> Learning About Reefs > About Coral
and Coral Reefs
Scientists divide natural objects
and living things into several categories—animals,
plants, fungi, bacteria, and protists. Although corals
contain protists and also make large mineral structures
that may be considered non-living, scientists have determined
that corals should be classified as animals.
One of the most critical distinctions
between plants and animals is that plants make their
own food, while animals depend on outside sources for
their nutritional requirements. Coral animals take advantage
of both forms of nutrition by hosting plant-like algae
(protists) in their tissues. The majority of the coral’s
energy needs are provided by tiny algae called zooxanthellae,
which live inside the coral and produce food using the
sun’s energy from carbon dioxide and water. The
protist algae and the coral animals live together in
a symbiotic relationship. This partnership allows corals
to live in nutrient-poor environments where sunlight
is a plentiful source of energy and the supply of planktonic
food is limited. Corals also use their tentacles to
capture tiny marine animals called plankton. At night,
the corals’ tentacles extend from their bodies
and wave in the water, collecting plankton to eat.
Corals are invertebrates; like shellfish,
they have no spinal column or internal bones. Coral
colonies are composed of many tiny cup-shaped animals
called polyps. Millions of polyps working together in
a cooperative colony generation after generation create
the limestone skeletons that form the framework of the
There are two general types of corals—hard
corals and soft corals. Hard corals take calcium and
carbonate out of seawater and turn it into an external
skeleton that forms beneath their tissues. This external
skeleton is deposited on top of the existing skeleton;
the corals grow upward and outward on top of this skeleton.
Corals are mainly colonies, budding new animals as the
thin layer of tissue on top of the external skeleton
expands in area. The process by which it is created
is similar to that used by clams, oysters and snails
to create their hard shells. Only the outer layer of
the reef is alive, as coral colonies grow and expand
and new corals build their skeletons on those of dead
corals from earlier generations. Millions of corals
have built colonies in this way over thousands of years
to form the structure of coral reefs we see in the oceans.
Soft corals, which look more like plants, are also animals.
The symbiotic algae that live inside
corals are one-celled protists called zooxanthellae.
They are sensitive to high light, variations in the
concentrations of salt, and especially, to high temperatures.
The algae use sunlight for photosynthesis, the process
all plants use to convert energy from the sun into food
energy. Corals (and the algae that live in the corals)
can only grow in a few regions of the world where the
water is shallow enough for sufficient sunlight, where
there is not too much silt in the water, and where the
water temperature is warm enough---but not too warm.
Corals live in areas where the temperature ranges between
25° and 29° Celsius (77° to 84° Fahrenheit).
Because they are sensitive to changes in these conditions,
corals are at risk of being damaged or destroyed. Corals
also face natural threats such as hurricanes, and the
impact of human activities such as over-fishing and
destructive fishing practices, coastal development and
Planet Earth is often called the “ocean planet”
because more than 70 percent of the earth’s surface
is covered by salt water. While there are very few places
left on the surface of the earth that have not been
explored, most of the ocean has not been visited by
humans – mainly because of its depth and inaccessibility.
The average depth of the ocean is more than 3,657 meters
(12,000 feet) and the deepest spot in the world is the
Marianas Trench in the Pacific Ocean, which is 10,911
meters (35,798 feet) deep. (Compare this with Mount
Everest, the planet’s tallest mountain, which
is 8,848 meters (29,029 feet) high. The seas are also
teeming with life. There is no ocean environment with
a greater diversity of species than tropical coral reefs.
Ecologists classify coral reef environments as an ecosystem
– a living community of plants and animals and
their physical environment connected by a vast and intricate
web of food chains. In the Coral Reef Adventure
film, viewers can see a broad diversity of tropical
reefs in the Pacific Ocean.
Coral reefs are beautiful vibrant
underwater cities, home to one hundred thousand different
species of sea creatures. Some, such as Australia’s
2,000-kilometer (1,243-mile) long Great Barrier Reef
are so large that they can be seen from outer space.
From the air, coral reefs may form large patches of
blue, green and brown shapes. The deep-water areas appear
dark blue in color while the shallow sandy patches appear
light blue-green. The living reef structure appears
brown. Up close, the thin veneer of living organisms,
both plants and animals that form the coral reef, is
visible in many colors.
