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Online Courses: E-Learning for Invertebrate Zoology
Biodiversity of Corals
Presented by
Prof. Dr. Fayez A. M. Shoukr
Professor of Marine Invertebrates,
Zoology Department,
Faculty of Science,
Keywords:
Biodiversity, Cnidarians, Corals, Hydrocorals,Fire corals,
Octocorals, Soft
corals, Hexacorals, Hard
corals.
Objectives:
1. Define the term biodiversity and describe the
diversity of life at the three biological levels.
2. Recognize a coral and know that corals are the most important
contributors to the structure of a reef.
3. Mention the different types of corals including hydrocorals (Fire corals),
octocorals, soft corals,
hexacorals, and hard corals.
4. Understand the phenomenon of polymorphism in the colony
of the hydroid coral: Millipora dichotoma which shows gastrozooids,
dactylozooids and medusea (gonozooids).
5. Enumerate the diversity of Alcyonarian corals
(Octocorals) which include organ-pipe corals (Stolonifera) e.g. Tubipora
musica, soft corals (Alcyonacea) e.g.Alcyonium,
6. Identify the diversity of Zoantharian corals
(Hexacorallian corals or Hexacorals) which include black or thorny corals e.g. Antipathes
dichotoma and true hard corals e.g.
7. Compare between the three groups of coral community of hard
species which include fungian corals, imperforate corals and perforate
corals.
8. Differentiate
between the different types of skeletons of corals.
9. Describe the structure of a polyp and skeleton of a hard
coral and know the
terms corallite,
corallum and others.
10. Understand the biology of corals through the study of the
digestive system & feeding with coral reproduction and life cycle.
11. Demonstrate the
symbiotic relationship in hermatypic
corals with the protistan algae (dinoflagellates) termed
zooxanthellae through the process
of photosynthesis.
12. Explain the process of calcification
in hermatypic corals.
13. Know
that corals are accumulated over thousands of
years and thus, never damage or remove or break
off
corals.
Introduction
Invertebrates, which represent more
than 95% of the known species on our lovely blue planet, are animals without
backbones. Corals are marine invertebrates showing great diversity and belong
to a large phylum of aquatic fauna namely Cnidaria (pronounced
nid-AIR-ee-ah, the C is
silent). In addition to such well-known organisms
such as jellyfish, sea anemones, the lesser-known sea fans, sea pens and the
fragile fern-like hydroids. There are two basic forms of cnidarians: the polyp form, such
as sea anemones & corals, and the medusa such as the jellyfish. Although
corals show a wide range of species with different shapes and sizes, they share
the same basic body plan of the polyp as a simple sack-like stomach with a
single mouth, encircled by a ring of tentacles. There are many corals to be found in the
Biodiversity
Biodiversity (biological diversity) is
defined as the variety of life forms (microorganisms, plants, and animals) on
Earth from all sources of habitats including terrestrial, marine and other
aquatic ecosystems and its interactions. This includes diversity within
species, between species and of ecosystems. Biodiversity describes the
diversity of life at three biological levels and can be subdivided into:
1) Species diversity: Species diversity is the number
of different species of living things living in an area. A species is defined as a group
of organisms (microorganisms, plants
or animals) that are similar and able to breed and produce fertile offspring
under natural conditions and sterile offspring produce between different species.
2) Genetic diversity: This refers to the total number
of genetic
characteristics in all of the individuals that comprise a particular species.
3) Ecological or Ecosystem
diversity: An
ecosystem consists of all living (biotic) and non-living (abiotic) things in a
given area that interact with one another. Ecosystem diversity is the variation of community types
with habitats and other abiotic environments present in a given area.
To
observe changes in biodiversity you need to know what organisms are found in the present taking into
consideration the effects of environment change on the species. Thus, taxonomists
increase our understanding of biodiversity and how it responds to environmental
change. Taxonomic knowledge is needed to identify where the areas of highest
biodiversity are, or where the rarest or extinct species are found. This
information is vital to deciding where to locate protected areas. Taxonomists
can help identify and record biodiversity and can pass this information on to
others in the form of field guides and databases. Depending on study of biodiversity,
the scientists can use animals around them for the development of new drugs
based on natural medicines.
