Coral reefs and atolls

Coral reefs are very unique, shallow marine geological formations. Over 150 million square kilometers area of the Earth is covered by excellent coral reefs. The largest coral reef is the Great Barrier Reef, located off the eastern coast of Australia. It stretches for more than 1500 Km. Some coral reefs are thousands of meters in thickness. Coral reef zones are the pleasant home to thousands of marine plants and animals. Corals themselves are good treasure houses with precious resources. Pirates used to hide their treasures in the sheltered coves of the coral reefs, in some places. The corals were first seen during 1815- by a naturalist. The name " bioherm " was given to the corals and coral reefs, initially, by William Thornbury. In this lesson, let us learn about the Corals, their origin, growth, types of Coral reefs and their distribution.

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CORAL REEFS & THEIR TYPES

By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
INDIA

INTRODUCTION

You all might have seen in some films and TV programmes, some scuba divers swimming in
bluish sea waters watching the marine life. These naturalists dip into the lagoonic waters with a
scuba diving tank or a snorkel and see all the colorful underwater features and marine life.
These features are very rare formations on the earth. Do you know what those features are?. Yes
, they are the Coral reefs. Coral reefs are the geological formations created by the marine
organisms in seas.

Coral reefs are very unique, shallow marine geological formations. Over 150 million square
kilometers area of the Earth is covered by excellent coral reefs.
The largest coral reef is the Great Barrier Reef, located off the eastern coast of Australia. It
stretches for more than 1500 Km. Some coral reefs are thousands of meters in thickness.
Coral reef zones are the pleasant home to thousands of marine plants and animals. Corals
themselves are good treasure houses with precious resources.
Pirates used to hide their treasures in the sheltered coves of the coral reefs, in some places.
The corals were first seen during 1815- by a naturalist. The name " bioherm " was given to the
corals and coral reefs, initially, by William Thornbury.

In this lesson, let us learn about the Corals, their origin, growth, types of Coral reefs and their
distribution.
The modules covered in this lesson are:

1. Origin of Corals and Coral Reefs
2. Types of Corals and Coral reefs
3. Reef Forming Environment
4. Distribution of Coral Reefs
5. Protection of Coral Reefs.

1. ORIGIN OF CORALS AND CORAL REEFS

Corals are gastrovascular marine organisms. Each one of these animals is known as a coral
"polyp". Coral Polyps are tiny, primitive marine organisms. A single polyp has a tube-shaped
body with a mouth which is surrounded by tentacles. The polyp of hard corals produces a stony
skeleton of calcium carbonate which form the base. Often the skeleton forms a cup-like structure
in which the polyp lives. Coral polyps in colonies make up the coral reefs. A coral polyp is a
soft, almost transparent animal which builds its own skeleton outside its body. Since it has no
backbone, it is an invertebrate.

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It is necessary to understand the parts of a coral polyp first. A coral polyp has three main parts.
They are its base, mouth and a middle portion. The base of the polyp is attached to its skeleton.
The mouth (or) open end, has tentacles around its edge. The middle section is shaped like a
tube. It has a hollow space inside and is known as the gut. A polyp is made up of two cell
layers as : the epidermis and the gastrodermis. The non-tissue layer between the gastrodermis
and the epidermis is called the mesoglea.

A coral reef starts with a single tiny coral animal floating in the water. When it finds a hard
surface (any rocky substratum) it settles on the rock and attaches to it. This becomes the parent
coral polyp. Then the polyp begins producing its hard skeleton. After the polyp grows for a
period of time, the little polyps begin budding out of its body. As the buds grow, they will
begin forming buds of their own.
The result of this repeated budding is a huge number of polyps living together in a group, or
"colony ".

The corallite is the part of the skeleton deposited by one polyp. The skeletal wall around each
polyp is called as theca. The coral anatomy also includes the calcareous plate-like structure
known as septa. The septa radiate from the wall to the center of the corallite.

Corals which produce a hard skeleton are called as stony corals. They have special cells on the
outskirts of their bodies which produce a limestone substance. This makes them hard, like a rock
.The polyps sit in the cup like portions of the skeletons.

