Examkin

Question

Difficulty: MediumEvaluating Passage Arguments and Claims

For decades, marine biologists analyzing benthic invertebrate populations at deep-sea hydrothermal vents operated under the passive transport model. This model posits that oceanic currents alone dictate the dispersal distance and colonization success of planktonic larvae migrating between isolated vent fields. Proponents emphasized that because hydrothermal larvae lack propulsion mechanisms capable of overriding deep-ocean current velocities, their spatial distribution must be a purely stochastic function of hydrodynamics.

However, recent oceanographic tracking combined with larval physiological profiling has challenged this framework. Researchers discovered that certain vent species regulate their vertical positioning within the water column by adjusting their buoyancy in response to minute temperature and chemical gradients. By ascending into faster-moving mid-water currents or descending into stagnant boundary layers, larvae actively maneuver between distinct hydrodynamic regimes.

Critics of this active-buoyancy hypothesis contend that larval metabolic reserves are insufficient to sustain prolonged vertical migration in nutrient-deprived abyssal waters, suggesting that observed vertical position shifts are merely artifacts of localized thermal turbulence. Yet this objection overlooks key metabolic adaptations. Biomass assays reveal that vent larvae possess unusually high lipid concentrations, allowing them to remain energetic for months without feeding. Thus, rather than disproving active dispersal, the metabolic evidence reinforces the claim that larval behavior significantly modulates population connectivity across distant vent systems.

Based on the passage above, which of the following, if true, would most seriously weaken the author's defense of the active-buoyancy hypothesis against the critics' objection?

  1. High lipid concentrations in hydrothermal vent larvae are consumed almost entirely during embryonic development prior to larval release into the water column.Answer
  2. B
    Hydrothermal vent species with lower average lipid reserves exhibit far less frequent colonization of distant vent fields than do species with higher lipid reserves.
  3. C
    Abyssal ocean currents surrounding hydrothermal vents have experienced substantial shifts in overall velocity over geological timescales.
  4. D
    Critics of active dispersal originally proposed that buoyancy regulation was the primary driver of larval distribution in upper surface waters.
  5. E
    Hydrothermal vent fields are situated at distances that exceed the maximum theoretical migration range of any planktonic organism.

Answer

The argument is most weakened by evidence showing that high lipid concentrations in hydrothermal vent larvae are consumed almost entirely during embryonic development prior to larval release into the water column.
The author counters the critics' objection by citing high lipid concentrations as evidence that larvae possess sufficient energy for prolonged vertical maneuvering. If these lipid reserves are consumed during embryonic development prior to larval release, they are absent during migration. Consequently, the high lipid levels cannot support active buoyancy control, leaving the critics' objection unrefuted and weakening the author's defense.

Step-by-Step Solution

1
Identify the author's defense and main claim in response to critics.
The author responds to critics (who claim larvae lack metabolic energy for vertical migration) by asserting that high lipid concentrations provide larvae with enough energy to remain active for months.
Evaluating a defense requires pinpointing the exact premise used to defeat the counterargument.
2
Determine what condition would undermine this specific defense.
If the lipid reserves are unavailable during the actual dispersal phase, the author's evidence fails to resolve the critics' objection.
A claim relying on a metabolic resource is invalid if that resource is exhausted before the relevant activity occurs.
3
Evaluate the choices to find the statement that demonstrates this breakdown in logic.
The statement that lipid reserves are fully consumed during pre-release embryonic development invalidates their use during active migration.
This directly breaks the functional link between high initial lipid levels and active larval buoyancy control.

Key Concept

Evaluating Passage Arguments and Claims
Rate this question

Topics

Evaluating Passage Arguments and Claims

More questions from this topic