Definition of a coral
Corals belong to Domain: Eukarya, Kingdom: Animalia, Phylum:
Cnidaria and Classes: Hydrozoa & Anthozoa. Thus, the coral is defined
as a group of cnidarian invertebrates, which indicates the presence of skeletal
material which is embedded in the living tissue or encloses the animal
altogether. Corals are marine tiny animals, called polyps, which
look like sea anemones and are cousins of jellyfish. At its free end is a mouth
surrounded by numerous tentacles with stinging cells. The polyp extracts
calcium carbonate (limestone) from seawater to build the skeleton that protects
the soft body of the coral. Each coral polyp secretes a stony cup around itself
as a skeleton. The polyps of corals divide as they grow and form colonies. As the coral colonies build up on top of each
other, they gradually form a coral reef. Corals are the most important
contributors to the structure of a reef. Individual coral colonies may be up to
1000 years old and coral reefs may be many thousands of years old which are
formed slowly over time. Usually, there are two main types of corals: soft
corals and hard corals. Soft corals look like plants or trees and consider non-reef
building corals. Hard corals look like stones or rocks and consider reef-building
corals. Their skeletons are made out of calcium carbonate.
Biodiversity of Corals
Domain: Eukarya
Kingdom: Animalia
Phylum: Cnidaria
Class:
Hydrozoa
1. Hydrozoan corals (Hydrocorals)
Order: Milleporina (Hydrocorallina)
(fire corals)
e.g. The fire corals: Millipora
dichotoma
Class: Anthozoa
2. Anthozoan corals
Subclass: Alcyonaria (Octocorallia)
2.1. Alcyonarian corals (Octocorallian corals or Octocorals)
2.1.1. Organ-pipe corals: Order:
Stolonifera.
e.g. Organ-pipe corals: Tubipora musica.
2.1.2. Soft corals: Order: Alcyonacea. e.g. Dead
man’s fingers: Alcyonium
digitatum, Pumping or
Pulsing corals:
fuscescens, Leather corals: Sarcophyton
sp.,
Tree corals: Dendronephthya sp.
2.1.3.
Horny corals: Order: Gorgonecea .
e.g. Sea fans : Gorgonia sp.,
Precious red corals :
Corallium rubrum, Sea whips : Eunicella sp.
2.1.4.
Blue corals: Order: Coenothecalia.
e. g.
Blue corals: Heliopora sp.
Subclass: Zoantharia (Hexacorallia)
2.2. Zoantharian corals (Hexacorallian corals or
Hexacorals)
2.2.1. Black or thorny corals: Order: Antipatharia.
e.g.
Precious
black corals: Antipathes
dichotoma.
2.2.2.
True hard corals: Order: Scleractinia (= Madreporaria)
e.g. Plate solitary coral (mushroom coral) :
Fungia fungites ,
Acropora palmata, Closed brain corals:
Favia sp., Brain corals: Meandrina meandrites
(Linneaus, 1767),
Tooth corals:
Galaxea sp.,
Finger
corals: Porites porites,
Needle corals: Seriatopora sp.,
Cat's paw corals: Stylophora pistillata (Esper, 1797).
1. Hydrozoan corals
(Hydrocorals)
Typical hydrozoans
have a complex life cycle with an alternation of generations between polyps namely hydroids and
medusa known as
hydromedusae. In life cycle, the characteristic planula larva attaches to the
bottom, metamorphoses into a polyp that reproduces asexually by budding or
fragmentation. A bud develops into the medusa that detaches from the polyp, and
swims away in plankton.
Usually, the medusae have a muscular band projecting inward from the margin
of the bell for
locomotion and frequently called velum. The gonads are ectodermal (epidermal) in origin. The
coelenteron is not subdivided by vertical septa (mesenteries) and lacking
stomodaeum. The tentacles of polyps are generally solid (not hollow).
