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KNOWN SPECIES:
About 9,000 living species are known.
SIZE RANGE:
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, Fungia (mushroom coral), is a solitary
coral that can extend 10 in. (25 cm) in diameter. b. Colonial coral
polyps are much smaller and average 0.04 to 0.12 in. (1-3 mm) in
diameter. Coral colonies also vary in size. Some corals form only
small colonies. Others may form colonies several feet (a few meters)
high. Star coral (Montastrea
annularis) colonies reach an average height of 10 to 13 ft.
(3-4m).
CHARACTERISTICS:
Body shape:
A coral polyp is a
tubular saclike animal with a central mouth surrounded by a ring
of tentacles. The end opposite the tentacles, called the base,
is attached to the substrate.
A coral polyp is a tubular saclike animal.
Color:
Natural pigments
in coral tissue produce a range of colors including white, red,
orange, yellow, green, blue, and purple. Colored calcareous
spicules (needle-shaped structures) give some octocorallians
their colors. Algae that
live within the tissues of some corals may make the coral appear
brown, green, or orange.
Tentacles:
The
tentacles are for
defense and for moving food to the mouth. Depending on the
subclass, a coral polyp's tentacles are arranged in multiples of six
or eight. The tentacles
contain microscopic stinging capsules called nematocysts. A
nematocyst is a bulbous double-walled structure containing a
spirally folded, venom-filled thread with a minute barb at its tip.
A tiny sensor projects outside the nematocyst. When the sensor is
stimulated physically or chemically, the capsule explodes and ejects
the thread with considerable force and speed. The barb penetrates
the victim's skin and injects a potent venom.
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A nematocyst contains a spirally folded, venom-filled thread. When
the sensor is stimulated, the capsule explodes and ejects the
thread.
Senses
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Nervous system:
Corals lack a brain but have a simple nervous system called a
nerve net. The nerve net extends from the mouth to the tentacles.
Chemoreception:
Polyps can detect certain substances such as sugars and amino
acids. This sense, similar to our senses of smell and taste,
enables corals to detect prey.
Nematocyst sensors:
Tiny sensors in the ends of nematocysts in polyp's tentacles
trigger the nematocyst to eject. These sensors are stimulated
either chemically or physically.
Attachment:
Most coral polyps attach themselves to a hard substrate and
remain there for life.
Symbiosis:
Reef-building corals have a mutualistic relationship with
zooxanthellae, a microscopic algae that live within the coral polyp's
tissues. Both the polyp and the zooanthellae benefit from this. For this
reason, reef-building corals are found only in areas where symbiotic zooxanthellae can take in
enough light for photosynthesis. Through photosynthesis, zooxanthellae convert carbon dioxide
and water into oxygen and carbohydrates (food for the coral). The coral polyp uses
carbohydrates as a nutrient. The polyp also uses oxygen for
respiration and in turns, returns carbon dioxide to the zooxanthellae. Through this exchange, coral saves energy that would
otherwise be used to eliminate the carbon dioxide. Nitrogen and phosphorus are cycled between zooxanthellae and
coral polyps. For example, zooxanthellae take in ammonia given off
as waste by the polyp, and return amino acids for the coral. Zooxanthellae also promote polyp calcification by removing
carbon dioxide during photosynthesis. Under optimum conditions, this
enhanced calcification builds the reef faster than it can be eroded
by physical or biological factors.
Toxins:
Certain toxic compounds in soft corals (Order Alcyonacea)
make the corals unappetizing and deter predators. Corals compete for living space on the reef,
so some soft corals
secrete toxins to eliminate competitors. Some reef-building corals
can actually digest the tissue of an invading coral.
Food:
Some corals eat zooplankton (tiny drifting animals) or
small fishes. Others consume organic debris. Many reef- building
corals derive their nutrition from
zooxanthellae.
Eating:
Coral polyps are generally nocturnal feeders. At night, they
extend their tentacles to capture food with the aid of nematocysts.
Some corals secret films or strands of mucus to collect fine
organic particles.
In reef-building corals, to mobile filaments originating from
the stomach cavity can capture larger food particles. These
filaments are also capable of digestion. The stomach cavities of colonial corals are interconnected,
and food obtained by one polyp can be passed to other polyps in the
colony. A polyp excretes solid wastes through its mouth.
Reproduction:
Corals can reproduce both sexually and asexually. An
individual polyp may use both reproductive modes within its
lifetime.
