Coral reefs, known as the 'rain forests of the sea,' are crucial ecosystems hosting roughly 4,000 fish species and providing significant economic benefits estimated at US$30 billion annually. These reefs are formed by coral polyps and their symbiotic algae, zooxanthellae, but are increasingly threatened by climate change, ocean acidification, and human activity. The document outlines the biology, formation types, and the reproductive mechanisms of corals, while emphasizing the urgent need to protect these biodiverse habitats.

1. Coral Reefs
THE MARINE SCOOP GUIDE TO…

2. Contents
Introduction
What is a coral reef?
Polyps and zooxanthellae
Reefs at risk
Coral biology
Summary of reef formations
New oceanic islands
Fringing reefs
Barrier reefs
Atolls and seamounts
Reef growth
Coral reproduction
Examples of coral species

3. Coral reefs, the “rain forests of the sea,” are
among the most biologically rich and productive
ecosystems on earth; some 4,000 species of fish
are found here (approximately one-quarter of all
marine fish species), along with a vast array of
other life forms—molluscs, crustaceans, sea
urchins, starfish, sponges, tube-worms and many
more. There are perhaps 1 million species found
in a habitat that covers a total of about 250,000
sq km (roughly the area of the United Kingdom).
They also provide valuable ecosystem benefits
to millions of coastal people. They are important
sources of food and income, serve as nurseries
for commercial fish species, attract divers and
snorkelers from around the world, generate the
sand on tourist beaches, and protect shorelines
from the ravages of storms. Latest estimates
suggest coral reefs provide close to US$30 billion
each year in goods and services.
"Coral reefs are the
most biodiverse of all
known marine
ecosystems, and
maintain much higher
genetic diversity than
tropical rainforests.”
John McManus, The International Coral
Reef Initiative
Introduction

4. Coral reefs are physical structures built by the actions of many tiny coral
animals that live in large colonies and lay down communal limestone
skeletons. Over millennia, the combined mass of skeletons build up into huge
reefs, some of which are visible from space. There are some 800 species of
reef-building or hermatypic corals and they have exacting requirements,
needing bright, clear, and warm waters.
What is a coral reef?
“Coral reefs are diverse underwater ecosystems
held together by calcium carbonate structures
secreted by corals.”

5. The individual coral animals, known as
polyps, have a tubular body and central
mouth ringed by stinging tentacles, which
can capture food. Living within their body
tissues are microscopic single-celled
dinoflagellate algae (zooxanthellae) that
need sunlight to survive. These algae
convert sunlight into sugars, which
produces energy to help sustain their coral
hosts. These same algae also provide the
corals with their vibrant colours.
Polyps and zooxanthellae
Kingdom: Animalia Phylum: Cnidaria Class: Anthozoa Order: Scleractinia

6. Coral reefs face a wide and intensifying array of threats—including impacts from overfishing, coastal
development, agricultural runoff, and shipping. In addition, the global threat of climate change has
begun to compound these more local threats to coral reefs in multiple ways. Warming seas have
already caused widespread damage to reefs, with high temperatures driving a stress response called
coral bleaching, where corals lose their colourful symbiotic algae, exposing their white skeletons. This
is projected to intensify in coming decades. In addition, increasing carbon dioxide (CO2) emissions
are slowly causing the world’s oceans to become more acidic. Ocean acidification reduces coral
growth rates and, if unchecked, could reduce their ability to maintain their physical structure.
Reefs at risk “Over 60% of the world’s coral reefs are under immediate threat”
Reefs at risk revisited 2011

7. Coral biology
If you have ever seen branching corals spreading their arms out like tree limbs, you can see why early
scientists thought corals were plants. But these tiny, soft-bodied creatures are carnivores, despite being
sessile, or fixed to one spot. Like their relatives, the jellyfish and the sea anemone in the Cnidaria
phylum, each individual coral, or polyp, has barbed, stinging cells called nematocysts it can extend to
capture prey like zooplankton or small fish.
Although those early scientists got it wrong, it's easy to understand their mistake. Coral almost could be
considered half-plant because of the zooxanthellae (pronounced zoo-zan-thelly) algae that live just
inside each polyp's cell walls. The zooxanthellae supplies the polyp with the by-products of
photosynthesis, which the polyp turns into proteins, fats and carbohydrates. In turn, the polyp shelters
the zooxanthellae and provides the carbon, nitrates and phosphates the algae need for photosynthesis.
Up to 90 percent of the energy produced by zooxanthellae's photosynthesis is transferred to the coral
host. This mutually beneficial arrangement is called symbiosis.
Coral polyps also use the energy supplied by their symbiotic algae to produce calcium carbonate, or
limestone. They secrete the limestone from their base, creating a protective skeleton and a hollow
chamber called a cup. The polyps retreat into their cup to hide when predators come looking for them.
Polyps rarely exist alone. They usually join other polyps to form a larger colony that acts as a single
organism. While the individual minuscule polyps grow to 1-3 mm on average, colonies can weigh tons.
Even a single branching coral comprises thousands of individual polyps. Over hundreds or thousands of
years, these colonies may connect to form a reef.

8. Summary of reef formations
Depending on their structural characteristics, coral reefs are generally described using three major reef
types:
1) Fringing reefs, the most common type of the reef forms, extend seaward from the shore, sometimes
separated by a narrow stretch of water or lagoon. They are generally found in shallow waters with the
reef flats becoming exposed during low water.
2) Barrier reefs, like the Great Barrier Reef in Australia, border the shoreline but are separated from the
land by a large lagoon that may be of considerable depth. Barrier reefs are frequently formed by a
network of individual reef systems, often encompassing fringing reefs extending from offshore islands.
3) Atolls are circular or horseshoe shaped coral islands that surround a central lagoon. Darwin
suggested that atolls were created after periods of transition caused by the subsidence of oceanic
volcanoes. This notion of transition from fringing reefs to an atoll remains widely supported by scientists
today.

