- A The blue-ringed octopus
- B The moray eel
- C The giant clam
- D The crown-of-thorns starfish Correct
Answer: D. The crown-of-thorns starfish
Why: It feeds by pushing its stomach out through its mouth and digesting the coral where it sits. In outbreak years the starfish can strip large stretches of reef faster than the coral can regrow.
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8Mass coral spawning across a reef is cued mainly by which combination?
- A A sudden drop in salinity after a storm
- B Day length acting on its own
- C The phase of the moon together with water temperature Correct
- D The arrival of the monsoon rains
Answer: C. The phase of the moon together with water temperature
Why: Corals release eggs and sperm into the water on only a few nights a year, and neighbouring colonies do it within minutes of each other. Flooding the water all at once means predators cannot eat more than a fraction of what is released.
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9Why do mangrove forests matter to a nearby reef?
- A They keep the surrounding water warm enough for coral growth
- B They trap sediment coming off the land and shelter young reef fish Correct
- C They supply the fresh water corals need to grow
- D They provide the calcium that corals build their skeletons from
Answer: B. They trap sediment coming off the land and shelter young reef fish
Why: Tangled mangrove roots hold silt in place that would otherwise cloud the water over the reef, and the shallows between them make a nursery too awkward for large predators. The same root network takes much of the force out of a storm surge.
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10Reef-building corals belong to the same animal group as which of these?
- A Jellyfish and sea anemones Correct
- B Sponges
- C Starfish and sea urchins
- D Clams and other molluscs
Answer: A. Jellyfish and sea anemones
Why: Corals are cnidarians, and like their relatives they carry stinging cells on their tentacles. A single coral polyp is essentially a small anemone sitting in a limestone cup.
Frequently asked questions about Coral Reefs and the Shallow Sea
How many questions are in Coral Reefs and the Shallow Sea?
Coral Reefs and the Shallow Sea has 10 questions and takes about 7 minutes to finish. It is rated Medium and sits in our Oceans & Marine Life category.
How is Coral Reefs and the Shallow Sea scored?
Each of the 10 correct answers adds 10 XP, up to 100 XP for a clean run. Wrong answers cost you nothing, and your full question-by-question review is shown at the end.
Are the answers to Coral Reefs and the Shallow Sea explained?
Yes. All 10 questions carry a written explanation of why the correct answer is right, so the quiz works as a study sheet as well as a test.
Can I replay Coral Reefs and the Shallow Sea?
Yes, as often as you like. The questions and the answer options are reshuffled on each run, and you can use Practice Mistakes afterwards to retry only the ones you missed. No account is needed to play.
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