CAT 2025 Slot 2 · VARC · RC passage 2: Mexican Tetra Cavefish: Loss and Enhancement

The full passage and all 4 questions (Q9–Q12) from CAT 2025 Slot 2, with answers and solutions.

The passage below is accompanied by four questions. Based on the passage, choose the best answer for each question.

Time and again, whenever a population [of Mexican tetra fish] was swept into a cave and survived long enough for natural selection to have its way, the eyes disappeared. "But it's not that everything has been lost in cavefish . . . Many enhancements have also happened." . . . Studies have found that cave-dwelling fish can detect lower levels of amino acids than surface fish can. They also have more tastebuds and a higher density of sensitive cells alongside their bodies that let them sense water pressure and flow. . . .

Killing the processes that support the formation of the eye is quite literally what happens. Just like non-cave-dwelling members of the species, all cavefish embryos start making eyes. But after a few hours, cells in the developing eye start dying, until the entire structure has disappeared. [Developmental biologist Misty] Riddle thinks this apparent inefficiency may be unavoidable. "The early development of the brain and the eye are completely intertwined—they happen together," she says. That means the least disruptive way for eyelessness to evolve may be to start making an eye and then get rid of it. . . .

It's easy to see why cavefish would be at a disadvantage if they were to maintain expensive tissues they aren't using. Since relatively little lives or grows in their caves, the fish are likely surviving on a meager diet of mostly bat feces and organic waste that washes in during the rainy season. Researchers keeping cavefish in labs have discovered that, genetically, the creatures are exquisitely adapted to absorbing and storing nutrients. . . .

Fats can be toxic for tissues, [evolutionary physiologist Nicolas] Rohner explains, so they are stored in fat cells. "But when these cells get too big, they can burst, which is why we often see chronic inflammation in humans and other animals that have stored a lot of fat in their tissues." Yet a 2020 study by Rohner, Krishnan and their colleagues revealed that even very well-fed cavefish had fewer signs of inflammation in their fat tissues than surface fish do. Even in their sparse cave conditions, wild cavefish can sometimes get very fat, says Riddle. This is presumably because, whenever food ends up in the cave, the fish eat as much of it as possible, since there may be nothing else for a long time to come. Intriguingly, Riddle says, their fat is usually bright yellow, because of high levels of carotenoids, the substance in the carrots that your grandmother used to tell you were good for your…eyes.

"The first thing that came to our mind, of course, was that they were accumulating these because they don't have eyes," says Riddle. In this species, such ideas can be tested: Scientists can cross surface fish (with eyes) and cavefish (without eyes) and look at what their offspring are like. When that's done, Riddle says, researchers see no link between eye presence or size and the accumulation of carotenoids. Some eyeless cavefish had fat that was practically white, indicating lower carotenoid levels. Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating whenever food arrives.

Question 9 · RC Detail and Fact Based Questions

All of the following statements from the passage describe adaptation in Mexican tetra cavefish EXCEPT:

  1. A

    "Since relatively little lives or grows in their caves, the fish are likely surviving on a meager diet of mostly bat feces and organic waste that washes in during the rainy season."

  2. B

    "It's easy to see why cavefish would be at a disadvantage if they were to maintain expensive tissues they aren't using."

  3. C

    "Even in their sparse cave conditions, wild cavefish can sometimes get very fat, says Riddle."

  4. D

    "'But when these cells get too big, they can burst, which is why we often see chronic inflammation in humans and other animals that have stored a lot of fat in their tissues.'"

Pick an option, then check.
Answer & solution

Answer: Option D

The test is simple: three of these describe something about the cavefish specifically, and one does not.

Option 1 describes how the cavefish survive on a meagre diet of bat feces and washed-in organic waste – a feature of how this fish lives in its cave.

Option 2 explains why the cavefish would be disadvantaged by keeping unused tissue, which is the reasoning behind losing the eyes – adaptation by subtraction.

Option 3 reports that wild cavefish can get very fat despite sparse conditions, tied to their gorging whenever food arrives – again a feature of this fish.

Option 4 is a general statement of biology: fat cells burst when too big, 'which is why we often see chronic inflammation in humans and other animals'. It names humans and other animals, not the cavefish. It is background that sets up the contrast in the very next sentence, where cavefish are shown to have fewer signs of inflammation.

Shortcut: scan each quoted line for its subject. Three have 'the fish' or 'cavefish' as subject; one has 'humans and other animals'. In an EXCEPT question of this kind the odd subject is the answer.

Hence, the answer is option 4.

