Coral reefs are structures built by tiny animals called corals that secrete calcium carbonate. They form in shallow tropical waters and provide habitat for many species. There are three main types of reefs - fringing reefs along shorelines, barrier reefs separated from land by lagoons, and atolls which surround lagoons where islands once were. Coral growth and subsidence over geological time can transform one reef type into another. Reefs need specific conditions like temperature, wave energy, light and water chemistry to survive.

2.  Coral reefs are accumulated dead skeletal matter
built up by carbonate-secreting organisms.

3.  Coral reefs are colonies of tiny living animals found
in marine waters that contain few nutrients.
 Most coral reefs are built from stony corals. Similar
to a sea anemone but with a hard skeleton.
 Built only underwater and
at shallow depths, coral
reefs need a stable
foundation upon which to
grow, provided by shallow
continental shelves and
submerged volcanoes.

4.  Southeast Asia accounts for 32.3% of that figure,
while the Pacific including Australia accounts for
40.8%. Atlantic and Caribbean coral reefs account
for 7.6%.

5.  Although corals exist both in temperate and
tropical waters, shallow-water reefs form only in a
zone extending from 30° N to 30° S of the equator.

6.  Fringing reef – Grows seaward from the rocky
shores of islands and continents.

7.  Barrier Reefs – Parallel shorelines of
continents and islands and are separated
from land by shallow lagoons.

8.  Atoll Reef - a coral island (or islands) that
encircles a lagoon partially or completely.

9.  Volcanic island becomes
extinct.
 As the island and ocean floor
subside, coral growth builds
a fringing reef.

10.  As the subsidence continues,
the fringing reef becomes a
larger barrier reef further
from the shore with a bigger
and deeper lagoon inside.
 Ultimately, the island sinks
below the sea, and the
barrier reef becomes an atoll
enclosing an open lagoon.

13.  Reef Flat - shallow zone of the back reef
located closest to the shore. The flat's
shallow waters experience wide variations in
temperature and salinity and contribute to
the accumulation of sediments. These
factors, along with occasional exposure
during low tides, tend to limit coral growth
on this section of the reef.

15.  buttress zone (outer part of fore reef) - coral
reef here is characterized by spurs, or
buttresses, of coral that jut out from the wall.
In between the spurs are deep grooves, or
channels, that help to mitigate the force of
oncoming waves by channelling spent waves
back out to sea where they collide with
oncoming waves, thus cancelling out their
strength. The grooves also drain sediment off
the reef.

17.  Reef wall – zone which receives less sunlight
but is abundant with corals because of the
reduced wave activity.

18.  Temperature - Coral reefs thrive in water
temperatures of 25° and 29° Celsius. Certain
corals have evolved to survive outside this
temperature range
 Wave Motions- Water movements help take
away waste and bring food.

19.  Nutrients - Corals survive only in clear water,
without much sediment, where light can breach the
surface of the water. Since there isn't much debris
floating around, the water is usually low in
nutrients. These waters would not naturally have
many animals living in them because of the lack of
nutrients, but since corals live here, many
thousands of animals survive by eating either the
coral, or the animals that eat the coral.

20.  Light- Light is incredibly important for coral reefs. The
reason corals are able to survive in such non-nutritious
water is because of the zooxanthellae algae produces food
for the coral as well as itself. In order to photosynthesize to
produce food however, the algae needs light. Without light,
this ecosystem cannot exist.

21.  Salinity- Corals tend to live in salty water,
surviving in a range of 27-40 parts per thousand
(ppt) but preferring 36 ppt.
 Depth- Corals live in shallow water and are usually
found to a depth of 60 meters but deep water
corals go far deeper to about 2000 meters.