calcium carbonate skeletons of the coral colony. Coral bleaching may be caused by the expulsion of the algae from the polyps either due to shortage of nutrients or algae may produce toxins under stress which affect the polyps. Coral bleaching may also be caused due to coral diseases, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures. Coral bleaching because of the warming has destroyed more than 90% of coral around Seychelles.
D. Coral Diseases: There are certain diseases which are reported to cause harm to the coral reefs e.g. white band disease (WBD) and black band disease (BBD) which kill coral tissue by growing in the form of a band around the coral and make them colorless and lifeless. BBD is caused by cyanophyte Phormidium corallyticum, while WBD is believed to be caused by a bacteria pathogen not known yet. These diseases weaken the
corals and damage them. BBD has a higher rate of infection in warmer water so seasonal temperatures affect the spread of BBD. The exact methods by which these diseases are transmitted are unknown.
There are a great number of threats to coral reefs, and most of the threats are related to humans directly or indirectly. Work must be done quickly to protect our threatened aquatic ecosystem. There is a great need to enforce strict rules and regulations to ensure that proper techniques of fishing are used without disturbing the coral life. Rules are of no use unless people are properly educated throughout the world so that they are willingly ready to protect natural resources and so the coral reefs.
Recently, Japan has begun planting baby coral on a Pacific atoll, to save sinking islets. It is a multi-million-dollar project in which several plants of juvenile corals near the uninhabited islets will be planted. So, it is a beginning not only to protect the islets from disappearance but also to preserve the coral community.
Opening of pinnate tentacles
Mouth
Pinnate tentacle
Epidermis
Gastrodermal tubes
Coenenchyma Spicules
Siphonoglyph Solenia
Coelenteron
Soft coral
1a
1b
Fig. 1a: Octocorallian polyp and coral structure.
1b: Different types of spicules found in mesogloea of octocorallian corals.
Expanded polyps (anthocodia)
Polyps withdrawn within Enteron Stalk of the
colony
Base for the attachment
Fig. 2: Alcyonium coral colony
Polyps
Pinnate tentacles
Coenosteum Solenia Blind cylindrical cavities
Fig. 3: Heliopora coral showing two polyps and coral skeleton in a section.
Expanded polyps
Polyp openings
Vertical tubes
Horizontal platforms
Fig. 4: Tubipora, a part of the colony.
Fully expanded polyps
Pores in which polyps are withdrawn
Base of the colony
Fig. 5: Corallium, a part of the colony.
Anthocodia
Main Branches
Mesh formed by cross connections between smaller branches bearing polyps called anthocodia.
Central axis
Stalk Basal disc
Fig. 6: Gorgonia colony
Mouth Epidermis
Tentacle
Gastrodermis
Stomodaeum
Gastrodermal canal of another polyp which is being budded off
Mesenteries Ridges Sclerosepta Corallite
Groove Basal plate
7a
7b
Theca
Secondary mesenteries
Primary mesenteries Retractor
muscles
Pharynx
Endocoelic Sclerosepta
Fig. 7a: L.S. of a Hexacorallian coral showing the growth of sclerosepta alternating with mesenteries.
7b: Diagrammatic section of a polyp showing formation of sclerosepta within endocoels.
8a 8b Fig. 8a. Fungia (a solitary coral)
8b. Diagrammatic section of a solitary coral showing sclerosepta and mesenteries.
Fig. 9: Madrepora (Acropora), a part of the colony.
Coenosarc
Polyps Corallites
Mesenteries Mouth Tentacles
Sclerosepta growing inwards Coelenteron
Basal plate
Sclerosepta Columella
Polyps
Corallites
Solid and stony coral
Fig. 10: Diagrammatic view of Astraea
11a
11b
Fig. 11a: Antipathes, a single expanded polyp. 11b. A few polyps in retracted form.
12a 12b
Mouth
Retracted polyps Tentacles
Capitate tentacles
Dactylopore
Gastropore
Dactylozooids
Medusa
Gastrozooid
Ampulla with medusa Pores
Coenosarcal canals
Tabulae Degenerating
canals
Basal disc
Fig. 12a: Millepora colony ( A hydrozoan coral). Fig. 12b: Millepora in section (magnified view).
