Categories and Structure of Coral Reefs
There are three basic kinds of coral reefs in the Caribbean: fringing reefs, barrier
reefs and atolls. Fringing reefs are coral reefs that grow in shallow waters and border
the coast closely or are separated from it by a narrow stretch of water. Fringing reefs
consist of several zones that are characterized by their depth, the structure of the reef,
and its plant and animal communities. These regions include the reef crest (the part of
the reef the waves break over), the fore reef (the region of medium energy), and the
spur and groove or buttress zone (the region of coral growth which includes rows of
corals with sandy canyons or passages between each row).
Barrier reefs are reefs that are separated from land by a lagoon. These reefs grow
parallel to the coast and are large and continuous. Barrier reefs also include regions of
coral formation that include the zones found in fringing reefs along with patch reefs
(small reefs), back reefs (the shoreward side of the reef), as well as bank reefs (reefs
that occur on deep bottom irregularities). Coral reefs also include reef flats (the are of
the reef not exposed), the reef crest, which runs parallel to the coast and is protected
from waves, and a coral terrace (a slope of sand with isolated coral peaks). These
features are followed by another coral terrace and a vertical drop into deeper waters.
The third type of coral reefs are atolls. Atolls are annular reefs that develop at or near
the surface of the sea when islands that are surrounded by reefs subside. Atolls
separate a central lagoon and are circular or sub-circular. There are two types of
atolls: deep sea atolls that rise from deep sea and those found on the continental shelf.
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Corals can obtain food in a variety of ways. Reef-building corals rely on the
photosynthetic products of zooxanthellae for the majority of their nutrients. However,
corals also capture zooplankton for food. Corals are suspension feeders. They utilize
two main methods of prey capture: nematocyst adhesion and mucus entrapment
(Sebens and Johnson, 1991). Nematocysts on the tentacles and mesentarial filaments
can be used to sting prey and move it into the mouth. Some corals will trap prey in
sticky mucus on their tentacles and move the prey into the mouth using the mucus
and cilia. Prey size is correlated to polyp size (Sebens and Johnson, 1991).
Most corals feed at night. This may be because night is when the zooplankton travel
into the water column and become available for capture. Keeping the tentacles
retracted during the day may also help corals avoid predation, protect themselves
from UV light, and avoid shading their zooxanthellae.
Prey can be brought into contact with the tentacles by direct interception (the flow of
water makes the particles impact the tentacles), by inertial impaction (momentum of
dense particles makes them deviate from the water flow and impact the tentacles),
and by gravitational deposition (gravity causes heavy particles to fall into the
tentacles) (Sebens and Johnson, 1991). Regardless of the method of capture, prey is
moved through the mouth, down the pharynx (gullet), to the gastrovascular cavity for
digestion.