Dr. Y. SAVITHRI

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ZOOLOGY – SEMESTER – I UNIT-II : COELENTERATA –

CORALS AND CORAL REEF FORMATION IN COELENTERATA

HANDOUTS

PREPARED BY

Dr. Y. SAVITHRI

LECTURER IN ZOOLOGY

GOVT. COLLEGE FOR MEN (AUTONOMOUS)

KADPA

PHYLUM : COELENTERATA

Introduction: Aristotle Knew the stinging qualities of coelenterates and considered these organisms as intermediate between plants and animals and termed them Acalephe or cnide (Gr., akalephe =nettle; cnodos = thread). They were included in the Zoophyta ( Gr; zoon= animal; phyton= plant) together with various forms from sponges to ascidians. The animal nature of coelenterates was established by Peyssonel (1723) and Trembley (1744). Linnaeus, Lamarck and Cuvier grouped the coelenterates under Radiata which included the echinoderms also because of their symmetry. Hatschek (1888) splitted Coelenterata into three distinct phyla – Spongiaria (Porifera), Cnidaria (Coelenterata) and Ctenophora. General characters of Coelenterata Leuckart (1947) separated the coelenterates from echinoderms and treated a separate phylum Coelenterata (Gr., koilos = cavity; enteron – intestine).  Coelenterates are Metazoa or multicellular animals with tissue grade of organisation.  These are aquatic, mostly marine except few freshwater forms like Hydra.  These are sedentary or free-swimming and solitary or colonial.  Individuals are radially or bi-radially symmetrical with a central gastro vascular cavity communicating to the exterior by the mouth.  Diploblastic animals body wall consists of an outer layer of cells called ectoderm and inner layer of cells the endoderm.  These animals exhibit the phenomenon of polymorphism.  Acoelomate animals because they do not possess a second body cavity, the coelom.  Short and slender tentacles provided with nematocysts; encircle the mouth in one or more whorls and helps in food capturing, ingestion, locomotion and protection.  These are usually carnivorous; digestion is extracellular as well as intracellular. Anus is not found.

 Nervous system is primitive, consisting of diffuse nerve net.  Respiratory, circulatory and excretory systems are wanting.  Reproduction is both by asexual and sexual methods. A ciliated planula larva usually present in the life history.  The life history exhibits the phenomena of alternation of generations or metagenesis in which the asexual polypoid, sessile generation alternates with sexual medusoid, free- swimming generations. Classification of Coelenterata

The phylum includes nearly 11000 known species but half of which are extinct. According to Hyman, Phylum Coelenterata has been divided into three classes, viz., Hydrozoa, Scyphozoa and Anthozoa.

Class: Hydrozoa (Gr: Hydra=water, zoon=animal

Hydrozoa are solitary and freshwater or mostly colonial and marine, sessile and free- swimming forms. They exhibit tetramerous or polymerous radial symmetry. Body wall consists of an outer ectoderm and an inner endoderm separated by non- cellular mesogloea. Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament. Skeleton or horny structure is horny peri- sarc in some forms, while coenosarc sec•retes a skeleton of calcium carbonate forming massive stony structure or coral in other forms.

They exhibit polymorphism, There are two main types of zooids, the polyp and medusa. Medusa is provided with true muscular velum. Many Hydrozoa exhibit alternation of generations. Reproductive products or sex cells are usually ectodermal in origin and discharged externally. Cleavage is holoblastic, embryo ciliated planula.

Ex: Hydra, Obelia, Porita, physalia (Portugese an of war), Velella (Little sail).

Class: Schyphozoa: Gr: Skyphos= cup, Zoon= animal

Scyphozoa include large jelly-fishes or true medusae are exclusively marine. Medusae are large, bell or umbrella-shaped, without true velum, free swimming or attached by an aboral stalk. Marginal sense organs are tentaculocysts having endodermal statoliths. Polypoid generation absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilisation or transverse fission. Gonads are endodermal and the sex cells are discharged.

Ex: Aurelia, Cassiopea, Rhyzostoma, Lucrnaria, Periphylla.

