---
title: Coral reef builders: polyps, bleaching, sand from parrotfish and gia… — how Mova does on the tests · Biology
description: Lesson 172 of the biology course of Mova's primary school: 9 of 11 tests right, exam 2 of 4, every question with the options, the student's choice, the right answer and why it chose.
---

<!-- md -->
[Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/school-run/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/)

<!-- crumbs -->
[home](https://md.mova.elamur.ai/free/en/) / [at school](https://md.mova.elamur.ai/free/en/school-run/) / [Biology](https://md.mova.elamur.ai/free/en/school-run/biology/) / 172

# Coral reef builders: polyps, bleaching, sand from parrotfish and giant clams

<!-- .vintro -->
**Tests:** 9 of 11 right · **Exam:** 2 of 4 right · **Kind:** ★ · **Understood:** 8 of 15 · **Run:** 2026-10-18, commit `f3c23ea72`

<!-- .vabout -->
Lesson 172 of the biology course of Mova's primary school, as Mova's small model took its tests: it had read this lesson and the ones before it and learned from their exercises. Each test shows the options in the order the student saw them, its choice, the right answer, the score of every option and why it chose. The lesson itself, sentence by sentence with trees and pragmatics: [the lesson in the corpora](https://md.mova.elamur.ai/free/en/corpora/school/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/).

<!-- .vabout -->
Every question of this lesson and each of its options parsed by Mova, with trees and pragmatics: [the lesson's questions in the corpora](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/).

<!-- div qa ok -->

### Test 1 · ★ · right ✓ {#t1}

<!-- .q -->
«diver here, touched a coral, feels like a rock so it's a rock right» What is right?

- **1.** No, it is a colony of tiny animals with limestone skeletons — **← Mova's choice** · **✓ the right answer** · score 7
- **2.** No, it is a plant like a tree, with roots, a trunk and branches of stone — no support
- **3.** Yes, rocks that grow flowers — no support
- **4.** Yes, corals are just rocks — no support

<!-- .why -->
**Why:** It took the question to ask which statement is true. It chose option 1 with score 7, the only option with support. Its support: lesson 172, line 5: «Corals are animals, not plants or rocks: colonies of tiny polyps that build hard skeletons of limestone and at night stretch out tentacles to catch tiny food.», reached through the option's own match among the sentences about the question's things. What matched: the same thing acted on «animals» (+3); the same circumstance (+2); the same light verb (be, have, do, get, go) (+2). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t1/)

<!-- /div -->

<!-- div qa no -->

### Test 2 · ★ · wrong ✗ {#t2}

<!-- .q -->
In SpongeBob, Patrick the starfish lives happily on a reef. Which real starfish is a danger to coral reefs?

- **1.** Every kind of starfish eats coral, all the time — **← Mova's choice** · score 5
- **2.** The crown-of-thorns starfish, which eats living coral — **✓ the right answer** · score 5
- **3.** Starfish eat only sponges, like SpongeBob's friend Patrick does — score 1
- **4.** Starfish never eat anything — score -1

<!-- .why -->
**Why:** It took the question to ask which statement is true. It chose option 1 with score 5, tied with another option at 5 (the first shown, or the one claiming less beyond what it knows, wins). Its support: lesson 172, line 14: «Crown-of-thorns starfish eat living coral and sometimes come in huge numbers; giant triton snails eat them, and divers remove them to help reefs.», reached through the option's own match among the sentences about the question's things. What matched: the same thing acted on «living coral» (+3); the same action «eat» (+2). The right option, 2, scored 5. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t2/)

<!-- /div -->

<!-- div qa no -->

### Test 3 · ★ · wrong ✗ {#t3}

<!-- .q -->
What would happen if the sea around a reef stayed too hot for many weeks?

- **1.** The reef would turn into sand at once — **← Mova's choice** · no support
- **2.** The corals would bleach and might starve — **✓ the right answer** · no support
- **3.** Nothing, corals love very hot water — no support
- **4.** The corals would grow faster than ever — no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t3/)

<!-- /div -->

<!-- div qa ok -->

### Test 4 · ★ · right ✓ {#t4}

<!-- .q -->
«my uncle says giant clams snap shut and drown divers, legit?» What is right?

- **1.** No, that is a myth: they close slowly and usually not fully — **← Mova's choice** · **✓ the right answer** · no support
- **2.** Yes, it happens every week, ask any diver on the reef — no support
- **3.** No, giant clams cannot close at all — no support
- **4.** Yes, they bite like sharks — no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t4/)

<!-- /div -->

<!-- div qa ok -->

### Test 5 · ★ · right ✓ {#t5}

<!-- .q -->
«sat on a white beach in the Maldives, someone said it's fish poo lol» Could that be right?

