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Difficulty: MediumWeaken

Marine Biologist: Deep-sea glass squid possess photophores on their undersides that emit bioluminescence matching the wavelength and intensity of downwelling sunlight. In laboratory trials, whenever overhead light levels fluctuated, the squid immediately adjusted their ventral light emission to match. Because this counterillumination eliminates the squid's dark silhouette against surface waters, the squid must have evolved this adjustable bioluminescence primarily to evade predators that hunt by looking upward.

Which of the following, if true, most seriously weakens the marine biologist's argument?

  1. A
    In laboratory conditions using artificial lighting, ventral photophores fail to replicate the exact spectral distribution of sunlight penetrating ocean waters.
  2. B
    Deep-sea predatory fish that regularly feed on glass squid possess specialized tubular eyes oriented almost exclusively toward the ocean surface.
  3. The primary prey of deep-sea glass squid consists of small fish whose upward-pointing eyes detect overhead silhouettes, prompting them to flee before the squid can capture them.Answer
  4. D
    All marine organisms residing at ocean depths below the maximum reach of downwelling sunlight lack ventral photophores entirely.
  5. E
    Several related species of shallow-water cephalopods rely on dynamic chromatophore pigmentation rather than bioluminescence for camouflage.

Answer

The finding that the squid's primary prey possesses upward-pointing eyes and flees upon detecting overhead silhouettes.
The argument concludes that adjustable counterillumination evolved primarily for defense against upward-looking predators based solely on the fact that it eliminates the squid's downward silhouette. The correct answer weakens this conclusion by presenting an alternative explanation: the squid's prey also looks upward to detect silhouettes and escape. If eliminating the silhouette is essential for successfully catching food, the trait may have evolved primarily for predation rather than defense.

Step-by-Step Solution

1
Identify the argument's premises and main conclusion
Premises establish that glass squid adjust ventral bioluminescence to match downward sunlight and eliminate their silhouette. Conclusion asserts that this trait evolved primarily to evade predators hunting from below.
Deconstructing the argument isolates the core causal claim: that predator evasion is the primary evolutionary reason for counterillumination.
2
Identify the underlying assumption or logical gap
The author assumes that because eliminating a silhouette conceals the squid from viewers below, the selective pressure driving this adaptation came from predators rather than another selective advantage, such as capturing prey.
Weakening a causal or teleological conclusion typically requires introducing an alternative explanation for the observed adaptation.
3
Evaluate the choices to find the one providing a strong alternative explanation
Demonstrating that the squid's prey relies on detecting overhead silhouettes to escape provides a direct alternative purpose: offensive camouflage for hunting.
An alternative evolutionary function directly undermines the claim that the trait evolved primarily for predator evasion.

Key Concept

Alternative Causal Explanations in Evolutionary Arguments
Estimated Time:1m 15s
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