Coral reefs are only found in tropical
and sub-tropical waters around the world. However, since
ocean currents can bring warm water to cooler places,
reef corals grow in surprising locations such as off
the coast of Texas and near Tokyo, Japan.
Coral reefs in different parts of the
world vary in the number and composition of coral species.
The number of coral species is highest on the Indo-Pacific
reefs featured in Coral Reef Adventure. For
example, the number of reef-building coral species on
the Great Barrier Reef exceeds five hundred whereas
a typical Caribbean reef has no more than fifty species.
A subject of great debate among scientist is why no
species are found in both the Caribbean and Pacific
Oceans – each has its own unique fauna.
ON CORAL REEFS
Coral reefs provide habitat for almost all forms of
life including fish, crustaceans (like crabs and lobsters),
seaweeds, reptiles, bacteria and fungi. As seen in the
film, coral reefs attract a diverse array of organisms,
each carving out its own home, or niche. A coral reef
is like a city. Little crabs dig their holes, fishes
find crevices to hide in, some animals even live on
top of other animals; every inch of the reef is covered
with life and every form of life has a job to do. For
example, fishes that eat seaweeds and other algae are
called herbivores, and herbivorous fishes constantly
crop the reef of fast-growing seaweeds that could smother
coral polyps. Many fishes such as jacks (Trevally) even
“commute” onto the reef from deeper water
in search of food or even cleaning. This is shown in
Coral Reef Adventure when the potato cod is being
cleaned, and by the cleaner shrimp in Michele’s
mouth. All of these organisms live in a delicate balance.
Despite covering less than 0.2 percent of the total
area of oceans, coral reefs are noted for some of the
highest levels of total productivity on earth and house
25 percent of all species of marine creatures.
TYPES OF REEFS
The famous biologist Charles Darwin was
the first to describe how the three main types of coral reefs
— fringing reefs, barrier reefs and atolls — are
formed. Fringing reefs hug the coastline and barrier reefs
grow farther away from the coast, with a lagoon in-between.
Darwin theorized that fringing reefs form on the sides of
islands that are sinking (called “subsidence”
by geologists) over many thousands of years. As the islands
sink to depths where reef-building corals cannot grow, the
fringing reefs turn first into barrier reefs and finally,
when the island sinks beneath the waves, only a doughnut-shaped
atoll is left. On reefs with a lagoon and an open ocean side,
the ocean-facing side is usually more varied in coral composition
than the lagoon side. Howard and Michele Hall made their dives
in Australia on a barrier reef, in Fiji and Tahiti on fringing
reefs and in Rangiroa on atoll reefs.
If a fringing coral reef was sliced like a
cake, looking at the side or profile, there would be a gently
sloping shallow Reef Flat near the shore. There is often a
lot of sand on the Reef Flat interspersed with patches of
bare rock, rubble and living coral. Moving farther out to
sea, the seabed rises until it reaches the area called the
Reef Crest where the ocean waves break on living corals. This
surf zone is an area of high physical energy and biological
activity. The reef corals that live here are adapted to the
high-energy environment and have high growth rates. Storms
can destroy them. Thus the area just landward of the Reef
Crest is often covered with broken fragments of corals and
accounts for the rise of the seabed towards the crest. Seaward
of the Reef Crest, the seabed slopes downward – sometimes
in wide terraced steps and other times with a steady downward
slope to about 9 to 18 meters (30 to 60 feet) in depth. In
this depth range on many reefs the reef profile suddenly “drops
off” heading almost straight down – for perhaps
30 meters (98 feet) or more. This is why scuba divers call
this area the “drop off.” In Coral Reef Adventure,
the deep dive Howard and Richard made was on such a steep
walled reef. Live reef-building corals are not found below
100 meters (328 feet); most do not occur below 30 meters (98
HOW CORAL REEFS
Old coral reefs may be over 30 meters
(98 feet) thick, but the living part is only a thin veneer
of corals and other organisms, perhaps only a meter (about
3 feet) thick on the surface. As the reef grows upwards, the
older parts die and are built upon. The oldest fossil reefs
in the world existed more than 150 million years ago. Some
of these fossil reefs are found in surprising places where
the ocean used to be – such as a farmer’s field
in Vermont, USA! Most living reefs are less than 25,000 years
old. When scientists want to know the history of a reef, they
can drill down through it, often for over 304 meters (1000
feet) and bring up the “cores” for detailed study
and carbon dating. Carbon dating is a way to tell how old
the reef is. By drilling and by studying places where old
fossil reefs have been exposed, such as construction sites,
geologists can also study the structure of the reef and determine
how the reefs were formed. Coral reefs grow like high-rise
buildings are built – the skeletons of large corals
form the frame of the building while the sand and coral rubble
fill in between the frame
WHAT IS A CORAL?