Corals exhibit sexual and asexual reproduction. The coral colony expands in size by budding.
Budding may be intratentacular or extratentacular. A common type of asexual reproduction in
corals is by fragmentation.
The adult members of the colony can produce a small coral animal. This can float away to a
longer distance in water. If it is not eaten, then this may start a new coral reef, several miles
away. Broken pieces of corals that land on a suitable substrate may begin growing and produce
a new colony.
After several years they form a great structure called the reefs. The rate of coral growth is very
slow. It takes a thousand years for a reef to grow just three feet.

Many coral species undergo mass spawn. Within a 24 hour period, all the corals from one
species and often within a genus release their eggs and sperm at the same time.

Depending on the species, coral polyps may measure less than an inch to several inches in
diameter (a few millimeters to several centimeters). One of the largest corals, the mushroom
coral is a solitary coral that can extend upto 25 cm in diameter. The Colonial coral polyps are
much smaller in size. They range from 1 to 3 mm in diameter. The Coral colonies also vary in
size. Some corals form only small colonies. Others may form colonies of several meters. And
also tall. The Star coral colonies reach an average height of 3-4m.
Natural pigments in coral tissue produce a range of colors including white, red, orange, yellow,
green, blue, and purple.

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A coral reef is composed of calcium carbonate, or limestone, derived from the water by the reef
organisms: colonies of coral polyps and coralline algae.
Most of this structure, the underlying foundation of the reef, is dead, made up of layer upon layer
of coral skeletons.


2. TYPES OF CORALS AND CORAL REEFS

Corals are members of a large, related group of marine animals called the Cnidaria.
Collectively it includes hydroids, jellyfish or medusae, sea fans, sea pens, soft corals, and sea
anemones.

The taxonomic classification of corals is as follows:

The Phylum is Cnidaria. Four classes of corals have been recognized so far. They are:
A. Class Hydrozoa which includes hydroids, Millepora and Stylaster.
B. Class Scyphozoa (jellyfishes)
C. Class Cubozoa (sea wasps)
D. Class Anthozoa which includes soft corals, sea fans and relatives , sea pens, sea
anemones , colonial anemones, true stony corals, black corals and tube anemones.

Corals are of different geological ages. There are Palaeozoic corals, Mesozoic corals and modern
corals. The Palaeozoic Corals are the oldest forms. They are found as fossils.

Corals are also classified into two other types as, perforate and imperforate corals. Perforate
corals have porous skeletons with connections between the polyps through the skeleton.
Imperforate corals have solid skeletons.

TYPES OF CORAL REEFS

Coral reefs can be classified based on their morphology, evolution, size, depth of surrounding
water, and relationship to nearby land. Morphology is the major criteria considered.

Coral reefs take a variety of forms, defined as follows:
* Apron reef — short reef resembling a fringing reef, but more sloped; extending out and
downward from a point or peninsular shore.
* Fringing reef — reef extending directly out from a shoreline, and more or less following
the trend of the shore.
* Barrier reef is the reef which is separated from the mainland or island shore by a
lagoon.(example the Great Barrier Reef). A barrier reef is almost parallel to a shore and
separated from it by a lagoon or other body of water. A barrier reef is usually pierced by
several channels that give access to the lagoon and the island or continent beyond it.
* Patch reef — an isolated, often circular reef, usually within a lagoon or embayment.
* Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll
lagoon.
* Table reef — isolated reef, approaching an atoll type, but without a lagoon.

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* Atoll reef — a more or less circular or continuous barrier reef surrounding a lagoon
without a central island. Coral Atolls are rings of coral that grow on top of old, sunken
volcanoes in the ocean. They begin as fringe reefs surrounding a volcanic island; then, as
the volcano sinks, the reef continues to grow, and eventually only the reef remains.

Coral reefs contain three distinct zones as the fore reef, reef crest, and the back reef or reef
lagoon. The following are the parts of a reef:
 The reef surface is the shallowest part of the reef. It is subject to the surge and the rise
and fall of tides.
 The off-reef floor is the shallow sea floor surrounding a reef. This zone occurs in
continental shelf regions.
 The reef face is the zone above the reef floor which holds the richest habitat.
 The reef flat is the sandy-bottomed flat located behind the main reef. It contains chunks
of coral.