Hydrozoans are mostly colonial i.e. with colony formation. Polymorphism
occurs in colonies of some species of hydrozoans, the zooids (polyps & medusae) being
specialized for functions such as feeding, defense and sexual reproduction. The skeleton is present
(exoskeleton) in polyps and absent in medusae. Most hydrozoans are marine among
benthos or plankton. The fire corals: Millipora dichotoma consider one
of the most commonly seen hydrocorals in the
Fire corals: Millipora
dichotoma (Linnaeus, 1758)
Fire corals belong to hydrozoans that produce a
massive calcareous external skeleton (exoskeleton) from calcium carbonate. It
is not really a true hard coral but a hydroid coral with a hydropolyp and heavy
coral-like calcareous skeleton. This well known fire coral is yellowish in
color and its skeleton forms encrusting colonies. It is found among coral reefs
all over the world and abundant in the
2. Anthozoan corals
Polyps of
anthozoans are dominant and medusae are absent or wanting. There is no
alternation of generations in their life cycle. Gonads are endodermal
(gastrodermal) in origin. Skeleton is present and either internal
(endoskeleton) as in octocorallians or external (exoskeleton) as in hexacorallians or absent as in anemones. Some
internal skeletons of octocorallians contain calcareous spicules. These
spicules are either scattered of fused. They stiffen and shelter the polyps.
Other octocorallians have internal skeletons made of protein. Reef-building corals (hexacorallians) 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 sclerosepta walls
extend up from the polyp's base and reinforce the skeleton. Coelenteron is subdivided by
vertical mesenteries with septal filaments. The mesenteries serve a digestive-
reproductive function. The anthopolyps are devoid of oral cone but with a
stomodaeum (pharynx or gullet) and siphonoglyphs. The cilia of the stomodaeum
create an upward current of water. The siphonoglyph is a heavily ciliated
groove that creates a downward current of water. Tentacles are hollow, with octomerous
or hexamerous arrangement. The polyps appear as colonial or solitary forms.
Mesoglea is cellular with incipient of fibrous connective tissue. These cnidarians
are found among benthos. e.g. soft corals, sea anemones, stony corals. The
anthozoan corals comprise the Alcyonarian corals (Octocorals) and the zoantharian
corals (Hexacorals).
2.1. Alcyonarian corals
(Octocorallian corals or Octocorals)
As their name suggests, members
of octocorals
have a body design arranged in eights. There are always eight tentacles and
eight mesenteries. The alcyonarian polyp has hollow pinnate tentacles (feather like), with
symmetrical lateral branches (pinnules). The mouth leads to a stomodaeum
(pharynx) which has one ciliated groove commonly known as the siphonoglyph. The
later consider the ventral side of the alcyonarian polyp. The pharynx held to
the body wall by eight endodermal mesenteries. The mesenteries are single (not
arranged in couples) and complete (macrosepta). The mesentery (septum) bears
strong longitudinal muscles termed retractors facing the siphonoglyph side and
found on the ventral faces of mesenteries. The pharynx has one pair (couple)
of directive mesentery. Below the pharynx, the inner edge of the mesenteries is
thickened into a cord known as the mesenteric filament. The mesenteric filament
lacks the 3 ridges (lobes) of the zoantharian polyps. The alcyonarian polyps are
usually supported by internal skeleton from calcareous loose spicules in the
mesoglea. Some alcyonarians have external skeleton. The Alcyonarian corals
(Octocorals) comprise organ-pipe corals (Stolonifera) e.g. Tubipora
musica, soft corals (Alcyonacea) e.g. Alcyonium,
2.1.1. Organ-pipe corals: (Order:
Stolonifera)
The skeleton, when present, consists of warty calcareous
spicules that may fuse to form compact tubes and platforms. The polyps arise
from a creeping stolon or basal plate. The fleshy body of the colony, known as
coenenchyme, has a system of canals. The polyps bear eight feathery (pinnate)
tentacles. Usually, the polyp is distinguished into two parts; a lower basal
part (anthostele) and an upper distal part (anthocodium). e.g. Organ-pipe corals : Tubipora musica . A common
2.1.2. Soft corals: (Order: Alcyonacea)
The soft
coral polyp is easily identified by the presence of tentacles with small
pinnate fringes & complete mesenteries in sets of eight. Also, the skeleton
is composed of internal
isolated calcareous spicules (endoskeleton) commonly called sclerites. They
do not have a hard (rigid) calcium carbonate exoskeleton. The advantage of non-rigid
skeleton of the soft corals is to allow colonies to live in areas of somewhat
high current strength. The spicules or sclerites are embedded in the mesogloea and composed of either
calcium carbonate or Gorgonin to give some stiffness and flexibility to soft
corals. The polyps of
the soft coral
colony are embedded in a fleshy gelatinous mass (coenenchyme), thus, soft
corals have a soft or leathery sense. The upper portion of the polyps, known as anthocodia,
protrudes from the coenenchyme, which has transverse canals. The soft
corals are very sensitive to their adjacent neighbors and when threatened the
polyps retract and produce important amounts of mucous for defense. In general,
the soft corals are ahermatypic (non-reef-building) and do not acquire
protistan symbionts.