While there are many different kinds of corals of different
shapes and sizes, all corals are invertebrate animals and
most live in colonies consisting of many interconnected individuals
called polyps. Corals are tiny animals, called polyps, that
are related to and look like sea anemones. Each coral secretes
a stony cup of limestone around itself as a skeleton. The
polyps divide as they grow and form coral colonies. As the
coral colonies build up on top of each other, they gradually
form a coral reef. There are over 4,000 different coral species.
The largest reef in the world, Australia’s Great Barrier
Reef, alone is home to 700 different coral species, but not
all of these grow fast enough to help build the reef.
As seen in Coral Reef Adventure, there
are several types of corals including soft corals, sea fans
and hard corals. Soft corals look something like asparagus
or broccoli and feel like soft shoe leather. Their skeleton
has been reduced to tiny rods that are found throughout their
tissue. Soft corals are some of the most beautiful animals,
featuring brilliant red and orange colors. Sea fans (known
as gorgonians by scientists) have a wiry, flexible internal
skeleton and lots of tiny polyps on the branches of the flat
fan-shaped structure. In Coral Reef Adventure, the
divers go behind one huge red sea fan that is several meters
(about six feet) across. About 600 species of hard corals
are called reef builders because they grow fast enough to
form the reef framework. Millions of corals have built their
skeletons in this way over thousands of years to form the
massive coral structures seen in the film. Some coral colonies
are very old; many are hundreds of years old.
All corals are animals,
but like lichens on land, which are part fungi and part algae,
they have a wonderful symbiotic relationship with algae. (In
this type of symbiosis, two different organisms each benefit
from living closely together.) Microscopic algae called zooxanthellae
(pronounced “zo-zan-thel-ee”) live within the
internal tissue of each coral polyp. The zooxanthellae are
the corals’ “solar panels,” providing the
reef building corals with enough energy to extract calcium
from the seawater to build their skeletons rapidly. The zooxanthellae
use sunlight for photosynthesis and produce oxygen and sugars
(like an energy bar!) which the corals can use. The coral
animal produces carbon dioxide and waste products (nutrient-rich
fertilizer) that the zooxanthellae use. This symbiosis is
so successful that fast-growing species of branching corals
can grow up to 20 cm (8 inches) per year in shallow sunlit
waters. The zooxanthellae are so numerous in the coral tissue
(up to several million per square centimeter) that they give
many corals their characteristic greenish-brown color. Coral
species that do not form symbioses with algae grow very slowly
and do not get very big; hence they do not contribute much
to the framework of coral reefs. Many other reef animals in
shallow sunlit waters, including anemones, soft corals and
giant clams, also have symbiotic relationships with these
algae. Since reef-building corals require sunlight to grow,
coral growth is limited by such factors as muddy water and
great depth. For example, there are no coral reefs below about
200 feet (61 meters) in Fiji – there is not enough light
for the zooxanthellae to carry out photosynthesis. This zone
is dominated by non-reef-building corals.
The “Coral Reef City” lives
in balance with the “Suburbs” -- seagrass beds
and mangrove forests that are usually located closer to shore.
Seagrass and mangrove ecosystems serve as nurseries for many
fish and shellfish species which later migrate to reefs; many
reef creatures spent their formative years in these other
habitats, protected by their calm waters and nourishing nutrients.
Some creatures continue to travel between the coral reefs
and seagrass beds their entire lives, transporting nutrients
between these ecosystems and connecting them. Mangrove trees
live at the boundary between land and sea and may grow to
15 meters (50 feet) high. Their root systems form an important
underwater ecosystem for marine organisms. Mangroves require
some fresh water, so are found near river mouths. Seagrass
beds are strictly marine and are often found in shallow protected
bays. Both mangroves and seagrasses help the reefs by trapping
terrestrial sediment that could smother the corals and reduce
the light needed by the zooxanthellae. Coral reefs protect
these habitats from the eroding action of waves.
