In this article we will discuss about Corals:- 1. Definition of Corals 2. Structure of Coral 3. Types 4. Formation.
Definition of Corals:
The hard external skeleton of certain solitary and colonial marine anthozoan cnidarians; composed chiefly calcium carbonate.
Structure of Coral:
Coral is secreted by some coral forming actinozoa. The skeleton of a solitary polyp is known as the corallite (Fig. 12.38) and many corallites combine to form the skeletal mass. The skeleton as a whole is known as the corallum. The structure of a corallum will be clear from the detailed structural organisation of a single corallite.
Structure of a Corallite:
Each corallite consists of a cup containing vertical ridges radiating from the centre to the periphery. The bottom of the cup beneath the polyp is designated as the basal plate. The wall of the cup enclosing the aboral portion of the polyp is termed as the theca. The ridges of the cup are called the skeletal septa or sclerosepta.
The formation of theca may be an independent process or may arise by the fusion of the outer ends of the sclerosepta and the theca thus formed are called the pseudotheca. The inner ends of the sclerosepta are fused to a central columnar skeletal mass, termed as the columella which may arise independently as the outgrowth of the basal plate or may be formed by the union of the central ends of the sclerosepta.
The columella formed by the last process is termed as the pseudocolumella. A secondary wall or epitheca may be formed outside the theca. The epitheca is separated from the theca by a space and this space is crossed by continuations of the sclerosepta called the costae. Small ridges called the pali are present between the columella and the main parts of the sclerosepta.
There are horizontal plates between sclerosepta, which, when incomplete, are called dissepiments and when complete, are known as tabulae. The sclerosepta are usually spiny. The sclerosepta present in a corallite are definitely related to septa and typically occur in hexamerous cycles: 6 primaries, 6 secondaries, 12 tertiaries; 24 quareternaries, etc. The sclerosepta are comrrtonly endocoelic in origin.
Hand (1966) opines that the arrangement of sclerosepta postulates that the corals are ancestors of the sea anemones.
Types of Coral:
The classification of coral is unsatisfactory. Corals are divided into several types depending upon the habit or association and upon the structure.
I. Solitary corals:
The typical examples of the solitary corals are the Fungia, (Fig. 12.39A), Flabellum, Caryophyllia, Balanophyllia, etc. They are solitary corals having disc or cup or mushroom-shaped corallites and measuring about 5 mm to 25 mm across.
The corallites are often without a theca and lie loose on the bottom and remain attached by a stalk. They may reproduce by longitudinal fission or by budding from any part of the body surface. The sclerosepta of the buds are formed from those of the parents.
II. Colonial corals:
The typical examples of the colonial corals are Oculina, Favia, Madrepora, Meandrina, Tubipora, (Fig. 12.39B, C), etc. Most of these corals are colonial and usually form low flat plates or spherical masses or cups or vases. They may be branched also. The branched forms have short stout or long selender branches.
The colonies are formed by asexual methods from a single sexually produced polyp. The colony is largely composed of calcareous secretion and the surface is only occupied by living individuals.
The polyps constituting the colony may be widely separated from each other with separate theca or the thecae of the different polyps may fuse together to form a common wall. The polyps are usually small, varying from 1 mm to 3 cm in length. The space between the thecae of coral colonies is occupied in living state by coenenchyme.
On the basis of its structural organisation, the corals can be divided into three types:
1. Imperforate or Aporose:
These corals have compact thecae and sclerosepta. The loculi are usually separated off by dissepiments or synapticulae. The examples are: Flabellidae, Turbinolidae, Oculinidae, Caryophyllidae, Faviidae.
The corallum is extremely porous and is of loose construction. The examples are: Eupsammidae, Acroporidae, Poritidae, Madrepora.
They may be either perforate or imperforate and have laminated septa joined by synapticulae. The corals belonging to the family Fungiidae are included under this category.
Formation of Coral:
The actual method of secretion of coral and the subsequent stage of formation are not very clearly understood. But the following account will give an idea about its formation. Secretion of coral does not commence until twelve primary mesenteries are attained by the polyp.
Secretion of calcareous materials which either immediately crystallise or crystallised by the interaction with chemical substances present in sea water, transforms into a calcareous mass. The calicoblast layer forms minute nodules at the base of polyp. The nodules unite to form prototheca which becomes gradually thickened and later assumes a cup-shaped structure.
At this stage the body- wall of the living polyp may or may not overflow the edge of prototheca. In the former case, the growth of the prototheca is brought about by the calicoblasts. The cup is now called the theca and the overflowed tissue is called the episarc.
The episare of the primary zooid thus overflowed on the substratum, gives rise to the successive layers of skeleton, called the “Coenenchyma”. By the process of asexual reproduction new zooids are developed near the primary one.
The episarc, independent theca and septa are gradually formed in secondary zooids also. The skeleton of different zooids remains interconnected with the primary one. Many corallites constituting the corallum are formed. Separate film of skeletal material is formed outside the theca and this layer is called the epitheca. Six to twelve ectodermal infolding’s are formed at the base of the polyp.
Secretion of skeletal material in those grooves takes place and sclerosepta are formed within the theca. Gradually the secondary and tertiary sclerosepta are developed in spaces between mesenteries and they alternate with the mesenteries. The edges of the theca become the cup.
The vertical space thus formed is filled with secretion which may be in the form of these plates. In many cases polyps may be connected by canals. The soft parts are washed away and the calcareous masses are left on the spot.