Class: anthozoa gr: anthos=flowers, zoon = animal or actinozoa: gr: actin = ray, zoon = animal. Solitary or colonial exclusively marine formsand exclusively polypoid. Medusoid stage is altogether absent. Body usually cylindrical with hexamerous, octomerous or polymerous biradial or radio bilateral symmetry. The stomodaeum is present, often provided with one or more ciliated grooves the siphonoglyphs. Gastro vascular cavity is divided into compartments by complete or incomplete septa or mesenteries. Mesenteries bear nematocysts at their inner free edges. Exoskeleton is formed from calcium carbonate which often form massive corals. The ripe sexual products are discharged into coelenteron.

Ex: Metridium(Sea anemone), Adamsia (sea anemone), Corallium (Precious coral or red coral), Gorgonia (Sea fan), Pennatula (Sea pen).

POLYMORPHISM IN COELENTERATA (GR: POLYS = MANY, MORPHE = FORM)

Polymorphism is one of the characteristics feature of Coelenterate animals. In coelenterata or in hydrozoa which may be single or colonial, here occur two main types of individuals or zooids- Polyp and medusae. Polymorphism (Gr., poly = many; morphe = form) is the occurrence of several different types of individuals or zooids in a single species during its life cycle or as members of the colony, the members perform different functions so that there is a division of labour amongst the members. However, polymorphism may be defined as the representation of a single organism by more than one kind of individuals or zooids which differ in their form and function.

Polyp: A polyp has a tubular body with a mouth surrounded by tentacles at one end. Other end is blind and usually attached by a pedal disc to the substratum.

Medusa: A medusae has a bowl or umbrella shaped body with marginal tentacles and centrally located mouth or manubrium.

Patterns of polymorphism:

Degree of polymorphism varies greatly in different groups of hydrozoa.

Dimorphic: Simplest and commonest pattern of polymorphsm is exhibited by many hydrozoan colonies like Obelia, Tubularia etc., They have two types of individuals or zooid namely:

Gastrozooids or hydranths are connected for feeding

Gonazooids or blastostyles with asexual budding forming sexual medusae or gonophores.

This phenomenon is termed as dimorphism.

Trimorphic: Besides gastrozooids and gonozooids they also possess a third type individuals the dactylozooids.

Polymorphic: Animals having more than three types of individuals are called polymorphic. some what greater degree of polymorphism is found in the encrusting colony of Hydractinia with five types of polyps each performing a specialized function.

Gastrozooids - feeding Dactylozooids - protection. Tentaculozooids - Sensory cells Skeletozooids - Spiny projections of chitin Gonozooids - Reproductive individuals. POLYPOID ZOOIDS: Gastro zooids Dactylo zooids Gono zooids

Gastrozooids :

The nutritive polyps are called gastrozooids. They alone take up nutrition in the colony. A mouth is present at the tip of the hypostome. Near the base of a gastrozooid usually a single, long and contractile tentacle arises. It shows batteries of nematocysts. Lateral branches are present called tentilla. Gastrozooids catch the prey and digest it. The digested food is thrown into the coenosarcal canal. In Pennatula the gstrozooids modified in to nectocalyx which are like buds on the body and helps in driving the water. In Renella nectocalyx are in bunches sometimes called as pseudonecocalyx. In Millipora many gastropores protrude out from the polyp. Each polyp with 4-6 tentacles and cnidoblast buds .

Dactylo zooids : They are called Palpons, feelers or tasters. They resemble the gastrozooids. They do not show mouth. Their basal tentacle is un branched.In Physalia, the tentacle is very long. In velella and Porpita the margin of the colony bears long and hollow tentacles. These zooids are protective in function. They bear batteries of nematocysts.

Gonozooids : The reproductive zooids. They have no mouth. In Physalia the gonozooid shows branched stalk, bearing clusters of gonophores (gonopalpon). Gonozooids produce medusae called gonophores. In Porpita and Velella dactylozooids are treated as gonodactylozooids.

MEDUSOID FORMS : Pneumatophores Nectopore or Nectocalyx or swimming zooid Bracts Gonophores

Pneumatophores : It functions as a float. It is an inverted medusan bell. The walls are two layered and highly muscular. The epidermal lining becomes glandular to form a gas gland. The gas gland secretes gas into the air-sac  The pneumatophore is small in Halistemma.  The pneumatophore is very large in Physalia.  It is disc-shaped in porpita.

Nectopore or Nectocalyx or swimming zooid: Nectocalyces or nectophores are bell-shaped medusoids with a velum, radial canals and circular canal, they have no mouth, manubrium, tentacles or sense organs, A nectocalyx is muscular and brings about locomotion of the colony by swimming. It is also referred to as nectophore or nectozooid.