- **1.** No, sand is made from salt — no support
- **2.** No, all sand comes from mountains, washed down by rivers to the sea — no support
- **3.** Yes, much of the sand there comes from parrotfish grinding up coral — **← Mova's choice** · **✓ the right answer** · score 2
- **4.** Yes, but only from whale poo — no support

<!-- .why -->
**Why:** It took the question to ask which statement is true. It chose option 3 with score 2, the only option with support. Its support: lesson 172, line 4: «Walk on a white tropical beach and much of the sand under your feet may have passed through a parrotfish, which crunches coral with its beak and poops out fine…», reached through the same lesson line as the sentence the question matched. What matched: the same doer «parrotfish» (+2); the same thing acted on «coral» (+1). Against it: a different action (counts against) (-1). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t5/)

<!-- /div -->

<!-- div qa ok -->

### Test 6 · ★ · right ✓ {#t6}

<!-- .q -->
Why would a reef not grow in deep, dark water off a tropical island?

- **1.** Corals hate salt water — score 0
- **2.** The water there is too warm — score 1
- **3.** Fish eat all the corals there, so none are left to grow — no support
- **4.** The algae inside the polyps need sunlight to make food — **← Mova's choice** · **✓ the right answer** · score 6

<!-- .why -->
**Why:** It took the question to ask why something happens (its cause). It chose option 4 with score 6, against 1 for the next best. Its support: lesson 172, line 6: «Coral reefs grow in warm, clear, shallow water, where the tiny algae living inside the polyps get enough sunlight to make food.», reached through the same lesson line as the sentence the question matched. What matched: the same action «make» (+4); the same doer «tiny algae» (+2). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t6/)

<!-- /div -->

<!-- div qa ok -->

### Test 7 · ★ · right ✓ {#t7}

<!-- .q -->
«3 parrotfish asleep on the reef, each makes a new bubble every night. how many bubbles over 2 nights»

- **1.** 6 — **← Mova's choice** · **✓ the right answer** · no support
- **2.** 2 — no support
- **3.** 3 — no support
- **4.** 5 — no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t7/)

<!-- /div -->

<!-- div qa ok -->

### Test 8 · ★ · right ✓ {#t8}

<!-- .q -->
A diver sees the sea turn pink with tiny floating bundles one night after a full moon. What is happening?

- **1.** The corals are spawning all at once — **← Mova's choice** · **✓ the right answer** · no support
- **2.** The reef is bleaching — no support
- **3.** Fish are laying their eggs all over the sand — no support
- **4.** A ship spilled paint — no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t8/)

<!-- /div -->

<!-- div qa ok -->

### Test 9 · ★ · right ✓ {#t9}

<!-- .q -->
«reefs are tiny, who cares if they die» What is a good answer?

- **1.** They are huge and cover the whole ocean, from coast to coast — score 5
- **2.** Reefs only matter to tourists who come to dive — score 2
- **3.** Nothing lives on reefs anyway — no support
- **4.** They cover under one percent of the sea floor but hold a huge share of sea life — **← Mova's choice** · **✓ the right answer** · score 6

<!-- .why -->
**Why:** It took the question to ask who or what does it. It chose option 4 with score 6, against 5 for the next best. Its support: lesson 172, line 15: «Coral reefs cover less than one percent of the ocean floor but are home to a huge share of sea life, and they protect coasts from waves like natural walls.», reached through the option's own match among the sentences about the question's things. What matched: the same action «cover» (+3); it fills the role the question asks for (+2); the same doer «coral reefs» (+1). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t9/)

<!-- /div -->

<!-- div qa ok -->

### Test 10 · ★ · right ✓ {#t10}

<!-- .q -->
«corals only live in hot seas, everyone knows that» What is right?

- **1.** No, they only live in rivers — score 2
- **2.** Yes, only in the Red Sea — no support
- **3.** Yes, always — no support
- **4.** No, some reefs grow in cold, dark water deep down, for example off Norway — **← Mova's choice** · **✓ the right answer** · score 15

<!-- .why -->
**Why:** It took the question to ask which statement is true. It chose option 4 with score 15, against 2 for the next best. Its support: lesson 172, line 16: «Not all corals live in warm water: some reefs grow in cold, dark water deep down, for example off Norway, and some are thousands of years old.», reached through the same lesson line as the sentence the question matched. What matched: the same action «grow» (+4); the same place «cold dark water» (+3); the same property (+3); the same circumstance (+3). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t10/)

<!-- /div -->

<!-- div qa ok -->

### Test 11 · ★ · right ✓ {#t11}

<!-- .q -->
Divers pull hundreds of crown-of-thorns starfish off a reef. Why?

