Corals and coral reef formation in Coelenterata

PRESENTED BY Dr. Y. SAVITHRI LECTURER IN ZOOLOGY GOVT. COLLEGE FOR MEN(AUTONOMOUS), KADAPA. Definition:

 Corals are animals belonging to the phylum Coelenterata.

 Calcareous or horny skeleton secreted by the class Anthozoa and few Hydrozoa animals of coelenterata is called as corals.

 Most of the corals belong to the class Anthozoa and few others to the class Hydrozoa.

 Corals are mostly marine colonial polypoid coelenterates living in the skeleton of calcium carbonate secreted by them selves. Some of them grow into huge mounds, while others are large and branched colonies.

 Vaughan (1917) defined coral reef as “a ridge or mound of lime stone the upper surface of which is near to the surface of the sea and which is formed of calcium carbonate by the actions of organisms, chiefly corals”.

Types of Coral Corals are solitary or colonial polypoid coelenterates living in a secreted skeleton formed of calcium carbonate depositions of their own. In coelenterata most of the anthozoans form corals which includes 2500 live and 5000 fossil species. They are red, blue and black in colour. There are three types of corals are there:

1. Hydrozoan corals

2. Octocorallian corals

3. Hexacorallian corals 1. Hydrozoan corals:

The hydrozoan corals belong to the genera, Millepora, Stylaster and Distichopora.

These are colonial and secrete massive calcareous exoskeletons. Within the exoskeletons, two types of polypoid forms, the gastrozooids and branched dactylozooids are found.

 The epidermis of these polypoid forms is modified and referred to as the calicoblast layer which secretes the calcareous exoskeleton.

 These corals are found with other corals forming huge mounds.

2. Octocorallian corals:

The Octocorallian corals belong to the following orders: Stolonifera - Tubipora (organ pipe coral) Alcyonacea - Alcyonium (Dead man’s fingers) Coenothecalia - Heliopora (Blue coral) Gorgonacea - sea fans like Gorgonia and Corallium. (Plant-like colonies) Order: Stolonifera: It includes colonial corals Skeleton is made up of calcareous spicules stained with iron, salts and consists of lateral plat forms. The vertical tubes are partitioned by cross plates. The polyps are lodged in the tubes. Ex: Tubiphora (organ pipe coral) Order: Coenothecalia: It includes only one species. The polyps form a very large coral skeleton called corallium. Ex: Heliopora (Blue coral)

Order: Alcyonacea: It includes soft corals. Skeleton is formed by calcareous spicules embedded in a massive mesoglea with the fusion of many polyps.

Ex: Alcyonium (Dead man’s finger) Order: Gorgonacea: It includes includes horny corals, The sea fans like Gorgonia. In this coral is a branched colony of polyps which secrete a horny proteinaceous material with calcareous spicules around the polyps.

3. Hexacorallian corals: This type of corals are stony corals or true corals form by madriporaria order. These are solitary and many of these are colonial coral forms. This group is the principal contributors of coral reefs. Coral Formation:

 The coral polyp develops from a planula which begins to secrete a skeletal rudiment or prototheca. The calcareous rudiment crystals mix with chemical substance of sea to form a calcareous mass. The calicoblast layer forms some small buds like structures at the base of the polyp and formed as a basal plate. All these cells unite to form protheca. Later larva develops radial folds which septa or sclerosepta and rim is built up as a thecal wall around the polyp. This layer on the top is called as mantle and the cells covered is episarc. Coral formation includes: A) coral polyp B) Coral skeleton C) Coral growth D) Forms of coral E) Fossil corals A. Structure of coral polyp

 The coral polyps are solitary (25cm dia) or colonial (1.3 dia).

 A basal disc is absent because the basal region of the polyp is surrounded by a calcarious exoskeleton

 Oral disc bears tentacles in cycle of six. Coral polyp extended from theca Mouth leads to pharynx. Mesenteries follow hexamerous plan and are restricted to upper part of the polyp. Mesentery fibres are with cnidoglandular band with cnidocytes.

Internal structure of coral polyp b. Coral Skeleton: The coral polyp secretes calcareous skeleton from the opposite sides of the column, pedal disc epidermis. This skeleton secreting epidermal cells are calicoblasts. It includes corallite and corallum

1. 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.

2. 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.

C. Coral growth

 The colony grows in size or thickness by the continuous budding of new polyps. Buds may arise from horizontal tubular interconnections or from the polyps themselves. Consecutive layers of skeleton are secreted as the new polyps are budded off, and further the skeletal layer increase in thickness, when polyps fully grow. D. Forms of corals

Based on the growth and arrangement the polyps are two types: They are : 1. Solitary corals 2. Colonial corals: 1. Solitary corals: The corallite is disk-like, cup-like or mushroom-shaped`. Measures 0.5 to 25 cm , with numerous sclerocepta without theca. These lies on the bottom by a stalk or peduncle.Ex: Fungia, Flabellum, Ceatophyllum.

