Corals are a group of marine cnidarians which are mostly colonial but some are solitary and occur only in polyp stages. Most of them belong to the order Madreporaria but some others belong to sub-class Octocorallia.
Besides a group of Hydrozoa, the Millipora and its allies also are called corals due to their skeletal structure but they have no real similarity with true corals. Since the true corals are the builders of coral reefs and island so only the Madreporarian corals are considered here.
Requirements and Distribution
The reef-building corals require warm, clear, shallow water. Therefore, they are confined to continental and island shores in tropical regions (latitude 28° N – 28°S). They flourish best at temperature between 22°C- 28°C.
As temperature falls with an increase in the depth of sea water, the reef building corals occur up to a depth of 30 metres, or at the most 50 metres. They inhabit water subject to strong wave action, because they cannot remove large amounts of sediment likely to accumulate on them in quiet waters. Excessive rains and fresh water are harmful for corals.
The reef building corals are found in two general regions : (i) the Caribbean waters including Florida, Bermuda, the Bhamas, and the West Indies; and (ii) the Indo-Pacific water from the east coast of Africa through the Indian Ocean and the Western Pacific as for as Hawaii. The second region specially abounds in coral reefs, and in fact the Pacific north-east of Australia is known as the coral sea.
1. Structure of Coral Polyp:
Coral polyp is a skeleton forming sea-anemone. Solitary coral polyps are of big size measuring upto 25 cm in diameter while most of the colonial polyps are very small averaging 1 to 3 mm in diameter. Coral polyp lacks pedal disc & its basal region is occupied by the skeletal cup. The oral disc is typical and bears tentacles in cycles of six. On contraction tentacles may fold over the disc or become incorporated into the disc. The circular or oval mouth is surrounded by flat, depressed or cortical peristome. Mouth leads into a short pharynx. All the septa (mesenteries) bear filaments that are often convoluted and usually protrude through the mouth in feeding. Each filament bears cnidoglandular band. Septal musculature is poorly developed. Mesenteries are more or less confined to upper part of the coral polyp. The lower part of the polyp is occupied by the folds into which the polyp base is forced by the skeletal ridges called sclerosepta. These folds extend to the polyp base between the skeletal ridges as blind pockets & are called Loculi. The single coral polyp develops from planula larva. It settles and by asexual budding becomes the parent of all other members of the colony.
2. Coral Skeleton: Coral skeleton is composed of calcium carbonate. It is secreted by the cells called calicoblasts of the aboral epidermis of the lower part of column and pedal disc. The skeleton of a solitary coral polyp or of each polyp of the coral colony is called corallite. The skeleton of the colony as a whole is termed as corallum.
(a) Structure of Corallite: Coral polyps remain fixed in a cup like exoskeleton, the theca. The skeleton of coral is made up of calcium carbonate and is formed by the precipitation of calcareous crystals in a colloidal matrix secreted by the ectodermal cells outside the body wall for the protection of the polyp. A typical polyp coral has the following parts:
i. Basal Plate: A basal plate lies between the polyp and the substratum. It is the bottom of the cup.
ii. Theca: It is a cup like structure from which the polyp projects outside and into which it can be retracted.
iii. Sclerosepta: These are calcareous ridges or partitions arranged vertically or projecting radially inwards and are connected at the base by basal plate and at the side with the cup like theca. They look like mesenteries but in fact lie between the mesenteries. The sclerosepta are commonly spiny, or thorny with toothed upper edges.
iv. Columella: It is a pillar like irregular central skeletal mass which may be either an independent outgrowth from the basal plate or one formed by the union of the central ends of the sclerosepta, then called pseudocolumella.
v. Epitheca: It is distinct calcareous layer which surrounds the base of the theca in a ring-likd manner.
vi. Costae: The space between the theca and epitheca is crossed by continuation of the sclerosepta called costae.
vii. Pali: Small ridges between the columella and the main parts of the sclerosepta arc termed as pali.
viii. Synapticula: These are the skeletal bars connecting adjacent sclerosepta.
ix. Dissepiments: Horizontal plates between sclerosepta are known as dissepiments. These are of small extent
x. Trabeculae: When the horizontal plates between sclerosepta are large and extend completely the corallite they are termed as trabeculae.
