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Difficulty: HardCommand of Evidence: Textual

Ecologist Dr. Nia Mshila and colleagues investigated the phenotypic plasticity of the marine snail *Nucella lapillus* in response to predatory cues from the green crab *Carcinus maenas*. In calm waters, snails exposed to crab effluents typically develop significantly thicker shells to resist crushing. However, Mshila hypothesized that in habitats characterized by high wave action, *N. lapillus* will prioritize shell shape (developing a wider aperture for stronger foot attachment to rocks) over shell thickness, even when exposed to crab cues, because hydrodynamic dislodgement poses a more immediate survival threat than predation. Which finding, if true, would most directly support Mshila's hypothesis?

  1. When reared in high wave action environments with crab effluents, *N. lapillus* developed wider apertures but showed no significant increase in shell thickness compared to snails reared in high wave action environments without crab effluents.Answer
  2. B
    Snails exposed to crab effluents in calm waters developed thicker shells and narrower apertures than snails in calm waters that were not exposed to these effluents.
  3. C
    In wave-swept environments, green crabs were observed to successfully capture and feed on snails with wider apertures at the same rate as they did on snails with narrower apertures.
  4. D
    Under laboratory conditions simulating only wave action without crab cues, *N. lapillus* developed wider apertures but did not change in shell thickness.

Answer

When reared in high wave action environments with crab effluents, *N. lapillus* developed wider apertures but showed no significant increase in shell thickness compared to snails reared in high wave action environments without crab effluents.
The correct answer directly supports the hypothesis by demonstrating that in high wave-action habitats, the presence of crab cues does not lead to shell thickening, while the snails still develop the wider aperture. This confirms that the snails prioritize the morphological change that prevents dislodgement over the one that protects against predation when both threats are present.

Step-by-Step Solution

1
Identify the core hypothesis to be supported.
The hypothesis is that in high wave action habitats, *N. lapillus* will prioritize shell shape (wider aperture) over shell thickness, even when exposed to predatory cues.
Understanding the specific trade-off and the condition (presence of both wave action and predator cues) is essential to evaluate the evidence.
2
Evaluate the choices to see which one provides direct evidence for this specific trade-off under the stated conditions.
The correct option shows that in wave-swept environments with crab effluents, snails change their shape (wider aperture) but do not increase shell thickness compared to control snails in the same environment. This directly demonstrates the prioritization of shape over thickness in the presence of predatory cues.
Comparing the experimental group (wave + cues) to the control group (wave only) isolates the effect of the predatory cue under hydrodynamic stress.

Key Concept

Command of Evidence: Textual
Estimated Time:2m 0s
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Command of Evidence: Textual

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