Corals: Structure and Types - Online Biology Notes

What is coral?

Corals (coral animals) are marine, mostly colonial, polyploid coelenterates, looking like miniature sea anemones and living in a secreted skeleton of their own.
Their calcareous or horny skeleton is also commonly known as coral.
Some corals grow into massive, solid structures, others form large, branched colonies.
Most of the corals belong to the class Anthozoa and a few to the class Hydrozoa of phylum coelenterata.

A typical coral polyp from is a small organism about 10mm long and 1 to 3 mm in diameter from a colony.
Solitary coral polyps are much larger reaching up to 25cm in a diameter.
A basal disc is absent because the basal region of polyp is surrounded by a calcareous exoskeleton.
Oral disc bears numerous tentacles, in several rows around an elongated, oval, or circular mouth.
Pharynx or stomodaeum is short and without siphonoglyphs.
Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
Body wall is without cinclides and nematocyst bearing structures (acontia).
Muscles are poorly developed while little is known about nervous system.
Living polyps are found only on surface layers of coral masses.
They feed at night both by raptorial and suspension feeding.
When not feeding, they withdraw into cup-like cavities of skeleton.

Skeleton of a solitary coral is known as corallite.
It is a calcareous exoskeleton secreted by epidermis.
In a colonial coral, corallites of individual poyps fuse together to form a skeletal mass, called corallum.
Each corallite is like a stony cup with a basal part or basal plate, and a cup wall or theca, enclosing the aboral portion of polyp.
Cavity of cup contains a number of vertical radiating ridges called sclerosepta, proceeding from theca towards the center of cup.
Inner ends of sclerosepta are fused to form an irregular central skeletal mass or columella.

In coral polyps, sexual reproduction takes place by fusion of gametes.
Zygote develops into a free-swimming ciliated planula larva which settles down and metamorphoses into a young coral polyp.
There is no medusa stage.
By asexual budding, single polyp becomes the parent of all other members of the colony.
The coral polyp begins to secrete a skeletal rudiment or prototheca.
It is secreted by ectoderm, first as a basal plate.
Following it, radial folds develop which secrete sclerosepta.
At the same time, a rim is built up as a thecal wall around the polyp, lying at the top.
Meanwhile further skeletal material is added into the gaps between sclerosepta of skeleton which usually alternates with mesenteries of the polyp.
Coral colony grows in size continuously by budding of new polyps, particularly along the margins and on surface layers of coral masses.
Variety in form of compound corals results due to various patterns of budding.

Order Hydrocorallina includes few genera, like Millepora, Stylaster and Distichopora, which are colonial and secrete massive branched calcareous exoskeletons.
These are found in coral reefs with other corals.
Skeleton is secreted by a modified epidermis, called calicoblastic layer.
Living within the skeleton occur two types of polyps, large feeding gastrozooids and defensive dactylozooids.

Order Alycyonacea includes marine, colonial, and soft corals.
A well-known genus is Alcyonium, popular as ‘dead man’s fingers’ because of its resemblance to a human hand.
It has an endoskeleton of separate calcareous spicules embedded in a massive mesoglea or coenenchyme.
Order Stolonifera includes the organ pipe coral, Tubipora musica, widely distributed on coral reefs in warm waters.
Skeleton is made of mesogloeal calcareous spicules forming parallel and vertical tubes, each occupied by one polyp, and connected together by lateral platforms.
Skeleton is dull red in color due to presence of iron salts.
Order Coenothecalia includes a single genus Heliopora, commonly known as blue coral.
Its massive calcareous, skeleton or Corallium is secreted by polyps living in large erect, cylindrical solenial tubes on the surface of skeleton.
Order Gorgonacea includes plant-like colonies of sea fans or horny corals.
In Gorgonea, colony branches in one plane only.
Its axial colony is made by horny proteinaceous material intermixed with calcareous spicules arranged around the polyps.
In precious red coral, Corallium nobile, the branching colony has canaliferous coenenchyme of cocnosarc containing dimorphic polyps.
Axial skeleton consists of spicules embedded in CaCO3 forming precious hard red coral which is used in jewellery.

Order Madrepora includes stony corals or true corals, which are the principal builders of coral reefs.
While some of them are solitary, most are colonial, assuming a great variety of forms.

Fungia, Flabellum, Caryophylla, etc., are the solitary corals or cup corals.
The corallite is disc-like, cup-like or mushroom-shaped in form and measure 5mm to 25cm across.
It is often used without a theca.

Most of stony corals are colonial with plate-like, cup-like, spherical, or vase-shaped skeleton (coralium).
Polyps live at the surface of the calcareous skeleton.
Typical examples of colonial madreporarian corals are Acropora, Oculina, Favia, Madrepora, Meandrina, etc.
Some of the colonies are branched.
In stag-horn coral, Acropora, there is always a primary polyp at the top of colony with lateral branches on either side.
In some corals, like Oculina, polyps remain widely separated, each occupying a separate theca.
In others, like Favia and Astraea, thecae are so close together as to have common walls.
In the brain-coral, Meandrina, polyps as well as thecae become confluent, occupying valleys separated by ridges, on the surface of corallum.