(b) Structure of Corallum: The skeleton of the colony as a whole is termed corallum. It consists of group of corallites which may consist of thousands of corallites. In a colony all the polyps remain interconnected by lateral hollow tubular horizontal folds. Each fold has an extension of the gastro-vascular cavity. In many colonial corals the polyps may be connected by canals coming from the polyp bases and passing through the openings in the loosely constructed theca. Thus the corals with many openings are called perforate (Madrepora) and those in which corallite is of solid texture and the polyps are connected by coenenchyme only over the upper rim of theca are called imperforate (Flabellum, Astraea). This distinction is not of much importance.
Forms of Corals
The growth pattern and the arrangement of polyps in the colony decide the particular form of coral species which are either solitary or colonial.
(a) Solitary Corals are large having disc, cup or mushroom-shaped corallites, and measure 5 to 25 cm across. They are generally without a theca Fungia, Flabellum, Caryophyllia are the examples of solitary corals. They lie on the bottom or are attached to the substratum by a stalk or peduncle.
(b) Colonial Corals are generally with plate-like, spherical vase-shaped, or cup-like corallum. Some of the colonies are branched like plants. Some may form cushion-like or rounded masses (Brain corals). In some corals (Oculina) polyps are widely spaced, each having separate theca. In others corallites or theca remain very close as to have common walls (Favia). In some colonies polyps as well as the thecae become confluent, occupying valleys separated by ridges on the surface of corallum (Maendrina).
(c) Fossil Corals: Fossil corals belong to the existing order Madreporaria and entirely extinct groups-Rugosa and Tubulata. Rugose corals were solitary and those of Tubulata were colonial consisting of tabulae – transverse platform in corallium. More than 6000 fossil species arc known.
(d) Existing Corals: Corals belong to class Hydrozoa and class Anthozoa. They are generally referred as Hydrozoan corals, octocorallian corals and hexacorallian corals.
i) Hydrozoan corals:
Some of the animals such as Millepora, Stylaster etc. belonging to the order Hydrocorallina are colonial and are surrounded by calcareous exoskeleton. The skeleton is secreted by ectoderm. The individual has two types of polyps namely gastrozooids and branched dactylozooids lodged within the exoskeleton. The dactylozooids are arranged around the central gastrozooid. They help in the formation of coral-reefs.
(ii) Octocorallian corals:
Include soft corals. The coral is formed of a colony of polyps with endoskeleton of separate calcareous spicules embedded in the massive mesogloea. In the colonial coral, Tubipora or organ-pipe coral, the skeleton is made of calcareous spicules consisting of vertical tubes connected together by lateral platforms. The vertical tubes are also partitioned by smaller cross plates. The tubes contain polyps. In Haliopora or blue coral, the calcareous spicules form a massive skeleton of corallum. In Gorgonia or sea fan, the colony branches in one plane and the axial skeleton is made up of horny material intermixed with calcareous spicules arranged around the polyps.
(iii) Hexacorallian corals:
These constitute the stony corals or true corals. They may be solitary or colonial and assume a great variety of forms. They are the main constituents of coral reefs.
Formation of Coral
The coral polyp develops from a planula which settles down and begins to secrete a skeletal rudiment or prototheca. It is secreted by ectoderm first as a basal plate. Following it, the larva develops radial folds which secrete septa (sclerosepta) and at the same time a rim is built up as a thecal wall around the polyp, laying at the top. Meanwhile, further skeletal material is added into the gaps between the septa. The septa of the skeleton usually alternate with the mesenteries of a living coelenterate.
Economic Importance of Corals
Stony corals are beneficial to man. They have been noted for their island building operations throughout the warmer seas of the world. Tropical islands such as West Indies and the South Pacific are composed of coral rocks. They may be of great strategic value in the Pacific Ocean for use of military bases and landing grounds.
Fossil corals were favourable sites for accumulation of petroleum deposits. Hard coral lime is also used for building purposes and decoration of architecture. Some corals, being very beautiful, are used as ornaments. The ‘Precious Red Corals’ are used for making jewellery. Some corals are used in interior decoration, indoor aquariums and rock gardens. Some of them form an attractive corner of the museums. Dried, dyed Sertularia are sold as ‘airfern’ for decorative purposes. Some corals have a medicinal value.