New York City SHSAT 2017

Many people dream about living on a coral island,
but probably few of us would be able to describe one with
any accuracy. Popular books and films create a romantic
image of these islands, but it is not always entirely
( 5 ) justified when the islands are seen from sea level. Beneath
the waves, however, the coral island is a fantastic and
very beautiful world, depending entirely upon a complex
web of interrelationships between plants and animals.
The environment of the coral reef is formed over
( 10 )thousands of years by the life cycles of vast numbers of
coral animals. The main architect of the reef is the
stony coral, a relative of the sea anemone that lives in
tropical climates and secretes a skeleton of almost
pure calcium carbonate. Its partner is the green alga, a
( 15 )tiny unicellular plant, which lives within the tissues of
the coral. The two organisms coexist in a mutually
beneficial relationship, with the algae consuming
carbon dioxide given off by the corals, and the corals
thriving in the abundant oxygen produced
( 20 )photosynthetically by the algae. When the coral dies,
its skeleton is left, and other organisms grow on top of
it. Over the years, the sheer mass of coral skeletons,
together with those of associated organisms, combines
to form the petrified underwater forest that divers find
( 25 )so fascinating.
Many aspects of coral reefs still puzzle scientists,
however. One mystery concerns the transformation of
coral reefs into islands. Many of today’s coral reefs are
attached to much larger land masses. The northeastern
( 30 )coast of the continent of Australia, for example, is
fringed by coral communities which have gradually
grown into vast ribbons of barrier reefs. Scientists
have proposed a theory to explain the transformation
of “fringing reefs” such as these into islands. It’s
( 35 )suggested that many of today’s reef islands resulted
from a rise in sea level at the end of the last Ice Age.
Land masses to which reefs were once attached were
gradually submerged by rising ocean levels. Scientists
also believe that certain reef islands resulted from
( 40 )volcanic activity. Reefs which originally surrounded
volcanic islands were transformed into atolls, or ring-