The tropical reef ecosystems are part of a
much larger marine habitat — the open ocean. A coral
reef is like an oasis in a desert. Many important open ocean
or pelagic fish such as tuna spend some time of their life
on or near a reef to feed on reef fish or to reproduce. For
example, the largest shark in the world, the whale shark,
eats small free-floating microscopic organisms called plankton
and, during part of the year, corals and reef fish larvae.
Learning about the connections in coral
reefs and among different marine ecosystems can help us understand
and appreciate other important marine habitats. If one habitat
is destroyed by pollution, for example, there are consequences
for other ecosystems because of these interconnections.
THE TROPICAL REEF ECOSYSTEM
People as well as marine creatures depend
on tropical marine ecosystems. In the film, Howard and Michele
Hall visit Fiji where the local Melanesian people are intimately
connected with their coral reefs. The Fijians have a taboo
system run by the Chiefs, which determines when certain types
of fish can be caught and where. This traditional reef management
system has worked well for thousands of years and has allowed
the Fijians to survive without over-harvesting their reefs.
The reefs give them fish for food, protection from storms
and rough seas, income from tourists that visit their island
to witness the beauty, and are part of their culture through
ceremonies to thank the reefs and waters. The Fijians are
just one of hundreds of cultures worldwide that rely directly
on the tropical marine system for survival. Even in spite
of the Fijians’ efforts to manage their coral reefs,
the reefs are still vulnerable to the impact of other human
activities like industrial development and global climate
Coral reefs provide many benefits to humans.
Throughout Southeast Asia, coral reef fisheries provide ten
to 25 percent of the protein for people living along the coast.
Coral reefs also have high economic value. Tourism is the
largest industry in the world and tropical destinations such
as Cancun and Jamaica rely on their coral sand beaches to
attract tourists. In the Indian Ocean, almost eighty percent
of the islands are built exclusively of reef material. For
the 20 million scuba divers of the world, a tropical reef
diving holiday is high on the ‘must do’ list.
Increasingly, coral reefs are largely
untapped resources for future pharmaceutical breakthroughs.
The best-known example of coral reef’s medicinal value
is AZT, a treatment for people with HIV infections, which
is based on chemicals found in a Caribbean reef sponge. Also,
more than half of all new cancer drug research focuses on
BY CORAL REEFS
Hurricanes, typhoons, El Niño, coral-eating
organisms, and diseases are natural disasters coral reefs
have been facing for millions of years. However, the imbalances
these events create are temporary and can actually help the
tropical ecosystem to maintain diversity. For example, hurricane
storm waves are particularly devastating for tall, branching
corals. However, broken fragments of branching corals can
start to grow where they land, creating a new colony. These
corals are fast growing and quickly form new reef framework
under suitable environmental conditions. Coral-eating creatures,
naturally kept in low numbers by predators, appear to help
the reef by opening bare space where new coral recruits can
settle and have a chance to grow, maintaining the diversity
of the reef.
Truly devastating threats to coral reef health
come when the impacts of natural events are compounded by
persistent human-induced stressors that include over-fishing
and destructive fishing practices, coastal development, sewage
and other pollution, and rising global temperatures.
Over-fishing and destructive fishing practices
According to the Reef Check monitoring program,
over-fishing is a major problem for coral reefs — and
for the ocean in general. Over-fishing works in a step-by-step
fashion to cause imbalances. First, fishing selectively takes
larger, predatory fish off the reef causing population explosions
of smaller herbivorous fish. When the larger fish become scarce,
the herbivorous fish are then targeted by fishermen. Without
the herbivores, seaweeds can over-grow the corals and smother
them. Destructive fishing practices include the use of cyanide
to stun fish for capture for the marine aquarium and live
fish trades and blast fishing to kill fish for food. These
fishing methods are not usually species-specific. Many organisms
are killed in the process and habitats that took thousands
of years to build are destroyed.
Coastal development threatens the reefs in
a number of ways. Tropical forests have a thin layer of soil
that is kept in place by dense forest. When land is cleared
for construction, this layer erodes quickly with any rainfall
and is transported by rivers to the sea. In areas with pristine
mangrove forests and seagrass beds, most of this sediment
will be trapped before reaching the reefs. But coastal construction
often removes mangroves and seagrass beds, which takes away
the sediment barrier (and also removes the reef’s nursery).
