Many aspects of coral reefs remain puzzling to scientists. One mystery concerns the relationship between Scleractinia, the coral type whose colonization produces reefs, and its symbiotic partner, a unicellular alga present in the coral's endodermic tissues. It is known that both organisms play an integral part in the formation of a reef's foundation by secreting and depositing calcium carbonate, which reacts with sea salt to form a hard limestone underlay. Scientists also know that, because of algal photosynthesis, the reef environment is oxygen rich, as similarly high amounts of carbon dioxide are rapidly removed. All of this accounts for the amazing renewability of coral reefs despite the erosion caused by waves. The precise manner in which one symbiotic organism stimulates the secretion of calcium carbonate by the other, however, remains unclear.

In addition to the unanswered question above, scientists have proposed various theories to explain the transformation of "fringing reefs" (those connected above sea level to land masses), into "barrier reefs" (those separated from shorelines by lagoons), and finally into island atolls. Although Darwin's view of the transformation is considered partially correct, some scientists feel that the creation of reef formations has more to do with the rise in sea level that occurred at the end of the last Ice Age than with a gradual submergence of the volcanic islands to which fringing reefs were originally attached. However, recent drillings at one atoll have revealed a substantial underlay of volcanic rock, which suggests that Darwin's explanation may be largely correct.

The term "coral reef" is something of a misnomer. The Scleractinia themselves generally comprise only 10 percent of the total mass of life forms of an average reef community. Algae, along with foraminifera, annelid worms, and assorted mollusks, can account for up to 90 percent of the reef mass. Moreover, the conditions under which reef growth occurs are determined by the needs of the algae, not those of the coral. Reefs flourish only in shallow, highly saline waters above 70 degrees Fahrenheit, because the algae require such an environment. Non-reef-building coral, meanwhile, occur worldwide.

The author suggests that coral reefs are able to survive the process of erosion

(A) primarily through the activities of algae.
(B) despite the high oxygen content of the reef environment.
(C) as a result of the combined relations of coral and algae.
(D) only if they have an above-surface connection to the shoreline.
(E) because of the volcanic rock at their base.



It can be inferred from the passage that Darwin

(A) believed that reefs became atolls through the sinking of volcanoes.
(B) should have expanded his studies of reefs to include those found at atolls.
(C) theorized that each reef formation was formed by an entirely different process.
(D) is less persuasive on the topic of reef formation in light of recent discoveries.
(E) was more interested in algae and coral than in other organisms living at reefs.



The passage does NOT discuss the relationship between

(A) algal photosynthesis and high oxygen content.
(B) Darwin's views and evidence supplied by recent research.
(C) volcanic rock and the life forms found at reefs.
(D) sea salt and calcium carbonate.
(E) wave action and the renewal of reefs.



Which of the following questions is most completely answered by the passage?

(A) What percentage of coral worldwide is of the reef-building type?
(B) How do rises in sea level affect reef formation?
(C) What are the requisite environmental conditions for coral reef formation?
(D) How does coral stimulate the calcareous secretions of symbiotic algae?
(E) What is the principal reason for the transformation of fringing reefs into atolls?



It can be inferred that the author would agree with all of the following statements EXCEPT:

(A) Coral cannot produce reefs without algae.
(B) The water around a coral reef contains high levels of oxygen.
(C) Darwin's theory about the causes of reef formation is at least partly accurate.
(D) The term "coral reef" should be abandoned by scientists.
(E) Coral are more resistant to cold temperatures than the algae that live in their tissues.