CORALS-FORMATION-TYPES

 

CORALS-INTRODUCTION

• Corals are developed by the coelenterate organisms.
• Corals are a deposit of calcium car­bonate.
• The coelenterate animal produces calcareous skeleton.
• But most coral pro­ducing polyps belong to the class Anthozoa.
• The true stony corals belong to the order Madreporaria of class Anthozoa. Many Solitary and colonial anthozoans produce corals which are sometimes brilliantly colored.
• The corals develop into coral rocks and coral islands.

 

 

Coral producing Polyp Structure:

• A coral-polyp resembles sea-anemone in its shape.
• The coral is its external shell.
• It is a product of the ectodermal cells, called Calicoblasts.
• The formation of the coral is not clearly followed, so far Calcareous crystals may be precipitated in the matrix and secreted outside the epidermis.
• Thus outer protective shell may be formed.

• A coral polyp will not show a pedal disc as its basal part is surrounded by the calcareous skeleton.
• The oral disc bears tentacles in cycles of 6.
• The circular mouth leads into a short stomodeaum which will not show siphonoglyphs.
• The mesenteries are restricted to the upper part of the polyp.
• The muscles are poorly developed.

 

FORMATION  OF CORALS

• The basal region of the coral polyp is fixed in a cup-shaped calcareous exoskeleton secreted by the epidermis of the base.
• On this basal plate a large number of radially arranged vertical septa are formed.
• They grow in height and they push the ectoderm up.
• An external wall is formed because of the circular up growth of the plate.
• The single cup like exoskeleton formed by an individual polyp is called a coralitte.
• It has the shape of the polyp.
• The majority of the corals are colonial, and the skeleton of an entire colony is termed coralium it may contain thousands of corallites.

Example for Coral formation

• In "Flabellum" the coral formation is based on these lines.
• The corallite is disc like. It is 5 mm to 25 cm. in length.
• The outer wall of the cup is made by stony calcium carbonate.
• It is called theca.
• The flattened bottom of the cup beneath the polyp is called the basal plate. The cavity of the cup develops a number of vertical septa or sclero-septa, proceeding form theca towards the center of the cup.
• Like mesenteries, they are typically arranged in cycles of six (6 primaries extending towards the center, 6 secondary’s, 12 tertiaries, 24 quartemaries etc.)
• The sclerosepta alternates with mesenteries Corallite lies entirely outside the polyp body.
• The polyp body is pushed up into ridges over the sclerosepta.
• Each ridge is covered by an internal portion of the body-wall Thus, the sclerosepta are external and lie outside the enteric cavity.
• Between the sclerosepta and the mesenteries, the polyp base is depressed into pockets called loculi.

• The inner ends of the primary sclerosepta are fused to form central column called columella.

In many corals, the theca is covered by a second calcareous wall called epitheca. The space separating the epitheca and theca, will show projections called costae.

Types of Corals and Examples: The true corals belong to Anthozoa but in class hydrozoa also some corals are seen.

 

I. Hydrozoan Corals :

1.Millepora :

• It is a massive, calcareous skeleton with two types opening
• a) Large gastropores, b) Smaller dactylopores.

Millipora is called stinging coral because it is the only coral with nematocysts, which causes pain to man.

2. Stylasterina :

• It is found in warm tropical water.
• The colony is tree like. It is also similar to millepora but has cup like gastropores.  There is a pointed style in the centre of each cup.

II.Authozoan Corals :

1.Alcyonium (dead man's fingers) :

• It is a marine colonial form living attached to stones.
• It is tree like. It grows to 4 to 10 cm height.
• The simplest form of skeleton is seen in this animal, which contains minute calcareous spicules.

2. Tubipora (organ-pipe coral) :

• It is a marine colony distributed in warm waters The spicules are fused to form a tube around the polyp.
• Many such tubes are united by horizontal platforms Again new polyps are formed They develop vertical tubes.
• This process is repeated. A big coral is formed.

3. Corallium (red coral):

• It is a marine sedentary form.
• It grows to 30 cm. It is highly branched.
• The polyps are white in colour. Autozooids'and siphonozooids are present.
• Herd and branched skeletal axis is formed by the union of many calcareous spicules in a matrix.
• Thus a coral is developed which is red in colour.

4. Gorgonia (Sea fan) :

• It is also called sea-whip.
• It is seen in tropical waters.
• It is tree like colony, it grows 80 cm. height.
• A central, rod like branched skeleton is formed by the ectoderm.
• Mesogloea also contains many calcareous spicules.

III.The corals belong to Madreporaria are true corals.

• The skeleton is entirely calcareous and is secreted by the ectoderm.
• The following are good examples.

1.Fungia (Mushroom coral) :

• It is marine form. The coral is discoid.
• The septa are numerous and are connected by small calcareous rods called synapticula.
• This coral is true coral.

2. Meandrina(Or Brain coral) :

• It is very big and grows up to 8 feet in diameter.
• It weighs in tons.
• The surface of the colony is marked by long curved grooves; hence it looks like human brain.
• In a living brain coral the polyps do not occupy separate cups.
• The mouths of these compound polyps will be separate.