Excess sediments cover corals, blocking the light necessary
for their symbiotic zooxanthellae and smothering polyps. Nutrients
from sewage pollution disrupt the balance between corals and
faster growing seaweeds. Under natural conditions, corals
are able to out-compete seaweeds because of the low nutrient
content in tropical waters. When outside nutrients are added,
faster growing seaweeds can take over and smother corals.
Pollution adds all sorts of unnatural
and potentially harmful substances to the reef system including
nutrients, pathogens and trash. Pollution has been implicated
in the apparent surge in coral diseases, especially in the
Global Warming/Rising Sea Temperatures
One of the most global threats to coral
reef ecosystems that is highlighted in Coral Reef Adventure
is rising seawater temperature. If the temperature is raised
above a critical level, the zooxanthellae leave, causing corals
to appear brilliant white. This condition is called “bleached”.
Prolonged bleaching can lead to coral death. According to
a report submitted to the US Coral Reefs Task Force in 1999,
“coral bleaching is most often associated with a significant
rise in sea surface temperatures. Activities such as the burning
of fossil fuels, changes in land use, and reduction in forest
cover are increasing the atmospheric concentrations of greenhouse
gases (e.g., carbon dioxide), altering radiative balances
and warming the atmosphere. Global climate change poses an
increasing threat to coral reefs. An increase in carbon dioxide
(CO2) in the atmosphere can reduce the ability of corals to
form limestone skeletons, slowing their growth and making
them fragile. Global mean sea-surface temperatures are projected
to increase approximately one to two degrees Celsius by the
year 2100. If the frequency of high-temperature episodes increases
as mean temperature gradually rises, corals will experience
more frequent and widespread disturbances.”
The human stresses on coral reefs occur at
every level: locally, through over-fishing and coastal development,
regionally through pollution affecting rivers, and globally,
through global warming which raises the temperature of the
ocean water. At each level, there are ways to reduce human
impacts and help corals flourish once again. Everyone can
have a hand in coral reef protection, from coastal towns choosing
to develop eco-tourism-based economies to people choosing
to eat sustainably caught fish.
The most important way to reverse the damage
to coral reefs is through what you are doing right now: education.
Learning about how our actions affect coral reefs and the
marine habitats we rely on is the first step to understanding
how important it is to protect these amazing ecosystems. Coral
Reef Adventure is also helping to bring these messages
to people everywhere. As Jean-Michel Cousteau says in the
film, we need to “teach our children [and adults] well”
and show them the beauty that is a coral reef.
Global Climate Change and our Life Styles
As the climate continues to warm, coral
bleaching events may become more frequent and severe. Even
reefs that are protected as marine sanctuaries are threatened
by global climate change. Greenhouse gas emissions are a major
contributor to global warming. Here are a few things that
you can do to help reduce greenhouse gas emissions:
|| Conserve energy
||Buy a fuel-efficient car and drive less often
||Reduce, re-use and recycle
||Contact your government representative and vote responsibly
on global warming issues
||Review the “Ways to Help Coral Reefs” section
of Websites of Interest on this website for more information
about everyday things that you can do to help keep coral
Coral reef management is a huge job that involves
identifying the threats facing coral reefs and developing
ways to monitor the reefs and human activities that affect
them. Learning how to monitor reefs through Reef Check training
is one example of how communities in tropical coastal areas
can be involved coral reef conservation. For details, go to
Monitoring can be an important first step in raising awareness
and changing destructive local and regional practices.
Marine Protected Areas
Creation of Marine Protected Areas (MPAs)
is a promising strategy that is being implemented worldwide
to protect biodiversity and help local communities manage
their resources. These are areas that regulate fishing and
extraction methods. The Great Barrier Reef Marine Park is
the largest coral reef MPA in the world, and the large size
and high abundance of fishes seen in the film are due to the
fact the Great Barrier Reef has been protected since the early
1970s. When allowed to recover, corals attract fish and the
fish spill over outside the MPA boundaries. MPAs also include
the added benefit of involving the local community in the
design, implementation, and enforcement of protected areas.
Groups are also working on creating better sustainable fishing
options, replanting lost mangroves, and diverting sewage from
being dumped into coastal marine habitats.
St. Lucia’s Marine Reserve
St. Lucia is an island in the Caribbean that
experienced increasing population pressures during the 1980s.
These threatened the health of St. Lucia’s fringing
reefs primarily due to pollution and over-fishing. In response,
government agencies and community groups formed the Soufriere
Marine Management Area (SMMA), a marine reserve, in 1994.