e.g. Dead man’s fingers : Alcyonium digitatum (Linnaeus,
1758). The colony is branched into blunt lobes. The distal portion of the
polyps termed anthocodia are scattered
over the distal region of the colony. The polyps are retractile into small pits
(pores) and appear monomorphic. Pumping or Pulsing Corals:
2.1.3. Horny corals :( Order: Gorgonacea)
The
skeleton of these octocorals is in the form of axial rod (axis) that branching
through ought the colony. It is composed of calcareous or horny material containing
gorgonin (proteins & mucopolysaccharides) or both, thus, generally called horny corals or gorgonian corals.
It is of ectodermal (epidermal) origin. The polyps arising as lateral
outgrowths from the coenosarc with tree- like colony. The fleshy part of the
colony, termed coenenchyme, has no canals. The polyps are scattered on the
surface of the colony branches and retractile inside the coenenchyme. e.g. Precious red corals : Corallium rubrum (Linnaeus,1758):
It has an upright-branched colony with axial red skeleton. The latter is
composed of internal calcareous spicules and calcium carbonate. The polyps show
dimorphism: (a) Autozooids (nutritive polyps), with 8 pinnate tentacles,
retractile and white in color, for feeding the colony.(b) Siphonozooids,
without tentacles but with a siphonoglyph for maintaining water circulation
through the colony. The
2.1.4 Blue corals: (Order: Coenothecalia)
The
polyps of these octocorals are dimorphic with autozooids arise from
large pores and siphonozooids from smaller pores in a perforated lobed
skeleton. The skeleton is massive and devoid of spicules. It has a blue color through the deposition of iron salts in calcium carbonate.
The blue corals which have this dense calcareous skeleton do not belong to the
order Madreporaria
(Scleractinia). The coenenchyme has cylindrical canals (solenial tubes). e.g. Blue corals: Heliopora sp.: These corals are common on coral reefs in the Indo-Pacific region. Colonies reach to several meters in diameter with tree-like, plate-like or
column-like structure. The living colonies are brown, or greenish - grey in
color.
2. 2. Zoantharian
corals
(Hexacorallian corals
or Hexacorals)
The zoantharian polyp has simple unbranched tentacles which
arranged on oral disc in one or more cycles of six or its multiples. The
stomdaeum (pharynx) usually has two siphonoglyphs, the ventral is termed sulcus
and the dorsal is sulculus. Siphonoglyphs direct water current inwards.
Mesenteries (septa) are usually arranged in pairs in the form of complete septa
(macrosepta), reaching the pharynx internally and incomplete septa (microsepta)
not reaching the wall of the pharynx. Usually, the number of septa is six or
its multiples (hexamerous).The longitudinal muscles, termed retractors; of the
paired septa (macrosepta & microsepta) face each other. Usually, the
pharynx has two pairs of directive septa at each side of the siphonoglyphs. The
pharynx directs water current upwards.
The retractors of the directive septa are on the opposite sides. The
septa bear much-convoluted filaments known as mesenteric filaments (septal filaments).
They appear trilobed in cross section, the middle lobe, termed the
cnidoglandular band. It consists of numerous large nematocysts (cnidocysts) and
digestive (enzymatic) gland cells for digestion and defense. The lateral lobes
(wings) are flagellated and termed flagellated bands which being phagocytic in
function. The zoantharian polyp has usually calcareous exoskeleton outside the
polyp (e.g. hard corals). The skeleton is never in the form of mesogloeal
spicules. Skeleton is absent in some forms e.g. sea anemones. Zoantharian polyps
are solitary or colonial. Zoantharian corals (Hexacorallian corals or
Hexacorals) include both of black or thorny corals: (Order: Antipatharia) e.g. Precious black corals: Antipathes dichotoma
and true hard corals: Order: Scleractinia (= Madreporaria) e.g. Plate solitary
corals:
Fungia fungites ,
2.2.1.