The common types of corals are classified into two major varieties as stony and soft varieties:

A. STONY CORALS:
1. Stony corals
2. Finger corals
3. Fungus coral
4. Brain coral
5. Tube coral
6. Rose coral
7. Staghorn coral
8. Lettuce-leaf coral.

B. SOFT CORALS:
1. Sea fan
2. Bushy soft coral
3. Sea feather
4. Black coral.


3. REEF FORMING ENVIRONMENT:

Corals build their reefs only in warm and shallow waters.
They do not grow well, where they are directly exposed to sun or rain for a long period of time.
They also do not grow where the water is too salty or too muddy.
Coral reefs are found in all oceans of the world, generally between the Tropic of Cancer and the
Tropic of Capricorn, because reef-building corals live in these waters.
Reef-building corals are found mainly in the photic zone (ie. Less than 50m depth), where the
sunlight reaches the ground and offers the corals enough energy.
The corals themselves do not photosynthesise, but they live in a symbiotic relationship with
types of microscopic algae that photosynthesise for them.
Because of this, coral reefs also grow much faster in clear water, which absorbs less light.

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The coral growth needs a clean and warm waters. The best temperature for coral growth is 18°C.
In general, reefs form where the following set of conditions are met:
 Warm water, not cooling below 18°C during the year.
 Wave-swept, high energy coast.
 Hard substrate.
 Normal marine salinity water.
 Absence of abundant clastic detritus.
 Shallow water (0 - 30 meters depth - photic zone)

Corals can be found from the water's surface to depths of less than 46 m, where sunlight
penetrates. Because, the reef- building corals have a symbiotic relationship with a type of
microscopic algae, sunlight is necessary for these corals to thrive and grow. Coral Reefs tend to
grow faster in clear water. Reef-building corals require warm ocean temperatures ranging from
20 to 28 deg. C. Reef development is generally more abundant in areas that are subject to strong
wave action. Waves carry food, nutrients, and oxygen to the reef; distribute coral larvae; and
prevent sediment from settling on the coral reef. Precipitation of calcium from the water is
necessary to form a coral polyp's skeleton. This precipitation occurs when water temperature and
salinity are high and carbon dioxide concentrations are low. Corals are major contributors to
the physical structure of coral reefs that develop only in tropical and subtropical waters.

OTHER LIFE ON CORAL SEAS

Coral reef zones are home to one quarter of all marine plants and animals.
Nearly a million species of fish, crabs, eels, mollusks, sponges, worms, grasses, algae, and other
marine animals live on reefs and use them as nurseries to protect their young.
Some inhabitants of coral reefs are the dugong (sea mammal), seahorse, moray eel, octopus, sea
turtle, crown-of-thorn starfish, giant clam, parrotfish, clownfish, lionfish, and scorpion fish.
1. Crustaceans or joint-legged animals: crab, shrimps and lobsters.
2. Conchs and other reef dwellers : snails, clam, Queen conch, Helmet conch, Fireworm.
3. Sponges : Basket sponge, Tube sponge, Vase sponge, Brush sponge, Red fire sponge and
Loggerhead sponge.
4. Reef Fish : The Grunt, the Snapper, Mutton snapper. The parrotfish, Angelfish, Goatfish,
Porcupine fish, Barracuda, Eel, Scorpion fish etc.


4. GLOBAL DISTRIBUTION OF CORAL REEFS

The majority of reef building corals are found within tropical and subtropical waters. These
typically occur between 30
0
north and 30
0
south latitudes. The red dots on this map show the
location of major stony coral reefs of the world.

The Indonesian/Philippines archipelago is the world’s greatest concentration of reefs and the
greatest coral diversity. Other area of reef concentration are found in the Great Barrier Reef of
Australia, the Red Sea and the Caribbean. Their diversity is more in all major Indo-Pacific
regions. This map shows the distribution of the world’s coral reefs (red). Various species of
corals are found in all oceans of the world, from the tropics to the polar regions.

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Coral reefs are generally found within 30*N and 30*S latitudes. Reef-building corals are
scattered throughout the tropical and subtropical Western Atlantic and Indo-Pacific oceans.
These regions are generally located within 30 degrees N and 30 degrees S latitudes.

