LSAT 159 | Section 3 | Logical Reasoning: Q6

LR Question 6 Explanation

QUESTION TEXT: A tiny crab that lives 550 meters below the surface …

QUESTION TYPE: Weaken

CONCLUSION: The crab probably hunts bioluminescent prey.

REASONING:

1. UV sensitivity at that depth is for something other than sunlight.
2. It’s probably to detect light from bioluminescent prey.
3. The crab isn’t bioluminescent, so it’s probably for prey.

ANALYSIS: The key assumption is that if the crab detects bioluminescent organisms, it’s to hunt them.

To weaken that assumption, we want a reason why UV sensitivity could be for something else. Maybe the crab needs to detect bioluminescent organisms to avoid predators, instead of to hunt prey. Or maybe this ability is random and doesn’t have any specific purpose.

___________

1. This proves that the trait is functional. But it doesn’t challenge the key assumption: what the trait is actually used for.
2. If anything, this could support the idea that any sensory edge (like detecting bioluminescent prey) is useful to hunting prey. It doesn’t give an alternative function for UV sensitivity.
3. This just says they don’t need UV to find each other. But the argument never claimed that’s what they use UV for.
4. This is irrelevant. We don’t care how long the adaptation takes, we’re concerned with what it’s used for.
5. CORRECT. This is a strong alternative function of the trait. If UV sensitivity is used to help the crab detect and avoid bioluminescent predators, it weakens the idea that the function of the trait is to hunt prey.