Black corals: (Order: Antipatharia)
Black corals have tree-shape with a branched horny axis made of protein. Its color is shiny golden
brown or dark brown or black. Thus,
this type of corals is frequently called black corals. The skeleton is of ectodermal
(epidermal) origin and is covered with tiny thorns (spines), thus, also called thorny corals. This is the key character of this type of
corals. There are no
calcareous spicules or sclerites in their skeleton. The polyps of these colonial forms are small and have a circlet of simple, unbranched, non-retractile
tentacles (6-24). The pharynx has one or two reduced siphonoglyphs.
The name Antipatharia is
derived from the old belief that wearing of these animals keeps the body against
diseases and the black
corals have been used for centuries as medicine and for charms. The skeleton is commercially used for jewelry or sold
as curios. Black
corals are found worldwide, but are most common in the Indo-Pacific and
2.2.2. True hard corals: Order: Scleractinia (= Madreporaria)
Hard corals (stony corals) are
mostly colonial which grow by asexual reproduction and sometimes appear
solitary. There is a hard calcareous exoskeleton comprising of calcium carbonate which secreted by the
ectoderm (epidermis). The skeleton of the scleractinian colony as a whole is called
corallum, while the skeleton of one polyp of an
individual scleractinian
is called corallite. The fleshy or bulbous polyps have numerous retracted
tentacles in cycles of six that are used for capturing food particles and sunlight.The
tentacles are armed with nematocysts, or stinging cells, that are used to sting
preys or for aggression.The
siponoglyphs are absent in the stomodaeum.The pedal disc is lacking. A large
number of hard corals enclose within the tissues of polyps symbiotic
protists know as zooxanthellae.These protists (algae) employ sunlight via a process
called photosynthesis to produce another food source for zooxanthellate corals.
One of the by-products of the photosynthetic process is that the polyps of hard
corals will secrete a calcium framework at their bases from which polyps
expands off.
True hard
corals are considering
the most important reef-building organisms which are commonly called hermatypic
corals (reef-building corals). The corals that do not build reefs are called
ahermatypic corals (non-reef- building corals) e.g. soft corals. Reef-building
corals secrete an external skeletal cup of calcium carbonate (limestone). Hard corals grow a limestone skeleton underneath, which pushes the
polyps upwards and thus gives birth to coral reefs and islands.Coral reefs are of economic value as
tourist attraction. Some hard corals are used as decorative pieces and lovely
jewelry. The shape, size, and surface of the
different corals usually give corals their common name. e.g. True hard corals: Plate solitary coral (mushroom coral) :
Fungia fungites ,
Structure of a hard coral polyp
The
anatomy of hard corals
show that they are made up of tiny organisms called polyps with a skeleton. The structure of the polyps and
the skeleton of the hard coral is a rather simple arrangement. A coral polyp appears like a sea
anemone as a tubular saclike animal with a central mouth surrounded by a ring
of tentacles. The polyp contains a pharynx, mesenteries
and mesentery (septal)
filaments, which contain nematocysts used for food capture. The pharynx emerges without siphonoglyphs .The
septal filaments are devoid of the lateral lobes or wings characterize a sea
anemone. The mesenteries include the gonads which contain
the reproductive cells. A polyp is made up or two cell layers: the
epidermis (ectoderm) and the gastrodermis (endoderm).The non-tissue layer
between the gastrodermis and the epidermis is called the mesoglea. The coral tissue
of polyps (epidermis, gastrodermis and mesoglea) that stretches over the
surface of the hard coral is called the coenosarc. The end opposite the tentacles, called the base, is
attached to the substratum.
The epidermis of the polyps secretes a calcareous coral
skeleton (cup or calyx) called a corallite. The
corallite is the part of the skeleton deposited by one polyp and the corallum
is the skeleton of the coral colony.The skeleton
originates as a thin basal plate beneath the polyp forming the bottom of the coral skeleton. The
skeletal wall around basal plate of each
polyp is called the theca. The theca of the corallite radiates inward vertical
partitions (ridges) called sclerosepta. The septa radiate from
the wall to the center of the corallite. These sclerosepta are secreted by polyp mesenteries and are therefore added in the same
order as the mesenteries. The sclerosepta occur in hexamerous cycles (6 primary, 6
secondary, 12 tertiary, and 24 quaternary). As a
result, septa of different ages are adjacent to one another, and the symmetry
of the scleractinian skeleton is radial or biradial. The inner ends of the primary
sclerosepta are fused in the center of the cup forming upright column called
the columella (the central axis of the corallite found below the
mouth). The
skeletal material around the corallites is known as the coenosteum. The polyp connected with
adjacent polyp in a colony by connecting canal passing through openings in the
theca at the polyp bases.