Bracts : They are also known as hydrophyllia. They are leaf like, helmet shaped. They serve to cover and protect other zooids of the colony.

Gonophores : Bearing gonads, male gonads produce sperm and female gonad produce ova

Hydrozoans exhibit remarked development of Polymorphism. Some of them are Physalia, Halistemma, Porpita

Physalia: Is commonly called as Portuguese man of war. This is a free floating pelagic colonial form. The medusa is modified in to a big pneumatophore or float which floats above the water. The underside of the float has cormidia. Each cormidium consists of a small dactyl zooids with a long slender tentacle, a large dactylozooid with an enormous nematocyst bearing fishing tentacles. A branched gonozooid with male and female gonophores is present. Halistemma: This is a floating form with long, thin peduncle (with nodes) having different zooid. Pneumatophore is at first anterior end of peduncle and helps the animal float on the surface of the water. The bottom of the float has asmmetric medusa, which are called nectocalyces which help in locomotion. Each nectocalyces is with nodes and bell shaped. Manubrium is abs ent.

Porpita: It has medusoid disc like large pnematohore and chitinous shell with many concentric gas chambers. On the ventro-central region is a single large gastrozooid which is surrounded by clusters of small gonozooids which bear sexual medusae. On the edge of it tantacle like dactylozooids armed with nematocysts.

ORIGIN OF POLYMORPHISM

There are many theories to explain the origin of polymorphism in coelenterates. Poly-organ theory: This theory was proposed by Huxley (1859), Eschscholtz (1829), E. Metschnikoff (1874) and Muller (1871).

According to this theory, a polymorphic colony is supposed to be a single medusoid zooid; its various components are regarded to be the modified organs of this medusoid zooid. The various parts of the zooid, i.e., manubrium, tentacles, umbrella, etc., multiply independently from one another and they have assumed different forms to perform different functions.

Poly-person theory: This theory was first proposed by Leuckart (1851), Vogt (1848), Gegenbaur (1854), Kolliker (1853), Claus (1863) and later strongly supported by E. Haeckel (1888), Balfour (1885) and Sedgewick (1888).

According to this theory colony is not a single individual but various parts of the colony are modified individuals which have changed their structure due to division of labour. They have all modified from the primitive zooid which was a polyp.

Medusa theory:

This theory was proposed by Haeckel (1888) as a compromise between the above theories. The theory says that the siphonophores formed from gastrula was a medusoid individual, from which zooids or persons appeared by budding from the subumbrella.

SIGNIFICANCE OF POLYMORPHISM

The phenomenon of polymorphism is essentially one of division of labour in which specific functions are assigned to different individuals. Thus, polyps are modified for feeding, protection and asexual reproduction, while medusae are concerned with sexual reproduction. This distribution of functions among diversified individuals and their subsequent modifications in coelenterates may have resulted from their initial simple organization and lack of organ specialization. Polymorphism gave the colonies competitive edge in protection and food gathering and eventual survival. polymorphism: colonies of some species have morphologically differing individuals each specialized for certain roles e.g. feeding, reproduction & defense etc.

CORALS AND CORAL REEF FORMATION IN COELENTERATA

Calcareous or horny skeleton secreted by the class Anthozoa and few Hydrozoa animals of coelenterata is called as corals. Corals are mostly marine colonial polypoid coelenterates living in the skeleton of calcium carbonate secreted by themselves. 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 include 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 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. 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.

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

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. 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. Coral Skeleton: The coral polyp secretes calcareous skeleton from the opposite sides of the column, pedal disc epidermis. These 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. 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.

2. Colonial Corals: Most of stony corals are colonial, Plate-like, cup-like, spherical or vase- shaped skeleton Ex: Acropora, Oculina, Favia, Meandrina etc 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. Types of Coral Reefs The coral reefs are of three types depending upon formation:  Fringing Reef  Barrier Reef  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.

Formation of coral reefs

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.

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.

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:

This 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.

WEB LINKS:

1. http://www.biozoomer.com/p/invertebrates-are-backbone-lacking.html 2. http://www.biologydiscussion.com/ 3. http://www.mcz.harvard.edu/Departments/InvertZoo/ 4. http://www.iaszoology.com/ 5. https://www.slideshare.net/ 6. http://digitalcommons.unl.edu/onlinedictinvertzoology/