There are two major zones:
a) the Western Atlantic reefs. These include areas such as Bermuda, the Bahamas, the
Caribbean Islands, Belize, Florida, and the Gulf of Mexico.
b) The Indo-Pacific ocean region. This region extends from the Red Sea and the Persian
Gulf through the Indian and Pacific oceans to the western coast of Panama. Corals grow
on rocky outcrops in the Gulf of California.

Let us see the world’s largest coral reefs.

1) The Great Barrier Reef is one of the largest reefs in the world based on both area
(134,363 square miles or 348,000 sq km) and length.
The others are:
2) Red Sea Coral Reef- in Red Sea near Israel and Egypt.
3) New Caledonia Barrier Reef- in the Pacific Ocean near New Caledonia
4) The Mesoamerican Barrier Reef- in the Atlantic Ocean near Mexico, Belize, Guatemala
and Honduras
5) Florida Reef- in the Atlantic Ocean and Gulf of Mexico near Florida
6) Andros Coral Reef- in Bahamas between the islands of Andros and Nassau
7) Zhongsha Islands- in the South China Sea
8) Saya Del Malha in the Indian Ocean
9) Great Chagos Bank in the The Maldives
10) Reed Bank in the South China Sea, claimed by the Philippines.


3. PROTECTION OF CORAL REEFS.

Coral reefs provide a natural habitat and massive protection for different species of fish.
They are not plants; they are living organisms that provide an environment for fishes to breed.
Without them, fishes in the ocean are homeless. Humans continue to represent the single
biggest threat to coral reefs. In particular, land-based pollution and over-fishing are the most
serious threats to these ecosystems.

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The live food fish trade has been implicated as one driver of decline due to the use of cyanide in
the capture of fish.
High levels of land development have also been threatening the survival of coral reefs.
Due to the increased demand for live reef fish in North America and Europe, the use of cyanide
for fishing has increased in the Indo- Pacific region. 85% of the of the world’s aquarium fish
are caught in this region and almost all of them are caught using cyanide. Cyanide is used to
stun the fish, in order to easily capture them for trade.
It is detrimental to the organs of fish, which would explain the 90% mortality rate of cyanide
captured fish.
Cyanide is also very destructive to the surrounding coral reef ecosystems. It kills corals and other
reef invertebrates. Corals are also harmed by the poor harvesting practices of the live fish trade.
Fishermen sometimes pound on the reef with crowbars and rocks to scare fish into nets or pry
corals apart to retrieve stunned fish.
Dynamite fishing is another extremely destructive method that fishermen use to harvest small
fish.
The procedure of dynamite fishing starts with a bottle that is filled with explosives made of
potassium nitrate, once the dynamite goes off the explosion brings about an underwater
shockwave causing the swim bladders of fish to burst making them float to the top. A second
blast is often set off after the first to kill any larger predators that are attracted to the initial kill of
the smaller fish.

This method of fishing does not only kill small fish but also claims the lives of many reef
animals.

During the 1998 and 2004 El Niño weather phenomenon, in which sea surface temperatures rose
well above normal, many tropical coral reefs were bleached or killed.

Coral Bleaching:

Warmer water temperatures can result in coral bleaching. When water is too warm, corals will
expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white.
This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a
bleaching event, but they are under more stress and are subject to mortality. In 2005, the U.S.
lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event.

Coral Diseases
* Coral diseases are rapidly spreading worldwide.
* Temperature changes such as El Nino are made responsible for lowered resistance of corals
to infections.
* Diseases without pathogen include Tissue Bleaching, White Band Disease, White Plague,
and White Pox.
* Diseases with pathogen include Black Band Disease, Red Band Disease, Black aggressive
Bands, Lethal organs disease, Yellow Band Disease, and more.

It is estimated that about 60% of the world’s reefs are at risk due to destructive, human-related
activities.

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The threat to the health of reefs is particularly strong in Southeast Asia, where an enormous 80%
of reefs are considered endangered.

Coral reefs are truly one of the most beautiful biomes. Future generations may not have the
opportunity to see them. Marine biologists who monitor reefs say that 10 percent of them are
already destroyed, and 70 percent could be gone in just 40 years from now unless we change our
ways. Humans should not destroy them for any reasons. A thing of beauty is joy forever. The
beautiful corals are the wonderful creations of nature. Let us protect corals.