Coral community
Usually, the hard
corals are classified into three groups:
(1) Fungian corals: They are solitary corals that have
disc or mushroom-shaped corallites. e.g. Plate coral or mushroom coral : Fungia fungites (Linnaeus, 1758). : It is a solitary coral, with a single polyp and single mouth, usually found on sandy bottom areas in the
(2) Imperforate corals (aporose): The colonial corals, in
which the corallites have solid skeleton with compact theca and without
pores, are called imperforate or aporose corals. e.g. Brain corals: Meandrina meandrites
(Linneaus, 1767). These colonial corals
appear with huge globular masses. The surface of the corallum looks like the
convolutions and twisting of the brain of a high vertebrate animal. The polyps found
in rows in depressions as a result of incomplete fission. Each polyp has many
mouths and bordered by a fringe of tentacles. Colonies
increase in size by asexual budding of additional polyps and successive
generations overgrowing one another. Closed brain corals: Favia sp.: These are colonial
corals with rather spherical corallum. The corallites have closely placed
polygonal cups. The colony develops by multiple fission of the polyps. Tooth corals: Galaxea sp.: These colonial corals have
high cup-shaped corallites, which are separated by equal intervals. The colony develops
by multiple fission of the well-separated polyps. The corallites appear with
separated upright high thecae.
(3)
Perforate corals (porous): These colonial corals in which the corallites have
thecae, which are extremely porous and of loose construction are termed
perforate or porous corals. e. g. Elk-horn corals:
Acropora sp.: These perforate corals have extremely porous corallum of
loose construction. These colonial corals have tree-like corallum. The branches
consist of small well-separated polyps and cylindrical cups. Acropora
has more than 200 species and constituting the major coral reef builders. It
considers the most abundant hard corals of the
Digestive system and feeding
The mouth found on the oral disc and surrounded by tentacles. It leads to the gastrovascular cavity or stomach cavity through the phyranx (gullet). The stomach cavity is divided by longitudinal partitions called
mesenteries. These mesenteries increase the surface area of digestion. The free
edges of the mesenteries bear long movable mesenteric filaments. These mesenteric
filaments can protrude through the mouth to capture food. Nematocysts on
mesenteric filaments and tentacles are used to
sting prey and move it into the mouth. Food is moved down, through the pharynx, to the coelenteron
for digestion.
Polyps of corals feed on zooplankton
(carnivorous) and consider suspension feeders. They utilize two main
methods of prey capture: nematocyst stings and sticky mucus with cilia. Most
corals feed at night. This may be because the zooplankton moves into the water
column at daytime and become available for capture at night. Also, reef-building
corals depend on the photosynthetic products of the symbiotic protistan
zooxanthellae for the majority of their nutrients.
Coral reproduction and Life Cycle
Corals
exhibit asexual and sexual reproduction.
1. Asexual reproduction (reproduction without
gametes): 1.1. Budding: Asexual reproduction occurs by longitudinal fission or
budding and the coral colony expands in size. Budding may be intratentacular, in
which the new bud forms from the oral discs of the old polyp, as in Diploria
sp.,or extratentacular in which the new polyp forms from the base of the
old polyp, as in Montastraea sp.1.2.
Fragmentation: A common type of asexual reproduction in corals is
by fragmentation. Broken pieces of
corals that ground on a suitable substratum may begin growing and produce a new
colony. This type of reproduction is common in branching corals like Acropora.
2. Sexual reproduction (reproduction
with gametes): The sex of hard corals is separate and also hermaphrodite species are
known. The corals from one species release their gametes (eggs and sperms) at
the same time. In some species, the eggs and sperms are released in a bag. They
float to the surface where they separate and fertilization takes place. The
zygote develops into the characteristic free-swimming larvae called planulae. They
attach to a suitable firm substratum and
metamorphose to a small coral polyp. They grow into a new colony by budding and secrete a hard calcareous skeleton.