Since the reserve was created, fish populations within the
management area have tripled. For more information see http://www.smma.org.lc
Vast areas of Philippine reefs are destroyed
due to intense blast and poison fishing. Nowhere is this situation
worse than on the island of Cebu in the Central Visayas. After
conducting a Reef Check survey, local residents realized the
terrible state of their reefs and made a decision to take
action. With help from additional service organizations, the
private sector and government agencies, the community established
a marine-protected area at Gilutongan Island in Cebu. In late
2001, only two years later, thousands of colorful reef fish
have returned to Gilutongan Reef. These fish are reproducing
and repopulating the surrounding damaged reefs. Additionally,
by charging divers a small visitor’s fee, the marine
protected area raised over $20,000 in 2001, a significant
source of income for the small island population.
FREQUENTLY ASKED QUESTIONS
What Is A Coral?
Despite the fact that corals look more like
rocks or plants, they are definitely animals. Coral colonies
are composed of many tiny, cup-shaped animals called polyps,
which are related to jellyfish. A single coral polyp may be
as large as a saucer or smaller than the head of a pin. Millions
of polyps working together in a cooperative colony generation
after generation create the limestone skeletons that form
the framework of the beautiful coral reef.
How Do Corals Start Out Life?
Corals begin life in tropical waters as free-floating
larvae. After a relatively short period of time, the larva
eventually attaches itself to a hard surface and becomes a
polyp. Polyps divide asexually and form colonies. Coral colonies
reproduce both sexually and asexually. In sexual reproduction,
the coral polyps release both eggs and sperm into the water.
(This is also known as coral spawning.) One type of asexual
reproduction occurs when fragments of coral are broken off
as a result of storm action. The broken pieces of corals usually
survive and continue to grow and produce a new colony. This
process is referred to as “fragmentation”.
What Do Corals Eat?
A coral polyp consists primarily of tentacles,
a mouth and a gut (think upside down jellyfish). Many corals
are passive feeders on plankton. Most corals also get nutrition
from microscopic algae (zooxanthellae) living within their
tissue. Coral polyps are generally nocturnal feeders and are
provided sugars made by their photosynthetic zooxanthellae
during the day.
Where Does The Framework Of A Coral Reef
Corals extract calcium and carbonate from seawater
to build an inner skeleton that is external to the coral.
This external skeleton lies underneath a thin layer of tissue.
Over the years millions of coral polyps in colonies create
the framework of the coral reef. Coral reefs grow very slowly.
It may take up to a hundred years for a coral reef to grow
one meter (around three feet).
What Is The Difference Between Hard And
Hard corals, also called reef-building corals,
produce a rock-like skeleton made of the same material as
classroom chalk (calcium carbonate). These skeletons and the
various shapes of different colonies form the familiar structure
of the reef. Hard corals rely on symbiotic algae (zooxanthellae)
living within their tissues for nutrition and energy to build
their skeleton. They must therefore live in shallow clear
water to allow sunlight to reach the algae. Soft corals look
like colorful plants or graceful trees and are not reef-building
since they do not produce the hard calcified skeleton of many
reef-building corals. However, soft corals do produce smaller
amounts of calcium carbonate that help them keep their shape.
Soft corals can be distinguished from hard corals by the fact
that soft coral polyps always have eight tentacles, while
hard coral polyps have multiples of six tentacles.
What Is Symbiosis?
Symbiosis is defined as the close association
between two or more interacting organisms, usually of different
species. The relationship is usually classified as belonging
to one of three types: mutualism (benefiting both partners),
parasitism (one partner, the parasite, benefits, at the expense
of the host), or commensalism (one partner benefits while
the other is unaffected). Changes in the physical environment
such as the amount of sunlight or salinity, or the temperature,
and in the biological community, such as the presence or absence
of other organisms and how they interact with the symbiotic
pair, may change the nature of the symbiotic relationship
from one type to another. Like organisms, symbiotic relationships
are responsive to the environment and can change over time.
What Is The Largest Coral Reef In The World?
As the name implies, the Great Barrier Reef,
located off Australia’s East Coast is the largest coral
reef in the world. This enormous reef is over 2023 kilometers
(1257 miles) long and covers more than 300,000 square kilometers
(about 186,000 miles). Home to more than 1500 species of fish,
dolphins, whales and sea turtles, the Great Barrier Reef is
actually a collection of more than 3000 smaller reefs. The
second largest reef lies off the coast of Belize, in Central
What Are The Main Types Of Coral Reefs?
