noes. As the volcanoes subsided beneath the sea, the rate of coral growth
matched the rate of volcanic subsidence.This kept the corals at a constant shal-
low depth for photosynthesis.
Coral reefs are among the oldest ecosystems on Earth and important
land builders.They form chains of islands and alter continental shorelines.The
major structural feature of the reef is the coral rampart, which reaches almost
to the water’s surface. It consists of large rounded coral heads and a variety of
branching corals. The fore reef is seaward of the reef crest, where coral blan-
kets nearly the entire seafloor. In deeper waters, many corals grow in flat, thin
sheets to maximize their light-gathering area.
In other parts of the reef, the corals form large buttresses separated by
narrow, sandy channels composed of calcareous debris from dead corals, cal-
careous algae, and other organisms living on the coral.The channels resemble
narrow, winding canyons with vertical walls of solid coral.They dissipate wave
energy and allow the free flow of sediments, which prevents the coral from
choking on the debris. Below the fore reef is a coral terrace, followed by a
sandy slope with isolated coral pinnacles, then another terrace, and finally a
near vertical drop into the dark abyss.
The coral polyp (Fig. 81) is a soft-bodied animal crowned by a ring of
tentacles surrounding a mouthlike opening.The tentacles are tipped with poi-
sonous stingers to attack prey swimming nearby. The polyp lives in a skeletal
cup or tube called a theca composed of calcium carbonate. Polyps extend their
tentacles to feed at night and withdraw into their thecae during the day or at
low tide to keep from drying out in the Sun. Because the algae within the
polyp’s body require sunlight for photosynthesis, corals are restricted to warm,
shallow water generally less than 100 feet deep.
Figure 81Coral polyps
seek protection in carbonate
cups from predators and
during low tide.SILURIAN PLANTS