Corals PDF

PHYLUM Cnidaria (Corals or Coelentrates) Members of this phylum include the sea anemones and jellyfish and exists as polyps, medusa, or alternate stages. The polyp stands on a base with mouth and tentacles extended upwards. The medusa floats with mouth and tentacles extending downward...

PHYLUM Cnidaria (Corals or Coelentrates) Members of this phylum include the sea anemones and jellyfish and exists as polyps, medusa, or alternate stages. The polyp stands on a base with mouth and tentacles extended upwards. The medusa floats with mouth and tentacles extending downwards Cnidaria are the simplest of the true metazoans. They are of an evolutionary grade higher than the sponges, since their cells are properly organized into tissues which are normally constructed on a radial plan The individuals are either solitary or colonial in habit and are of distinctly different but essentially homologous types , polyp and medusa. Polyp  Cnidaria are characterised by a life cycle in which successive generation are of different kinds. This alternation of generation is typical of less specialized cnidaria but may be suppressed entirely in the more advanced forms. The two types of individuals are the fixed polyp and the free medusa alternate successively so that the polyp gives rise to asexual to the medusa, which reproduces sexually so that their zygotes produce the polyps.  Three important groups of hard corals in the fossil record are a. Tabulates b. Rugosans c. Scleractinians. Tabulates corals are colonial. They possess well-developed tabulae but poorly developed septae. They range from middle Ordovician to Permian and became extinct at the end of the Palaeozoic.  Rugose corals can be either colonial or solitary. They have well-developed septae in sets of four (tetracorals). On the outside of the corals are coarse ridges called rugae. The solitary rugosans have a cup or cone shape and are often called horn corals. They range from middle Ordovician to Permian and were most abundant in the Devonian period.  CLASSIFICATION There are three classes 1. Hydrozoa 2. Scyphozoa 3. Anthozoa. CLASS Anthozoa Corals of this class have a polyp stage only, with no medusoid stage in the life cycle.  Though some corals are soft and have no calcareous skeleton, hard corals secrete aragonitic skeletons. The skeleton is tube-shaped and has walls that extend upwards as the polyp grows. The tube is called corallite. Hard corals are either solitary or colonial. Solitary forms are not attached to any other corallite. Colonial corals have their corallites attached to one another forming colonies of various shapes and sizes.  Anthozoans are grouped into three classes. 1. Ceriantipatharia 2. Octocorallia 3. Zoantharia. Subclass Zoantharia They are solitary corals living on various substrates and at depths of 0 – 2750m. In regions of strong tidal currents it is attached to pebbles or boulders but in weak currents on the outer shelf it can live in a variety of substrates.  The soft basal tissue secretes an aragonitic cup or corallum which is short and horn- shaped. It has an outer wall epitheca within which are numerous radially arranged septa between which lies the mesentaries. The first formed part prosepta are larger and more pronounced than the metasepta intercalated between them. A coral with part of epitheca removed showing bilateral symmetry.  Four orders have been defined for the Anthozoans; 1. Rugosa 2. Tabulata 3. Scleractinia 4. Heterocorallia  Order Rugosa They are an almost exclusively Palaezoic group of solitary and colonial corals. They show bilateral symmetry arising because the numerous metasepta are inserted in four loci alone. The soiltary forms build straight, curved conical or coni-cylindrical coralla and are commonly designated horn-corals. Colonial species built dendritic or massive compound coralla of many closely packed septate corallites of both cylindrical and prismatic nature.  The name Rugosa is commonly applied to this order calling attention to the rough exterior of many of the corallites. The skeletal elements form the basis of classification of the Rugosa and are of two main types. 1. Vertical elements ( septa, axial structure) 2. Horizontal elements ( tabulae, dissepiments) The septa are thin vertical plates arranged in a characteristic biradial pattern which develops to maturity through the ontogeny of the corals.  SEPTAL GROWTH In the early growth of the organism a single proseptum divides it, soon becoming separated into cardinal (C) and counter (K) proseptum. Two other pairs of prosepta follows: The alar (A) adjacent to the cardinal septum and the counterlateral (KL). There is a pause in the development of the animal.  