Corals reproduce sexually by either internal or external
fertilization; the reproductive cells are borne on mesenteries
(membranes) that radiate inward from the layer of tissue that
lines the stomach cavity. Internally fertilized eggs are brooded by the polyp for days
to weeks. Free-swimming larvae are released into the water and
settle within hours.
b. Externally fertilized eggs develop while adrift. After a few
days, fertilized eggs develop into free-swimming larvae. Larvae
settle within hours to days.
Some corals are hermaphroditic (having both male and female
reproductive cells). Others are either male or female. Both sexes
can occur in a colony, or a colony may consist of individuals of the
same sex. Synchronous spawning occurs in many corals. Polyps release
eggs and sperm into the water at the same time. This spawning method
disperses eggs over a larger area. Synchronous spawning depends on
four factors: time of the year, water temperature, and tidal and
lunar cycles. Spawning is most successful when there is little variation
between high and low tides. The less water movement over the reef,
the better the chance that an egg will be fertilized.
b. At least one-third of the reef-building corals of the Great
Barrier Reef are synchronous spawners. These corals spawn
(release eggs) annually in the spring. Spawning occurs on the
third through sixth nights after a full moon. Larvae usually
settle in four to ten days.
Once the larva settles on a substrate, it develops into a
polyp. Some scientists believe that most larvae settle within 2,000
ft. (600 m) of the parent reef. Others contend that some larvae
travel longer distances. Research is ongoing.
In Asexual reproduction
- environmental disturbances may dislodge some polyps or
portions of colonies from the parent colony and deposit them on
another part of the reef.
Also newly developing coral colonies split and form
separate colonies. Often a polyp that was originally produced by sexual reproduction initiates growth
of a colony asexually by budding. Budding occurs when a portion of
the parent polyp pinches off to form a new individual. Budding
enables the polyp to replicate itself several times and at the same
time maintain tissue connections within the colony. Later, the same
polyp may reproduce sexually.
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Budding occurs when a portion of the parent polyp
pinches off to form a new individual.
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Growth:
Coral colonies growing in shallow water are often heavily
branched. In contrast, deeper water corals often grow in sheets or
plates. These flattened forms allow for more efficient use of lower
light intensities in deeper waters. The growth rate of corals and coral reefs depends on factors
such as light intensity, water temperature, salinity, turbidity,
food availability, competition for space, and predation. Upward
growth of coral colonies is generally between 0.5 to 4 in. (1-10 cm)
a year.
ANATOMY:
Colonial corals:
Individual coral polyps within a colony are connected by
common tissue.
Skeleton:
Octocorallians have an internal skeleton. Some internal
skeletons contain calcareous spicules. Spicules are either scattered
of fused. They stiffen and protect the polyps. Other octocorallians
have internal skeletons made of protein.
2. Reef-building corals secrete an external skeletal cup of
calcium carbonate. This skeletal cup protects the polyp: when the
polyp contracts, it's almost completely inside the skeletal cup. The
stomach cavity of reef-building corals also contains radiating
calcareous walls. These walls extend up form the polyp's base and
reinforce the skeleton.
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When the polyp contracts, it's almost completely
inside the skeletal cup.
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Digestive system:
The mouth leads into the stomach cavity. The stomach cavity is partitioned by longitudinal membranes
called mesenteries.
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The stomach cavity is partitioned by longitudinal
membranes called mesenteries.
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Mesenteries increase the surface area of the stomach cavity,
which aids in digestion.
b. The edges of the mesenteries in reef-building corals support
long mobile filaments. These mesentery filaments can protrude
through the mouth to capture food.
Mesenteries also contain the
reproductive cells.
Respiration:
Gas exchange takes place through the body
surface.
Longevity:
Little is known about the lifespan of corals. Generally, coral
colonies may live for several decades to centuries.
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WHERE THEY LIVE:
Various species of
corals are found in all oceans of the world, from the tropics to the
polar regions.
Reef-building
corals are scattered throughout the tropical and subtropical Western
Atlantic and Indo-Pacific oceans, generally within 30 degrees N and
30 degrees S latitudes. Western Atlantic
reefs include: Bermuda, the Bahamas, the Caribbean
Islands, Belize, Florida, and the Gulf of Mexico. The Indo-Pacific
ocean 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 some areas of the Gulf of
California.
HABITAT:
Although various
types of corals can be found from the water's surface to depths of
19,700 ft. (6,000 m), reef-building corals are generally found at
depths of less than 150 ft (46 m), where sunlight penetrates.
Because reef- building corals have a symbiotic relationship with a
type of microscopic algae, sunlight is necessary for these corals to
thrive and grow.
Reefs tend
to grow faster in clear water. Clear water allows light to reach
the symbiotic algae living within the coral polyp's tissue. Many
scientists believe that the algae, called zooxanthellae,
promote polyp calcification.
