MPSC 2022 Prelims Paper-I · Question 17 of 100

Correct — option (4). All four statements describe properties that coral reefs are ordinarily credited with, so the row that accepts every one of them is the answer. Statement (a) calls them the oldest living communities on earth; reef-building organisms have been constructing limestone frameworks in warm shallow seas since the Palaeozoic, and a coral reef is regularly described in this way because the community, if not any particular reef, has persisted through immense stretches of geological time. Statement (b) calls the reef a high biomass producer, and that is one of its defining features: gross primary productivity on a reef is among the highest measured for any ecosystem, comparable with a tropical rain forest, and the reason is the partnership at the centre of the system, in which the coral polyp houses microscopic algae called zooxanthellae within its own tissues and receives from their photosynthesis the greater part of the food it needs. Statement (c), printed in broken English as a rich floral and fauna diversity, asserts the reef's biological richness, and the standard figure makes the point: coral reefs occupy far less than one per cent of the ocean floor and yet shelter something like a quarter of all marine species, which is why they are called the rain forests of the sea. Statement (d) calls them shallow water tropical ecosystems, and that follows directly from the partnership described above: because the zooxanthellae must photosynthesise, reef-building corals are confined to clear, sunlit water, generally shallower than about fifty metres, with temperatures between roughly twenty and thirty degrees, normal sea salinity and a firm foundation, which restricts them to the tropical belt. With no statement failing, option (4) is the answer.

• (1)Only (a) and (b) — This row accepts the first two statements and drops the last two, and both of the dropped statements are sound. The richness of reef life is the property for which reefs are best known — a quarter of all marine species on a fraction of one per cent of the sea floor — and their restriction to warm, shallow, clear water is the condition that makes reef formation possible at all. Candidates arrive at a row like this by a particular route: they verify statements until they meet one they are unsure of and then stop, taking the row that covers what they have checked. In a question of this shape that is a losing method, because the rows are constructed so that every partial selection is available. Each statement has to be judged, and only then can a row be chosen.
• (2)Only (a), (b) and (d) — This row accepts the first, second and fourth statements and excludes only the third, the one asserting rich floral and faunal diversity. It is the most dangerous row in the question, because the exclusion looks deliberate and the statement excluded is the one printed in the worst English — a rich floral and fauna diversity is not a sentence a textbook would write, and a candidate who has been trained to distrust oddly worded options will suspect it. The wording is a defect of the printing, not of the fact. Coral reefs are among the most diverse communities in the sea, supporting fish, molluscs, crustaceans, echinoderms, sponges, turtles and algae in enormous variety, which is exactly why they are compared with tropical rain forests. Judging a statement by its grammar rather than by its content is how this row collects its victims.
• (3)Only (c) — This row accepts only the third statement and rejects the other three, which requires a candidate to believe that reefs are neither ancient, nor highly productive, nor confined to shallow tropical water. Each of those rejections is wrong, and the last is the most clearly so, since the dependence of the coral polyp on its symbiotic algae ties reef growth to sunlight and therefore to shallow, clear water in the warm belt of the oceans; that is why the world's reefs lie in a band on either side of the equator and why the deep and the cold seas have none of this kind. The row is chosen by candidates who recognise the diversity of reefs as the one thing they are certain about and treat certainty on one point as a reason to reject everything else. Confidence in one statement is not evidence against the others.

A coral reef is built by colonies of tiny animals, the coral polyps, which secrete skeletons of calcium carbonate and grow one upon the other until a limestone framework accumulates. The reef-building or hermatypic corals live in symbiosis with single-celled algae, the zooxanthellae, which live inside the polyp's tissues, photosynthesise, and pass much of what they make to their host while using the host's waste products in return. That partnership explains almost every feature of reefs. It explains their extraordinary productivity in seas that are otherwise nutrient-poor, because nutrients are recycled within the system instead of being lost; it explains their confinement to clear, shallow, sunlit water, since the algae need light; and it explains their vulnerability, because when the water is too warm for too long the polyps expel their algae, lose both their colour and their principal food supply, and the reef bleaches and may die. The conditions reef corals require are warm water of about twenty to thirty degrees, depths generally less than fifty metres, clear water free of silt, normal salinity and a firm substrate. Reefs take three classic forms — the fringing reef attached to the shore, the barrier reef separated from it by a lagoon, and the atoll, a ring of reef enclosing a lagoon with no island in the middle — and Darwin explained the sequence by the slow subsidence of a volcanic island beneath an upward-growing reef. India's reefs lie in the Gulf of Kachchh, the Gulf of Mannar, the Lakshadweep atolls and the Andaman and Nicobar Islands.