9. New oceanic islands
Classic coral reef formations change through time. They begin with a brand new tropical island
(produced by an oceanic hot spot or at a plate boundary) and gradually change through thousands of
years from a fringing reef, to a barrier reef, to an atoll, and finally to an extinct reef as a seamount or
guyot.
A new oceanic island in the tropics will have no terrestrial or marine life when it is first formed. This new
island will be made entirely of crustal material (rock) with little or no evidence of life forms. If there is no
more volcanic activity and the island cools there will be a succession of life forms that may arrive by
flying, floating or swimming. These forms may stay and establish themselves if conditions there are
good for them. Over time a thriving community will be found on land as well as in the ocean.

10. Fringing reefs
A fringing reef forms along the shoreline of most new tropical
islands. This is because the reef building coral animal is one
of the few marine organisms that can survive in the warm,
nutrient-poor surface tropical waters. In fact, the reef
building coral (hermatypic) can only survive in warm, clear
ocean waters (such as would be common around a new
oceanic island). The coral animal reproduces sexually with a
microscopic planktonic larval form that is abundant in
tropical waters during coral reproduction. Most of these
larvae never grow to adults because there is no place for
them to exist in the warm surface ocean waters on a solid
surface. They need to attach to a solid surface that will stay
in the warm, sunlit, clear tropical water and most places are
already taken … but not on a new island. So a coral reef
forms along the edge of this new island, right next to the
island and is called a fringing reef - the first step in 'classical'
coral reef formation.

11. Barrier reefs
A barrier reef forms as the oceanic island begins to sink into
Earth's crust due to the absence of volcanic island building
forces, the added weight of the coral reef, and erosion at the
surface of the island. As the island sinks, the coral reef
continues to grow upward. The coral animals lay down
tremendous amounts of calcium carbonate (in part due to
their symbiotic zooxanthellae) in the form of their skeletons
(called corallites). If conditions are good they can usually
keep pace with the sinking island. Their living tissue remains in
the upper part of the reef in the warm, clear, tropical water.
The lower part of the reef is composed of the calcium
carbonate skeletons left by the reef building coral. This
upward growth creates a lagoon of water between the top of
the reef (that started as a fringing reef) and the sinking crustal
island. The lagoon fills in with eroded material from both the
reef and the island and is a haven for marine forms that
require protection from waves and storms. This barrier reef is
generally not a complete circle as it may be broken, here and
there, by storms.

12. Atolls and seamounts
An atoll forms when the oceanic island sinks below the
surface of the ocean but the coral reef continues to grow
upward. The atoll is usually circular in shape but a broken
circle (due to the history of storms). Eroded reef material may
pile up on parts of the reef, creating an area above sea level,
an island called a 'cay.' Cays may become stabilized enough
(often through plants) to provide a permanent island for
buildings. Cays may also be unstable and move across the
reef or disappear.
A seamount or guyot forms when the coral reef cannot keep
up with the sinking of the island. Seamounts and guyots are
below the surface of the ocean and may be home to a large
number of species depending on their location and depth.
Seamounts are rounded on the top and guyots are flat-
topped. The flat top of the guyot is from its existence at the
surface for some time and being planed down by waves (thus
the flat top).

13. Reef growth
Reefs grow in one of two ways. One way is to periodically add on to their limestone base. They simply
secrete more calcium carbonate under and around their current cup, creating the framework of the
reef and causing it to grow both upwards and outwards. They also grow by reproducing. Corals may
reproduce either asexually, by dividing and producing identical clones, or sexually, by sending out
eggs or sperm.
Either way, new coral polyps settle towards the ocean bottom until they find a hard substrate to call
home, either combining with a pre-existing coral colony or starting one of their own. In addition to
being connected at their bases, coral polyps link to one another laterally by a thin tissue called the
coenosarc. The coenosarcs and the polyps form the visible living part of the reef, while the limestone
base forms the non-living part.

14. Coral reproduction
Corals can reproduce asexually and sexually. In asexual reproduction, new clonal polyps bud off from
parent polyps to expand or begin new colonies. This occurs when the parent polyp reaches a certain
size and divides. This process continues throughout the animal’s life.
About three-quarters of all stony corals produce male and/or female gametes. Most of these species
are broadcast spawners, releasing massive numbers of eggs and sperm into the water to distribute their
offspring over a broad geographic area. The eggs and sperm join to form free-floating, or planktonic,
larvae called planulae. Large numbers of planulae are produced to compensate for the many hazards,
such as predators, that they encounter as they are carried by water currents. The time between
planulae formation and settlement is a period of exceptionally high mortality among corals
Along many reefs, spawning occurs as a mass synchronized event, when all the coral species in an
area release their eggs and sperm at about the same time. The timing of a broadcast spawning event is
very important because males and female corals cannot move into reproductive contact with each
other. Because colonies may be separated by wide distances, this release must be both precisely and
broadly timed, and usually occurs in response to multiple environmental cues.

15. Coral reproduction continued
The long-term control of spawning may be related to temperature, day length and/or rate of
temperature change (either increasing or decreasing). The short-term (getting ready to spawn) control
is usually based on lunar cues. The final release, or spawn, is usually based on the time of sunset.
Planulae swim upward toward the light (exhibiting positive photo taxis), entering the surface waters and
being transported by the current. After floating at the surface, the planulae swim back down to the
bottom, where, if conditions are favourable, they will settle. Once the planulae settle, they
metamorphose into polyps and form colonies that increase in size. In most species, the larvae settle
within two days, although some will swim for up to three weeks, and in one known instance, two
months.

16. Examples of coral species