Light-absorbing
adaptations enable some reef- building corals to live in dim blue
light.
Reef-building
corals require warm ocean temperatures (68 to 82 F, or 20 to 28 C).
Warm water flows along the eastern shores of major land masses.
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. These
conditions are typical of shallow, warm tropical waters. 5. Most corals grow
on a hard substrate.
Coral Reefs
Reef Composition: Hard corals build by secreting calcium carbonate skeletons.
Boring organisms such as sponges, worms, and bivalves; along
with grazers such as parrotfish and sea urchins break down the coral
skeletons. Borers and grazers usually attack dead coral. The
resulting sediment settles into spaces in the reef.
Coralline algae, encrusting bryozoans, and minerals cement the
dead organic matter, stabilizing the reef structure.
REEF FORMATION:
At one time it was mistakenly thought that coral grew at the
bottom of deep tropical seas and succeeding generations grew on top
of the dead calcium carbonate skeletons. This idea was dispelled by
dredging operations that indicated that reef corals were able to
grow only in shallow water.
Naturalist Charles Darwin's theory of coral formation is
widely accepted. This theory recognizes three types of reefs: the
fringing reef, the barrier reef, and the atoll.
The Fringing reefs border
shorelines of continents and islands in tropical seas. Fringing
reefs are commonly found in the South Pacific Hawaiian Islands,
and parts of the Caribbean.
The next type is the barrier reef, which occurs farther
offshore. Barrier reefs form when land masses sink, and fringing
reefs become separated from shorelines by wide channels. Land
masses sink as a result of erosion and shifting crustal plates of
the earth. (Crustal plates lift or sink the seafloor and adjacent
land masses.) Sinking and rising can also occur because of
rising or falling sea levels as a result of climate change. Barrier reefs are common in the Caribbean and
Indo-Pacific. The Great Barrier Reef off northern Australia in the
Indo-Pacific is the largest barrier reef in the world. This reef
stretches more than 1,240 miles (2,000 km).
If the land mass is a small island, it may eventually
disappear below the ocean surface, and the reef becomes an atoll.
Atolls are reefs that surround a central lagoon. The result is
several low coral islands around a lagoon. Atolls commonly occur
in the Indo- Pacific. The largest atoll, named Kwajalein,
surrounds a lagoon over 60 miles (97 km) long.
All existing coral reefs have been formed since the last of three
glacial periods in the Pleistocene epoch, 10,000 years ago. Seawater
trapped as ice in enormous glaciers caused sea level to fall.
Consequently, all previously formed coral reefs probably died from
exposure. When the glaciers melted, sea level rose to its current
position and present-day reefs began to develop.
source: G. Anderson
ECOSYSTEM:
The coral reef ecosystem is a diverse collection of species
that interact with each other and the physical environment. The
sun is the initial source of energy for this ecosystem. Through
photosynthesis, phytoplankton, algae, and other plants convert
light energy into chemical energy. As animals eat plants or other
animals, a portion of this energy is passed on.
Coral reef animals:
Sponges have been a part of the coral reef ecosystem from
early on. Several species of these porous animals inhabit reefs.
Sponges provide shelter for fishes, shrimps, crabs, and other small
animals. They appear in a variety of shapes and colors.
Sea anemones are close relatives of corals. Indo-Pacific reef
anemones are known for their symbiotic relationships with clownfish
and anemonefishes. An anemone's tentacles provide refuge for these
fishes and their eggs. In return, anemonefishes may protect the
anemone from predators such as butterflyfishes. Anemonefishes may
even remove parasites from their host anemones.
Bryozoans also encrust the reef. These microscopic invertebrates
from branching colonies over coral skeletons and reef debris,
cementing the reef structure. The reef is also home to a variety of worms, including both
flatworms and polychaetes. Flatworms live in crevices in the reef.
Some polychaetes such as Christmas Tree Worms and Feather Duster
Worms bore into coral skeletons. Other familiar species include Bristleworms.
The coral reef ecosystem is a diverse collection of
species that interact with each other and the physical environment.
Sea Stars, Sea Cucumbers, and Sea Urchins live on the reef.
The Crown-Of-Thorns Sea Star is a well- known predator of coral
polyps. Large numbers of these Sea Stars can devastate reefs,
leaving behind only the calcium carbonate skeletons. In dead reefs,
recently killed by the Crown-Of-Thorns Sea Star, larger food and
game fish are almost totally absent. Even deep-sea fish populations
may be affected by this breakdown in the food chain.
Shrimps, Crabs, Lobsters, and other crustaceans find
protection from predators in crevices or between coral branches.