Fungia Flabellum

2. Colonial Corals: Most of stony corals are colonial Plate-like, cup-like, spherical or vase-shaped skeleton Ex: Acropora, Oculina, Favia, Meandrina etc

Meandrina Acropora E. Fossil corals

 The order madriporaria includes more than 6000 fossil species of coral. Rugosa and Tabulata includes complete extinct forms. The fossils of these corals formed great in palaeozoic era.. Which were replaced by madriporarian corals. CORAL REEFS

 Coral colonies grow continuously in size by budding and form extensive masses known as Coral reefs.

 According to T. Wayland Vaughan (1917)’coral reef is a ridge or mound of limestone whose upper surface is near to the surface of sea

and which is formed chiefly of CaCo3 secreted by coral polyps.

 Principal builders of coral reefs are stony corals, important contributors are the hydrocorallines and alcyonarians.

 Foraminiferan protozoa also take part in the formation of coral reefs.

 Reef building coral requires warm shallow water (above 20◦C). Most of them develop to 10-20 mm in a year. The present coral reefs are formed 15000-30000 years ago.

Coral fauna: The coral reef is a shelter for several animals. The cracks crevices and masses of corals attract a host of other animals. This includes crabs, shrimps, barnacles, worms, starfishes, brittle stars etc. No part of the earth has this incredible variety of life as a coral reef.

TYPES OF CORAL REEFS:

 The coral reefs are of three types depending upon formation:

1. Fringing Reef

2. Barrier Reef

3. Atoll Reef Fringing Reef  Coral reef lying close to the shore of some volcanic islands or part of some continent are termed as fringing reef.  A Fringing reef may extend out to a distance of a quarter mile from the shore with the most active zone of coral growth facing the sea.  The seaward zone is commonly called the edge or front.  A shallow water channel, 50 to 100 meters broad, lies between the reef-edge and shore. At low tide, water of channel recedes at quickly exposing a flat bottom surface called reef flat.  It is largely composed of coral colonies and other animals.

Barrier Reef  Barrier reefs are like fringing reefs but they are locted some distance away from the shore.  The stretch of water, separating the barrier from land, may be half a 16 km or more in width. It is called a lagoon.  It is 20 meters to 100 meters deep and suitable for navigation.  Ex: Grear brrier reef along the North-estern coast of Australia. (about 200 km long and up to 150 km from shore.

Atolls  An atoll also termed a Coral island or Lagoon island.  It is a ring-like or horse-shoe shaped reef that encircles a lagoon but not an island. The lagoons varies from a few to about 90 km across.  It may be complete or broken by a number of channels of which only a few are navigable.  Outer side of the reef slope off rather steeply into the depth of ocean.  The atoll of Bikine, famous for atomic and hydrogen bob tests, lie in the Pacific Ocean.

ATOLL REEF FULANGA Atoll Formation: Step by step FORMATION OF CORAL REEFS

 1. Darwin- Dana Subsidence theory: Darwin (1831) proposed that each atoll takes approximately the shape of formed island. He also noticed that known coral reefs are found regions where sinking of land probably taken place. He explained that in a previous age, corals grew as fringing reef on the sloping shore of an island in a shallow tropical sea subsidence of the island then commenced. Thus slowly coral could grow upward so as to keep its surface same to that of ocean. The coral of seaward edge grew more rapidly for better food supply, there by the original fringing reef converted in to a barrier reef separated by a deep water channel the lagoon. This island became smaller and finally disappeared thus rising barrier reef became a ring shaped atoll. This theory is accepted one and supported by James Dwight Dana and WM Davis. 2. Stutchbury’s volcanic crater theory:

He supposed that the atolls of Pacific were built upon the summits of extinct volcanies. The crater of the volcano became the lagoon, while its elevate rim of the land for growth of coral reef. But most diverse shapes of atolls, limited depths of lagoons, the almost same elevation and the great number of craters in a single archipelago, make hypothesis incredible. 3. Simper Murray solution theory:

According to british Ship challenger Murray (1873-76) mounds are frequently built uon the ocean floor by the limestone skeletons of animal and other sediments, which rise up to right level the corals grow on them. A barrier reef results from the better growth of coral at the outer edge. Atoll is formed at the inner coral rock. Submerged bank theory:

Says that coral growth occurs on a flat existing surface, during or after the submergence of such a surface. It is presently accepted theory as the large number of submerged banks, drowned valleys, submerged cliffs occurs at proper depths in coral seas.

ECONOMIC IMPORTANCE OF CORAL REEFS

. Corals of the remote geological past formed reef structures were highly favourable sites for the accumulations of petroleum deposits.

. Coral reefs serve as habitats for many plants and animals like sponges, Molluscs, Echinoderms, Fishes etc.,

. Some corals are highly priced for their decorative value, Corallum rubrum is considered to be a precious coral in India and China.

. The red coral and organ pipe coral are used in some indigenous system of medicine in South India.

. Chunks of coral skeleton belonging to species porites are used as building material.

. Coral skeleton serve as raw material for the preparation of lime, mortar and cement because of their calcium carbonate and magnesium carbonate content.

. Reefs serve as good nursery grounds for commercially important fishes.

FIZZI ISLANDS OF PACIFIC OCEAN AND THOSE SITUATED IN BAHAMA ISLANDS OF ATLANTIC OCEAN REGION ARE THE BEST KNOWN ISLANDS OF THE WORLD.