Coral reefs are among the most heavily examined topics in the environment section, because a single reef question can test ecology, physical geography and conservation at once. MPSC sets them as statement questions of this kind, as questions about the conditions required for reef growth, and as questions about bleaching and its causes. This item is a good example of a difficulty this paper creates for itself: the English column is unevenly written, and statement (c) in particular reads as a rich floral and fauna diversity, which is not grammatical. A candidate who treats bad grammar as evidence of a planted error will reject the statement and choose the row that excludes it, which is precisely the row printed at option (2). The two judgments must be kept apart — whether a sentence is well made and whether what it says is true are different questions, and only the second is being examined. The other habit this question rewards is completeness: in a set of four statements with a full range of partial rows on offer, every statement must be judged before any row is read, since a partial verification will always find a row that matches it.

• Reef-building corals live in symbiosis with zooxanthellae, single-celled algae housed in the polyp's tissues, whose photosynthesis supplies most of the coral's food and drives the reef's high productivity.
• Reef corals require warm water of about twenty to thirty degrees, clear and shallow water generally less than fifty metres deep, normal salinity and a firm substrate, which confines reefs to the tropical belt.
• Coral reefs occupy far less than one per cent of the ocean floor but support roughly a quarter of all marine species, which is why they are called the rain forests of the sea.
• Coral bleaching occurs when prolonged warming causes the polyps to expel their zooxanthellae, so that the colony loses both its colour and its main food supply and may die if the stress continues.
• The three forms of reef are the fringing reef attached to the shore, the barrier reef separated from it by a lagoon, and the atoll enclosing a lagoon; India's reefs are in the Gulf of Kachchh, the Gulf of Mannar, Lakshadweep and the Andaman and Nicobar Islands.

• Rejecting a statement because it is badly written, when the grammar of a sentence and the truth of what it asserts are separate questions
• Verifying statements only until the first uncertain one and then choosing the row that covers what has been checked, when every partial row is on offer
• Supposing that reefs can grow at any depth, when the dependence of the symbiotic algae on sunlight confines reef corals to shallow, clear water
• Treating high productivity and high diversity as the same claim, when one concerns the rate at which the system fixes energy and the other the number of species it supports

Coral reefs reach MPSC papers as statement sets of exactly this shape, as one-line questions on the conditions required for reef growth or on the meaning of bleaching, and as location questions asking where India's reefs are found. The Commission likes the topic because it joins the environment syllabus to current affairs through climate change, and because the symbiosis at the centre of it explains so many separate facts that a single well-set question can test whether the candidate has understood the mechanism or merely collected the descriptions. Variants ask what zooxanthellae are, why reefs are called the rain forests of the sea, or which reef type an atoll represents. A short note on the symbiosis, the four growth conditions, the three reef forms and the Indian reef locations covers all of them.

No directly related past PYQ was found.

• practice — not a real PYQ

  Coral bleaching takes place when

  • (a)the polyps expel their symbiotic zooxanthellae under prolonged thermal stress
  • (b)silt from rivers settles on the reef and covers it
  • (c)the salinity of the surrounding sea water falls below normal
  • (d)fishing removes the larger predatory fish from the reef

  Answer(a) The polyps expel their symbiotic zooxanthellae under prolonged thermal stress — the colony then loses both the colour the algae give it and the food their photosynthesis supplies, and it dies if the stress continues long enough. Silt, reduced salinity and overfishing all damage reefs in other ways, but bleaching is specifically the loss of the algal partner, which is why it is so closely tied to warming seas.

• practice — not a real PYQ

  Which of the following best explains why reef-building corals are confined to shallow water ?

  • (a)The pressure of deep water crushes the coral skeleton
  • (b)Their symbiotic algae need sunlight for photosynthesis
  • (c)Deep water is too saline for coral polyps
  • (d)Coral larvae cannot swim below fifty metres

  Answer(b) Their symbiotic algae need sunlight for photosynthesis — the zooxanthellae living within the polyp's tissues supply most of the coral's food, so reef growth is limited to depths that light can reach, generally less than about fifty metres of clear water. This single requirement also explains why reefs need water free of silt, since suspended sediment cuts off the light as effectively as depth does.