Corals, Types of corals, coral skeletons

Corals

1. Corals Dr. Sonia Bajaj Assistant Professor Department of Zoology Shri Shankaracharya Mahavidyalaya ,Junwani ,Bhilai

2. Coral Corals are invertebrate animals belonging to a large group of colourful and fascinating animals called Cnidaria. Coral is a deposit of lime or calcium carbonate formed by coelenterates. Skeleton is secreted by a modified epidermis, called calicoblastic layer . Corals 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 coelenterate. The basic morphological unit is a clear polyp body with a central mouth surrounded by a ring of stinging tentacles. The scientific name for coral is Anthozoa . Alcyonium Millepora Gorgonium

3. hard corals or soft corals Hard corals form a colony and serve as a primary component of a coral reef. A coral reef consists of hundreds or thousands of coral polyps that are connected to each other. The coral polyps secrete calcium carbonate deposits to form a hard skeleton . This deposit renders the corals protective and structural support to the colony. The coral reefs are an important marine structure because they provide an underwater ecosystem for various small marine animals. Some of them are in ecological partnership with zooxanthellae, which are dinoflagellate endosymbionts and are sometimes responsible for the coloration of the reef . Soft corals do not produce coral reefs and lack the hard skeletons. Symbiosis with zooxanthellae is also infrequent among soft corals . Soft corals, which include seas fans, sea feathers and sea whips. corals eat- Most of a corals diet is obtained from zooxanthellae, they can also ‘fish’ for food too. During feeding a coral polyp will extend its tentacles out from its body and wave them in the water current where they encounter small fish, plankton or other food particles. The surface of each tentacle has thousands of stinging cells called cnidoblasts, and when small prey floats or swims past, the tentacles fire these stinging cells, stunning or killing the prey before passing it to the mouth .

4. Structure of coral polyps: Corals are made up of many tiny animals called polyps. Most polyps are less than one centimeter across. They are related to sea anemones and have stinging tentacles . Thousands of polyps live together in groups. Each coral polyp has a hard skeleton. The skeletons of all the polyps in a group form a coral's structure . A coral grows as more polyps add their skeletons. But because polyps are so small, this growth is very slow. A coral may take a whole year to grow just a few centimeters ! 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.

5. Structure of coral skeleton: Skeleton of a solitary coral is known as corallite. It is a calcareous exoskeleton secreted by epidermis . Corallite : A single polyp skeleton is called as corallite resembles with polyp. This disc is cone shaped measuring 1-25 mm dia. The outer wall is solid, stone like made of calcium carbonate and called as theca . corallum: This denotes an entire coral colony skeleton or a group of corallites, and has many thousands of corallites . In living colony small delicate polyps occupy only the upper surface of the Corallium and can withdraw completely in to their protective corallites, which emerges at night to feed. The bottom of the cup or 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.

6. Formation of coral skeleton: 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. It radial folds develop which secrete sclerosepta. At the same time, a rim is built up as a theca wall around the polyp, lying at the top. 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 .

7. Types of corals Hydrozoan corals: 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 - 1. large gastrozooids or nutritive zooids 2. Small dactylozooids or Protective zooids.

8. 2. Octocorallian corals/ Anthozoan corals: Order Alycyonacea includes marine, colonial, and soft corals . A well-known genus is Alcyonium ( D ead 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 (Stoney corals) , 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 (blue coral) Its massive calcareous, skeleton or Corallium is secreted by polyps living in large erect , cylindrical colonial tubes on the surface of skeleton. Order Gorgonacea includes plant-like colonies of sea fans (horny corals) In Gorgonea , colony branches in one plane only. Its axial colony is made by horny proteinaceous materia l intermixed with calcareous spicules arranged around the polyps. Axial skeleton consists of spicules embedded in CaCO3 forming precious hard red coral which is used in jewellery.

9. 3. Hexacorallian corals: Order Madrepora (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. 4. Solitary corals: 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. 5. Colonial corals: 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.

10. Economic important of corals Healthy coral reefs contribute to fishing and tourism, providing millions of jobs and contributing to economies all over the world . Coral skeletons are also helpful in making ridges that may act as natural barriers against sea erosion and cyclonic storms . Coral skeletons serve as raw material for the preparation of lime, mortar and cement because of their calcium carbonate and magnesium carbonate content . Corals of the remote geological past formed reef structures that were highly favorable sites for the accumulation of petroleum deposits . Large quantities of corals are shipped every year for the curio trade.

11. References Coelenterate-R.L . Kotpal Invertebrate Zoology-N. Arumugam Invertebrate Zoology-Park Haswell & Williams Invertebrate Zoology- V.K. Agarwal