Question 10 · RC Inference Questions

Which one of the following best explains why the "apparent inefficiency" is "unavoidable"?
  1. A
    Mexican tetra cavefish are similar to non-cave-dwelling variants in their early stages of development.
  2. B
    The inefficiency resulting from eyelessness is compensated by enhancements like more tastebuds in Mexican tetra cavefish.
  3. C
    The lack of light in the caves kills the eye cells in the developing Mexican tetra cavefish embryo.
  4. D
    The caves have poor and inconsistent availability of food and nutrition for Mexican tetra cavefish.
Pick an option, then check.
Answer & solution

Answer: Option A

Locate the inefficiency first. It is that cavefish embryos build an eye and then destroy it – wasted effort, on the face of it.

Now locate the reason the passage gives. Riddle says 'The early development of the brain and the eye are completely intertwined – they happen together', so 'the least disruptive way for eyelessness to evolve may be to start making an eye and then get rid of it'. Because brain and eye development cannot be separated, the cavefish embryo has to begin exactly as a surface fish embryo does.

Option 1 states that shared early development – cavefish are like their non-cave-dwelling relatives in the early stages. That is the reason the waste is unavoidable.

Option 2 is about compensation through tastebuds and other enhancements. That is paragraph 1's point about gains, and it explains nothing about why the eye must be built before being destroyed.

Option 3 claims darkness kills the eye cells. The passage describes a developmental programme in the embryo, not an environmental effect of light; the cell death happens 'after a few hours' of development.

Option 4 is about food scarcity. That explains why keeping an eye is costly – paragraph 3's point – but not why the eye must be started at all.

Shortcut: the word 'unavoidable' asks for a constraint, not a benefit. Options describing advantages (2), environment (3) or cost (4) answer a different question; only one option names a constraint.

Hence, the answer is option 1.

Question 11 · Strengthen and Weaken Arguments

Which one of the following results for the cross between surface fish (with eyes) and cavefish (without eyes) would invalidate Riddle's inference from the experiment?
  1. A
    Only eyeless offspring had yellow fat.
  2. B
    Some offspring with eyes had white fat.
  3. C
    Some eyeless offspring had white fat.
  4. D
    Some offspring with eyes had yellow fat.
Pick an option, then check.
Answer & solution

Answer: Option A

State Riddle's inference exactly: researchers 'see no link between eye presence or size and the accumulation of carotenoids'. To invalidate it, a result must establish precisely such a link.

Fix the colour code the passage supplies: high carotenoids give bright yellow fat, low carotenoids give practically white fat.

Option 1 – only eyeless offspring had yellow fat – means every high-carotenoid fish lacks eyes and every fish with eyes lacks carotenoids. That is a perfect correspondence between eyelessness and carotenoid accumulation, which is exactly the link Riddle denies. It invalidates the inference.

Option 2, some offspring with eyes had white fat, is consistent with no link – having eyes does not rule out low carotenoids.

Option 3, some eyeless offspring had white fat, is in fact what the passage already reports: 'Some eyeless cavefish had fat that was practically white'. Far from invalidating, it supports the inference.

Option 4, some offspring with eyes had yellow fat, also fits no link – eyed fish can have high carotenoids too.

Shortcut: a 'no correlation' claim is broken only by an option containing an absolute quantifier. Three options here begin with 'Some', which can never establish a correlation; only one begins with 'Only'.

Hence, the answer is option 1.

Question 12 · RC Inference Questions

On the basis of the information in the passage, what is the most likely function of carotenoids in Mexican tetra cavefish?
  1. A
    To help the fat cells store nutrients.
  2. B
    To render bright yellow colour to the cavefish.
  3. C
    To control inflammation from the bursting of fat cells.
  4. D
    To act as a substitute for eyes.
Pick an option, then check.
Answer & solution

Answer: Option C

The passage closes on exactly this point: 'Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating whenever food arrives.'

Link that to paragraph 4, which explains where the inflammation comes from – fat cells that get too big and burst. Option 3 puts the two together: carotenoids control the inflammation caused by bursting fat cells. That is the function the passage settles on.

Option 1 confuses two separate findings. The fish are 'exquisitely adapted to absorbing and storing nutrients', but that is a genetic adaptation reported in paragraph 3 and is not attributed to carotenoids.

Option 2 mistakes an effect for a function. The yellow colour is how the researchers noticed the carotenoids; colouring the fat is not what the carotenoids are for.

Option 4 is the hypothesis the passage explicitly tests and discards. It was 'the first thing that came to our mind', and the cross-breeding experiment showed no link between eyes and carotenoids.

Shortcut: when a passage narrates a hypothesis, a test and a conclusion, the answer to a 'most likely function' question is the sentence after 'Instead' or 'However' – that is where the surviving explanation sits.

Hence, the answer is option 3.

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