Reefs are generally classified into the following
|| Fringing reefs,
the most common type of reef, form along a coastline.
They grow on the continental shelf in shallow water.
||·Barrier reefs grow
parallel to shorelines but are farther from shore and
are usually separated from the land by a deep lagoon.
They are so called because they form a barrier between
the lagoon and the seas, protecting the coastline.
||Coral Atolls are rings of
coral reef growing on top of old sunken volcanoes in the
ocean. They begin as fringing reefs surrounding a volcanic
island; then, as the volcano sinks, the reef continues
to grow, and eventually only the reef remains. There are
over 300 atolls in the South Pacific. Atolls contain islands.
Do Any Animals Eat Corals?
One of the most important predators of corals
is the Pacific Ocean’s Crown of Thorns Sea Star. It
is estimated that a single Crown of Thorns Sea Star can eat
from 2 to 6 square meters (6 to 20 square feet) of corals
per year. Many fish species such as parrotfish, butterfly
fish and tangs also include corals as part of their diet.
Attentive divers and snorkelers hear the crunch of hungry
parrotfish as they chew up their delectable meal that includes
the skeleton. Other coral predators include some types of
marine snails and marine slugs, known as nudibranchs. Interestingly,
these coral predators digest the animal tissue and release
the symbiotic algae (zooxanthellae) in their feces. The nudibranchs
may also keep the nematocysts (stinging structures) and symbiotic
algae for their own nutritional use.
Why Are Coral Reefs Important To Humans?
Coral reefs are among the most biologically
diverse ecosystems on earth. Second only to tropical rain
forests in the number of species they harbor, they are sometimes
called the “rainforests of the sea”. Although
coral reefs only occupy about 0.07 percent of the ocean floor
(an area roughly the size of Texas), they are home to as many
as one quarter of the world’s marine species. Coral
reefs offer important income sources for their human neighbors
through tourism and fishing, which provide both subsistence
and trade. Recently, scientists have begun to discover that
coral communities may contain valuable medicines that may
one day lead to treatments for cancer and HIV. For coastal
communities, coral reefs also play an important role in protecting
their coastlines from storms.
Why Are Coral Reefs In Danger?
Coral reefs are among the most beautiful ecosystems
in the world but are also among the most susceptible to human
impacts and are damaged or destroyed with alarming ease. Practices
such as over-fishing, the use of dynamite or poison to capture
fish and dropping boat anchors on corals have produced enormous
damage. Even an accidental touch from divers and snorkelers
can significantly damage the delicate coral polyps. Pollution,
silting from land-based construction, and fertilizer runoff
have led to damage to coral reefs worldwide by blocking the
sunlight corals require for photosynthesis by their symbiotic
algae. Rising sea temperatures from global warming can also
destroy corals by ending the symbiotic relationship with zooxanthellae.
Hurricanes and earthquakes, which can also lead to significant
damage to the reefs, are nonetheless generally viewed as a
natural cycle of the ecosystem. However, when a coral reef
has been damaged from human effects, it may have a more difficult
time recovering from natural disasters such as hurricanes
ISSUES OF IMPORTANCE
A major environmental threat to coral reefs
is increased temperature of ocean water. One of the natural
causes of increased ocean temperature is El Niño, a
huge weather pattern that affects the trade winds in the Pacific
Ocean and changes rainfall patterns around the world. The
increased temperature puts stress on the highly sensitive
corals, causing them to expel the algae that live inside the
coral polyps. This process makes the white coral skeleton
visible, and is called bleaching. Corals can survive occasional
bleaching incidents, but they cannot recover from repeated
or prolonged stress because they depend on the symbiotic algae
for nutrition. If the algae are deprived of sunlight for too
long and the coral polyps cannot capture enough plankton to
survive, the corals will eventually die.
Another form of stress that the natural environment
causes for corals is related to the depth of the ocean. Because
the algae inside coral polyps depend on sunlight, corals cannot
survive at depths where sunlight cannot reach. It is believed
that if global warming causes the polar ice caps to melt,
ocean depths will increase more rapidly than corals can grow
preventing corals from getting the sunlight they need to live
and make their skeletons.