- **1.** The starfish block out the sunlight that the reef fish need to see — no support
- **2.** The starfish come in huge numbers and eat the living coral — **← Mova's choice** · **✓ the right answer** · score 10
- **3.** They want to eat them for dinner — score 6
- **4.** Starfish are endangered and need saving by divers — score 3

<!-- .why -->
**Why:** It took the question to ask why something happens (its cause). It chose option 2 with score 10, against 6 for the next best. Its support: lesson 172, line 14: «Crown-of-thorns starfish eat living coral and sometimes come in huge numbers; giant triton snails eat them, and divers remove them to help reefs.», reached through the same lesson line as the sentence the question matched. What matched: the same action «come» (+4); the same place «huge numbers» (+4); the same doer «crown starfish» (+2). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/t11/)

<!-- /div -->

<!-- div qa no -->

### Exam question 1 · formal language · wrong ✗ {#e1}

<!-- .q -->
Which of the following statements most accurately characterises the biological nature of a coral?

- **1.** It is a single giant organism of the seaweed family — **← Mova's choice** · no support
- **2.** It is a species of marine plant — no support
- **3.** It is a mineral formation devoid of life — no support
- **4.** It is a colony of minute animals, termed polyps, which secrete skeletons of limestone — **✓ the right answer** · no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/e1/)

<!-- /div -->

<!-- div qa no -->

### Exam question 2 · formal language · wrong ✗ {#e2}

<!-- .q -->
Identify the principal cause of the phenomenon designated coral bleaching.

- **1.** The consumption of coral by parrotfish — **← Mova's choice** · no support
- **2.** The deposition of salt crystals upon the reef — no support
- **3.** An excess of sunlight in water of moderate temperature during the winter months — no support
- **4.** The expulsion of symbiotic algae consequent upon excessive water temperature — **✓ the right answer** · no support

<!-- .why -->
**Why:** No option found support in what it remembers, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/e2/)

<!-- /div -->

<!-- div qa ok -->

### Exam question 3 · formal language · right ✓ {#e3}

<!-- .q -->
Select the response consistent with the established facts concerning the giant clam.

- **1.** The assertion that it entraps the limbs of divers is unfounded, since it closes slowly and incompletely — **← Mova's choice** · **✓ the right answer** · score 7
- **2.** It weighs not more than one kilogram — score 5
- **3.** It is known to seize and drown divers with regularity — no support
- **4.** It possesses no shell whatsoever — no support

<!-- .why -->
**Why:** It took the question to ask which statement is true. It chose option 1 with score 7, against 5 for the next best. Its support: lesson 172, line 10: «The giant clam is the biggest clam in the world and can weigh more than two hundred kilos; the story of giant clams trapping divers' legs is a myth, because th…», reached through the same lesson line as the sentence the question matched. What matched: the same action «close» (+4); the same property (+3). · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/e3/)

<!-- /div -->

<!-- div qa ok -->

### Exam question 4 · formal language · right ✓ {#e4}

<!-- .q -->
Ascertain the origin of a considerable proportion of the white sand upon certain tropical beaches.

- **1.** Fragments of coral ground and excreted by parrotfish — **← Mova's choice** · **✓ the right answer** · no support
- **2.** Particles of salt deposited by evaporation of the sea — no support
- **3.** Shells of the giant clam crushed by waves — no support
- **4.** Pulverised volcanic glass of black colour — no support

<!-- .why -->
**Why:** Nothing it remembers matched the question, so it guessed: it took the first option shown. · [the question and its options parsed, with trees and pragmatics](https://md.mova.elamur.ai/free/en/corpora/questions/biology/172-coral-reef-builders-polyps-bleaching-sand-from-parrotfish-and-giant-clams/e4/)

<!-- /div -->

<!-- pn -->
- ← previous: [171. The Southern Ocean: blue whales, colossal squid…](https://md.mova.elamur.ai/free/en/school-run/biology/171-the-southern-ocean-blue-whales-colossal-squid-and-life-under-the-ice/)
- next →: [173. Coral reef partners and tricksters: clownfish, c…](https://md.mova.elamur.ai/free/en/school-run/biology/173-coral-reef-partners-and-tricksters-clownfish-cleaners-mimics-and-puffers/)
- ↑ back to [Biology](https://md.mova.elamur.ai/free/en/school-run/biology/)