- CORAL REEFS—corals consist of both algae (zooanthellae) and tissues of
animal polyp. Corals obtain calories through the algae via photosynthesis
and also by extending tentacles to obtain plankton from the water. The most
common type of coral reef is the fringing reef. This type of coral reef grows
seaward directly from the shore. They form borders along the shoreline and
surrounding islands. When a fringing reef continues to grow upward from a
volcanic island that has sunk entirely below sea level, an atoll is formed.
Atolls are usually circular or oval in shape, with an open lagoon in the
center. Barrier reefs are similar to fringing reefs in that they also border a
shoreline. However, instead of growing directly out from the shore, they are
separated from land by an expanse of water. This creates a lagoon of open,
often deep water between the reef and the shore (e.g., the Great Barrier Reef
off of Australia).
Figure 4.8 Types of coral reefs
Threats to coral reefs include:
■ Rising seawater temperature as a result of climate change (i.e., when
temperatures are too high, the relationship between corals and their
symbiotic microalgae breaks down).
■ Increased ocean acidity because of increased levels of carbon dioxide. The
higher seawater acidity dissolves the coral skeletons, which make up the
structure of the reef, thus making it more difficult for corals to grow.
■ Overfishing, fishing using cyanide and dynamite, pollution from sewage
and agriculture, massive outbreaks of predatory starfish, invasive species,
and sedimentation from poor land use practices.
