Dircection of Questions 1 to 5: Read the carefully and answer the questions that follow:
There are a great many symbiotic relationships in the marine environment. A popular one, often noted for the striking beauty of the juxtaposition, is that of the sea anemone and the clown fish. The anemone has poison tentacles which—when they contact passing fish—paralyze the fish and drag the prey in for a meal. The clown fish uses the anemone's tentacle "garden " as a safe haven while attracting prey for the anemone to capture, for it alone is immune to the sting of the anemone.
Another symbiotic relation that remains the subject of scientific puzzlement concerns the relationship between Scleractinia, the coral type whose colonization produces reefs, and their symbiotic partners the zooxanthellae, the unicellular algae present in the corals' endodermic tissues. It is known that each symbiont plays an integral part in the formation of a reef's protective limestone foundation. The coral polyps secrete calceous exoskeletons which cement themselves into an underlayer of rock, while the algae deposit still more calcium carbonate, which reacts with sea salt to create an even tougher limestone layer.
It is also known that, due to the algal photosynthesis, the reef environment is highly oxygen—saturated, while the similarly high amounts of carbon dioxide are carried off rapidly. All this accounts for the amazing renewability of coral reefs despite the endless erosion caused by wave activity. However, the precise manner in which one symbiont stimulates the secretion of calcium carbonate by the other remains unclear.
Scientists have also proposed various theories to explain the transformation of "fringing reefs, " those connected above sea level to land masses, into "barrier reefs " that are separated from shorelines by wide lagoons, and then into free—floating atolls. Though the theory postulated by Charles Darwin is considered at least partially correct, some scientists today argue that the creation of the reef forms has more to do with the rise of sea level that accompanied the end of the Ice Age. However, recent drillings at Enewetak atoll have uncovered a large underlay of volcanic rock, which suggests that Darwin's explanation may have been more valid after all.
Even the name given to the reefs is something of a misnomer. The Scleractinia themselves generally comprise no more than 10 percent of the biota of the average reef community: zooxanthellae can account for up to 90 percent of the reef mass, along with foraminifera, annelid worms, and assorted mollusks. Moreover, reefs can flourish only in shallow, highly saline waters above 70?F., because the algae require such circumstances; yet non—reef—building corals occur worldwide under various environmental conditions, from the Arctic to the Mediterranean, home of the red coral prized for jewelry. The most likely reason that the term "coral reefs " persists is that the brilliant variety of coral shapes and colors makes aesthetic considerations more vivid than biological ones.
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