Six primary septa (prosepta) thus far developed. Cardinal counter Two alar and Two counterlateral. All major septa formed after the prosepta are metasepta and they appear at definite places in the four unequal quadrant marked off by the cardinal, counter, and alar septa.  The rugosa passes through a stage with six sclerosepta, all subsequent major septa are added in pairs in the cardinal and counter quadrants. In the cardinal quadrant metasepta appear on each side of the cardinal septum In the counter quadrant, they develop on the counter side of the alar septa. After the prosepta are developed, metasepta and minor septa are added in several different ways and in variable numbers so that the number of septa in the adult corallite can range from dozen to more than a hundred.  REJUVENESCENCE In solitary cylindrical forms the corallites is often found to be constricted at regular intervals, leaving a broad or narrow shelf where the septate older calice is exposed. Above this it may expand again, before once more being constricted. These constricted bands probably represent periods of famine during which the polyp resorbed its own tissue in order to stay alive and shrank away from the edges, becoming smaller while retaining its form. The next period of increased food supply permitted growth to begin once again. This is rejuvenescence  calice Rejuvenescence in the corals  EVOLUTION IN THE RUGOSA AND GEOLOGIC HISTORY The guiding principle controlling the evolution of the rugosa was always towards the development of a strong and firm skeleton. For instance , the complication of the trabeculae increased skeletal strength, the carinae held the polyp more firmly and the axial column gave greater support in the central region.  The oldest known tetracorals are found in rocks of lower Ordovician age. The rugosa attained its maximum in both abundance of individuals and number of species in Mid Palaeozoic (Silurian and Devonian) seas. They declined to extinction at the end of the Permian. Fossil rugosa are most abundant in calcareous rocks i.e limestone, argillaceous limestone and calcareous shales  They form extensive biostromes , but rarely form bioherms. Many are remarkable index fossils.  Order Tabulata The tabulate corals are an extinct, exclusively Palaeozoic group of compound corals. They are always colonial, never solitary and usually have small corallites. They have prominent tabulae, but other skeletal elements in particular the septa are reduced or absent.  FORM OF CORALLUM The corallum is built up by individual polyps which may or may not be directly connected to each other. Cerioid forms (eg Favosites) had polygonal corallites all in contact. Cateniform colonies (eg. Halysites) have elongated corallites joined end to end in a wandering palisade. Fasciculate tabulates (eg. Syringopora) have cylindrical corallites which may be dendroid or phaceloid and may be provided with connecting tubules.  Coenenchymal tabulates (eg. Heliolites) have no dividing wall between the corallites but instead a common mass of complex tissues. The coenenchyme originates as a marginarium and forming a dense calcareous mass in which the corallites are embedded.  EVOLUTION AND ECOLOGY OF THE TABULATA The earliest Tabulata are known in the Chazyan (oldest of three epochs of the Ordovician) of North America. By Trenton (Mid Ordovician) times they had spread to many parts of the world and the first Heliolithina and Halysitina had appear. The Favositina from their first appearance in the Middle Devonian became a very dominant group throughout the Silurian and Devonian, though the Heliolithina were also important.  With the Carboniferous, the Favositina which has undergone many change, lost their dominance and became subordinate to the Syringoporina. These remained the important groups until the end of the Permian. Large tabulates are found in coral- stromatoporoid reefs and were relatively important, though they were not really frame builders since they had no proper means of attachment.  Smaller tabulates tended to occur in deeper waters and fasciculate genera usually lived in quieter environment. Ordovician and Lower Silurian tabulates of smaller sizes are often found with early solitary Rugosa, forming a characteristic association in calcareous mudstone. Tabulata are not normally found in very high energy environments and sometimes occur in useful marker bands. CNIDARIA AS REEF BUILDERS Fringing Reef Barrier Reef Atoll