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 0 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. 1 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. 2 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). 3 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. 4 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. 5 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 gastro- zooids. 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 . 6 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. 7 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