Crustaceans are also predators. The Coral Crab crushes Sea Urchins
and Clams with its strong claws. The Banded Coral Shrimp is an
example of a cleaner shrimp. It removes parasites and dead skin from
reef fishes.
Octopuses, Ssquids, Clams, Scallops, Marine Snails, and Nudibranchs are all
Molluscs that live on or near the reef. Many
feed by filtering food particles from the water. Carnivorous Snails
are capable of drilling holes into Clams or other shelled animals
and then eating them. One of the largest Molluscs on the reef is the
Giant Clam. This clam may reach a length of 4 ft. (1.2 m). Both schooling and solitary fishes are essential residents of
the reef ecosystem. Fishes play a vital role in the reef's food web,
acting as both predators and prey. Their leftover food scraps and
wastes provide food or nutrients for other reef inhabitants.
Some species of sharks, skates, and rays live on or near the
reef. Others swim in to eat. Shark species include Lemon,
Nurse,
Pacific Blacktip,
White-tipped Reef, and
Zebra
Sharks. These
sharks as well as rays generally eat crabs, shrimps, squids,
clams, and small fishes. Parrotfish use chisel-like teeth to nibble on hard corals.
These fish are herbivores and eat the algae within the coral. They
grind the coral's exoskeleton to get the algae, and defecate sand.
A single parrotfish can produce about five tons of sand per year.
Wrasses comprise a large group of colorful cigar-shaped
fishes. Some species are known as cleaners, and set up cleaning
stations along the reef. When a larger fish aligns itself at one
of these cleaning stations, a cleaner wrasse removes parasites
from the fish. Eels are one of the reef's top predators. These fishes live
in crevices in the reef and venture out at night to hunt and feed.
They have sharp teeth set in a powerful jaw. Eels eat small
fishes, octopuses, shrimps, and crabs.
e. Other fishes found on the reefs include angelfishes, butterflyfishes, damselfishes, triggerfishes, seahorses, snappers,
squirrelfishes, grunts, pufferfishes, groupers, barracudas, and
scorpionfishes.
Some sea turtles frequent reef areas. Green, loggerhead, and
hawksbill sea turtles live in the warm waters of the Great Barrier
Reef.
10. Sea snakes are rarely found on reefs but do inhabit the
waters around reefs in the Indo-Pacific. They possess small fangs
but inject a potent venom.
HUMAN INTERACTION:
Ocean pollution poisons coral polyps.
Fertilizer runoff and untreated sewage introduce added
nutrients to coastal ecosystems. These elevated nutrient levels
promote algae growth. Deforestation degrades more than just land habitats. When
tropical forests are cut down to clear land for agriculture,
pasture, or homes, topsoil washes down rivers into coastal
ecosystems. Soil that settles on reefs smothers coral polyps and
blocks out the sunlight needed for corals to live.
Coastal development and dredging ravages reefs. Fishing with dynamite, cyanide, or bleach has killed coral
reefs in the Indo-Pacific region. Between 1986 and 1991, half of the
coral reefs in the Philippines have been demolished by these and
other destructive fishing methods.
Besides fishes, fishermen harvest a variety of exotic seafood
from the reef including conchs and lobsters. Overharvesting could
lead to these species' demise. Careless handling of nets, lines, and
lobster traps has led to some reef damage.
International seashell and aquarium trades have put a strain
on coral reefs and reef inhabitants. Careless water recreation damage reefs. Divers and snorklers
that stand on, sit on, or handle corals can injure the delicate
polyps. Dropped boat anchors can gouge the reef and crush corals.
NATURAL DISASTERS:
Changes in sea level are detrimental to established corals and
reefs. A drop in sea level exposes corals.
A rise in sea level decreases the amount of available
sunlight and may inhibit growth.
Coral diseases can wipe out entire strands of coral reefs.
Diseases may be connected to the sea level rise and nutrient level
increase.
Coral bleaching occurs when coral expels its symbiotic zooxanthellae. As a result, the coral loses its coloration. Without
zooxanthellae, the coral polyps have little energy available for
growth or reproduction.
Major tropical storms can strip corals from miles of reef
habitat.
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Class--Anthozoa: include
corals, sea anemones, sea pens, and sea pansies. These animals are
either solitary or colonial polyps that live attached to a substrate
(surface). Of the 9,000 known anthozoan species, corals comprise
about 2,500 species. They are further divided into three subclasses: Octocorallia, Zoantharia,
and Tabulata (extinct colonial corals). The term "corals," refers to both Octocorallians and Zoantharians.
Fire or stinging
coral is not a true coral. It is a hydrocoral (Class Hydrozoa).