A polyp colony Thicket Coppice
Bank 13a
Reef slope Reef edge
Dead corals and shells
Sea ward slope
Coral reefs Island
Heavy coral growth zone
13b Fig. 13a: Development of coral reefs
13b: Generalized diagram of formation of coral reefs.
Sea water
Island
Fringing reefs
Barrier reefs
Lagoon Fig. 14. Fringing reefs and Barrier reefs
Fig. 14. Fringing reefs and Barrier reefs
Island Coral reef
Coral reef
Outer flat Inner flat
Boulder zone Fringing reefs
Reef edge
Seaward slope
Fig. 15. Section showing Fringing reefs Fig. 15. Section showing Fringing reefs
Island Coral reef
Coral reef
Lagoon Fringing reefs
Low tide High tide
Barrier reefs
Fig. 16: Section through Barrier Reefs
Water channels
Atoll coral reefs
Sea water
Coral reef Coral reef
Fig. 17. Section through Atoll
Coral reefs growing up
Island
Growing
corals Subsiding
island
Island
Lagoon
Coral reefs Cora reefs
Fig. 18. Darwin’s theory to explain origin of coral reefs.
Island
VIII Bibliography
1. W. D. Russell-Hunter (1968), A biology of lower Invertebrates, The Macmillan Company, Collier-Macmillan limited, London.
2. M. D. L.Srivastava (1964), A text book of Invertebrate Zoology, Central book depot, Allahabad.
3. Robert D. Barnes (1986) Invertebrate Zoology, 5th Edition, ISBN 0-03022907-3 Saunders College Publishing, Address editorial correspondence to: 210 West Washington Square, Philadelphia, PA 19105
4. Edward E. Ruppert, Richard S. Fox, Robert D. Barnes (2004), Invertebrate Zoology, a functional evolutionary approach, 7th edition.. Thomson Learning Academic Resource Center i-800-423-0563. ISBN0-03-02982-7
5. R.S.K. Barnes, P. Calow, P. J. W. Olive, D.W. Golding, J.I. Spicer (2001), The Invertebrates: a Synthesis, 3rd edition; Blackwell Science Ltd Osney Mead, Oxford OX2 0EL, 25 John Street, London WCIN2BS
6. N.P.O. Green, G.W. Stout, D.J. Taylor (1996), Biological Science, 2nd edition Edited by R. Soper, Cambridge University Press,
7. Adam Sedgwick (1990) A student’s text book of zoology, Vol. I The Protozoa, by professor J. S. Dunkerley, Manchester University, low price publication.
8. T. Jeffery Parker and William A. Haswell, (1962) A text book of zoology, 6th edition, vol. I Macmillan & Co.Ltd, New York, St. Martin Press.
9. Richard A. Boolootian and Karl A. Stiles (1976), College Zoology, 9th edition.
Macmillan Publishing Co., Inc. New York, Collier Macmillan Publishers, London.
10. Jan A. Pechenik (2002), Biology of the Invertebrates, 4th edition. Tata McGraw-Hill Publishing Company Limited, New Delhi.
11. E. N. K. Clarkson(1986), Invertebrate Palaeonotology and evolution 2nd edition, Allen & Unwin(Publishers) Ltd, 40 Museum street, London WCIAILU, UK
12. Ashok Verma (2005), Invertebrates Protozoa to Echinodermata. Narosa publishing house, 22, Darya Ganj, Delhi Medical association Road, New Delhi 110002.Brown, B.E., Odgen, J.C. 1993. Coral Bleaching. Scientific American, 269:64-70.
13. Pechenik, J. A. 1991. Biology of the Invertebrates. Wm. C. Brown Publishers, Dubuque, IA, pp. 91-92.
14. Rowan, R. and D. A. Powers. 1991. A Molecular Genetic Classification of Zooxanthellae and the Evolution of Animal-Algal Symbioses. Science, Vol.
251:1348-1351.