Although these environmental risks raise concern
for coral reefs, there are some indications that corals may
be able to adapt to some forms of stress. In one study cited
in National Geographic, July 2001, scientists found evidence
that corals might possibly be able to replace their zooxanthellae
with different strains of zooxanthellae better able to survive
under the new environmental conditions.
The Pacific Marine Environmental Laboratory, a division of
NOAA, has a very informative web page with more details about
El Niño, including an animated image and maps. It can
be found at http://www.pmel.noaa.gov/tao/elnino/el-nino-story.html
Full text of the National Geographic
article can be found at http://news.nationalgeographic.com/news/2001/07/0725_coralbleaching.html
Impact Of Human Activities On Coral Reefs
There is a long list of human activities that
can negatively impact the health of coral reefs, beginning
with destructive fishing methods. Blasting dynamite near the
reef to bring fish to the surface can destroy the entire reef,
reducing it to rubble. Another destructive practice involves
divers squirting cyanide onto tropical fish to stun them and
take them for collectors. In addition to unnecessarily killing
many fish, the cyanide poison damages live coral polyps in
Tourism also has direct and indirect effects on coral reefs.
Snorkelers and divers who are not trained in proper behavior
around reefs can trample delicate soft corals, damage reef
structures and injure fish by attempting to feed them. Anchors
from boats can drag across the surface of the reef, breaking
or damaging parts of the structure. Some divers break off
pieces of living hard corals or uproot soft corals to take
as souvenirs of their trip. Many tourism companies and environmental
organizations have developed programs to educate reef visitors
to protect reefs. Divers are encouraged to treat coral reefs
with care and respect.
The basic problem of over-fishing permanently
changes the ratios of fish species in the area of the reef,
affecting food webs and the entire ecosystem. Removal of herbivorous
fishes may allow seaweeds to overgrow corals, blocking sunlight
and access to planktonTop
An indirect impact of tourism is caused by
over-development of coastal areas near reefs. In some locations,
shoreline property has been filled in to provide solid ground
for building or harbors have been dredged to open waterways
for large ships. Deforestation of inland areas causes runoff
into waterways that lead to the ocean. All of these practices
produce enormous amounts of particles in the water. The suspended
particles smother corals and prevent sunlight from passing
through to the zooxanthellae in corals and seaweeds that keep
the reef alive.
Development along coastlines may also lead
to increased amounts of sewage being dumped into the ocean.
This material changes the nutrient balance in the water and
can cause plankton and seaweed blooms. Fertilizer runoff and
water pollution from inland waterways also contribute to the
problems. Coastal power plants release large amounts of water
into the ocean, often raising local water temperatures and
stressing marine life.
Medical and Scientific Research
Coral reefs contain many species that make
chemicals that might be used to relieve pain or even cure
diseases. Doctors and scientists are studying many forms of
coral life for their medical potential. From natural poisons
that can be used as painkillers to natural sunscreen compounds
in corals that can prevent sunburns, researchers are finding
numerous benefits to humans in coral reefs.
The Australian Institute of Marine Science
has patented a sunscreen product based on the natural sun
protection chemicals found in corals. The product, which is
still being tested, absorbs ultraviolet light efficiently
without irritating skin or causing an allergic reaction. Scientists
are able to create the substance synthetically so that they
do not have to damage existing reefs to extract it. Another
coral reef species that is being investigated for skin care
properties is the soft coral sea whip. Compounds called pseudopterosins
may be able to reduce swelling and relieve irritated skin.
Scientists at the U.S. Food and Drug Administration have developed
a method of using a coral product called hydroxyapatite (HA)
as a bone-building substance for people with broken bones.
The material can be injected into the area of the break. HA
has similar properties to human bone and fuses to the existing
bone, forming a strong bond over time. HA was approved by
the FDA in 1992.
Even some poisonous creatures whose toxins
are highly dangerous in their natural environment might be
adapted for medical purposes. A snail that lives near coral
reefs contains a chemical poison that might be useful as a
painkiller. Studies have shown that the poison might be even
stronger than morphine in relieving pain. Medicines derived
from the chemical structure of the poison may have great potential
for patients suffering from severe pain.
Full text of sunscreen article: http://www.abc.net.au/science/news/stories/s102327.htm
Full text of FDA article: http://www.fda.gov/fdac/features/1998/198_deep.html
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