Some species of corals such as Acropora
brood their larvae inside coelenteron and
release brooded larvae.
Symbiosis & Calcification in hermatypic corals
The hermatypic corals
(reef-building corals) have a mutualistic relationship with single-celled yellow-brown algae or zooxanthellae (pronounced zo-zan-THEL-ee) termed symbiosis. Zooxanthellae are microscopic dinoflagellate protists that
live within tissues (gastrodermis) of coral
polyps. Zooxanthellae of various corals have been found to belong to at least ten
different species e.g. Symbiodinium microadriaticum..
Both of
the coral polyps
and the zooxanthellae benefit from this symbiotic relationship and depend on one another for
survival. The
zooxanthellae gain protection (shelter) and admission to light inside the cnidarian tissues. Because
of the need for light, zooxanthellate corals (reef-building corals) live only
in sea waters less than 100 meters deep where symbiotic zooxanthellae can take
in light for photosynthesis. Symbiotic zooxanthellae play an important role in the biology of reef-building
corals. The coral gains
a source of food (carbohydrates), oxygen and removal of waste products through
the process of photosynthesis of zooxanthellae. Through photosynthesis,
zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates.
The coral polyp uses carbohydrates as a nutrient. Also, the polyp uses oxygen
for respiration and in turns, returns carbon dioxide to the zooxanthellae. Thus,
zooxanthellae through photosynthesis recycle waste and respiratory byproducts from the
coral and recombine these into carbohydrates that utilized by the coral
polyp.
On the other hand, photosynthesis
in zooxanthellae aids in
the formation of the CaCO3 in the process called calcification of corals. Calcium (Ca++) is taken
indirectly from the seawater. Calcium combines with bicarbonate (Hco3-),
largely produced in metabolism, to form calcium bicarbonate Ca (Hco3)
2 and then calcium carbonate (CaCo3) or limestone precipitates
forming coral skeleton. The efficiency of calcification depends on the
effective removal of carbonic acid (H2Co3), which depends
on reactions catalyzed by carbonic anhydrase. CO2 of corals is removed
through photosynthesis by the zooxanthellae, this helps in precipitation of
CaCO3.Thus, zooxanthellae
help coral calcification by removing carbon dioxide during photosynthesis.
The transformation of dissolved calcium
carbonate (Ca CO3) into
solid limestone enabled the coral polyp to form its calcium carbonate skeleton.
When this happens the polyp takes the solid limestone, and positions it down
under its body in a skeleton that has a certain outline. The pattern in which
the skeleton is laid depends on what species of coral polyp it is. Every
different species of coral polyp lays down a different skeletal pattern.
Through the labor of the minute coral polyps, coral reefs build up masses of
limestone, sometimes building whole islands, and lining the ocean bottom near
shorelines. Millions of years later, these deposits of calcium carbonate
(limestone) may get pushed up by the earth's forces and become land. Calcium
carbonate is also a valuable mineral, used for the construction of buildings,
fertilization of soil and many other purposes.
Several organisms use different forms of calcium carbonate in their
bodies. Although humans do not get it from sea water like coral polyps do, we
also use calcium carbonate to create teeth and the bones of our skeleton. On a
coral reef, molluskan invertebrates such as snails and clams use a calcium
carbonate to produce their shells, while some sponges, soft corals and sea
cucumbers (Echinodermata) utilize it for formation of spicules in their
bodies.
Conservation of corals
The corals forming coral
reefs have accumulated over thousands of years to form the largest living reefs
in the world. Thus, never remove or break off a piece of coral during your
journeys and diving by scuba and snorkeling on coral reefs. Also, when you are
in a boat, vessel or yacht never anchor near a coral reef because the anchor
could get caught in the coral and destroy it. When reef walking, never walk on
corals and attach to the sandy areas.
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Correspondence To:
Prof. Dr. Fayez A. M.
Shoukr
Professor of marine Invertebrates,
Zoology Department,
Faculty of Science,
How to Cite
This Site:
Shoukr, F. A. (2006): Biodiversity of Corals. Electronic
internet document
available at
http://www.corals.8m.net/
Published by the
author, web page
established July,
2006.