The corals of the Persian Gulf endure summer water temperatures that would bleach and kill members of the same species almost anywhere else, a resilience long credited to heritable changes in the coral animals themselves. Transplant studies seemed to settle the matter: Gulf colonies moved to cooler aquaria retained their tolerance, suggesting a fixed trait rather than one acquired through exposure. Yet a coral is not a single organism. Each polyp harbors photosynthetic algae, and a transplanted colony takes its algae with it, so the persistence of tolerance after transplantation is just as consistent with the trait residing in the partner. Experiments that instead exchange algal partners between colonies point toward that second possibility, since heat tolerance in such trials has tended to follow the alga rather than the host. None of this renders the coral animal irrelevant-not every host can sustain a given alga-but it does suggest that the search for the source of the Gulf's resilience began by examining only half of the organism.

The primary purpose of the passage is to

A. reconcile two conflicting accounts of a biological trait
B. argue that findings long taken to confirm an explanation do not exclude another possibility
C. describe the experimental methods used to measure a trait
D. defend a long-standing hypothesis against a recent challenge
E. survey rival explanations for a phenomenon without endorsing either



In the context of the passage, the sentence “Yet a coral is not a single organism” functions primarily to

A. narrow the conclusion just stated by identifying a case to which it does not apply
B. offer a further reason to believe that the trait is heritable
C. introduce a fact that calls into question an inference drawn from the transplant studies
D. summarize the outcome of the algal-exchange experiments
E. concede a consideration that the author later dismisses as unimportant



The author would most likely agree that the transplant studies described in the passage

A. were flawed because the aquaria were not cool enough to induce bleaching
B. are beside the point, because the tolerance in fact resides wholly in the algae
C. demonstrated that the tolerance is acquired through exposure rather than inherited
D. could not by themselves determine whether the tolerance resides in the coral or in its algae
E. showed that algae from the Gulf cannot survive in cooler water