Chapter 14

1. Chapter 14 Coral Reefs

3. Coral reefs rival that other great tropical community, the rainforest in their beauty, richness and complexity • Basic physical structure of both communities is produced by organisms

4. Three-dimensional framework that is home to an incredible assortment of organisms

5. The Organisms that Build Reefs • Coral reefs are made of a vast amounts of Calcium carbonate (CaCO3), limestone that is deposited by living things • Reef-building organisms are corals

7. Reef Corals • Coral is a general term for several different groups of Cnidarians, only some of which build reefs • The polyps produce calcium carbonate skeletons • Billons of these tiny skeletons build a massive reef

9. Sclarctinian corals are the most important reef builders – “True corals” or stony corals • Hermatypic corals are reef building corals – have zooxanthelae • Aheratypic corals –do not build reefs – lack zooxanthellae

10. Zooxanthellae • Contained in most reef building corals • Help corals make their calcium carbonate skeletons – speed up the process • Single celled, photosynthetic algae that live within animal tissues

11. The Coral Polyp • Coral polyps are not only small but deceptively simple in appearance • Look like little sea anemones • Consist of an upright cylinder of tissue with a ring of tentacles on top • Use their nematocyst-armed tentacles to capture food, especially zooplankton

12. The tentacles surround the mouth, the only opening to the sac-like gut • Reef building corals are colonies of many polyps connected by a thin sheet of tissue

14. How a colony starts • A planula (planktonic coral larvae) settles on a hard surface • Larva metamorphoses into a polyp • The single founder polyp divides over and over to form the colony • The digestive systems of polyps usually remain connected, and they share a common nervous system

15. How the Reef Grows • Coral polyps lie in a cup-like skeleton of calcium carbonate that they make themselves. • The polyps continually lay down new layers of calcium carbonate, building up the skeleton beneath them so that it grows upward and outward

16. The skeleton forms nearly all of the bulk of the colony and can take many different shapes • The actual living tissue is only a thin layer on the surface • The calcareous coral skeletons form the framework of the reef

17. Coral Nutrition • Zooxanthellae nourish the host coral as well as help it deposit its skeleton • Zooxanthellae perform photosynthesis and pass some of the organic matter on to the coral • Essentially feeds the coral from the inside

18. Coral Food • Most corals will feed when they get a chance • Voracious predators of zooplankton • Reef as been described as a “wall of mouths” • Corals catch food with their tentacles or in sheets of mucus

19. Cilia move the food to the mouth • Corals also have long, coiled tubes (Mesenterial filaments) attached to the wall of the gut – these secrete digestive enzymes – the filaments can be moved to the outside of the body where the coral can digest and absorb food particles outside the body • Corals can also absorb dissolved organic matter (DOM)

20. Other Reef Builders • Corals cannot build the reef alone • Algae are essential to reef growth • Zooxanthellae are essential to reef growth • Encrusting coralline algae grow in rock hard sheets over the surface of the reef – depositing considerable amounts of calcium carbonate – sometimes more than corals

21. Coralline algae also keep the reef from washing away • The stony pavement formed by these algae is tough enough to withstand waves that would smash the most rugged corals • The algae form an algal ridge around the reef that protects it

22. Conditions for Reef Growth • Corals have very particular requirements that determine where reefs develop

23. Light and Temperature • Shallow water so light can penetrate because the zooxanthellae depend on light • Rarely develop in water deeper than 50 m • Only on the continental shelves, around islands or on top of seamounts • Clear waters • Warm water – 20oC (68oF)

24. If the water is too warm it is bad for the corals • Bleaching – first outward sign of heat stress or of stress of other kinds – corals expel their zooxanthellae

27. El Nino • Brings unusually warm water to many parts of the ocean • Causes widespread coral bleaching and mortality • Reef scientists are increasingly concerned that bleaching is becoming more frequent and more intense as a result of global climate change

29. Salinity, Sediments and Pollution • Most reefs are vulnerable to high levels of sediment unless there is enough wave or current action to wash the sediment away • Most reefs around the world have been damaged by human activities like mining, logging, construction and dredging that greatly increase the flow of sediment onto the reef

30. Corals are also sensitive to pollution of many kinds • Low concentrations of chemicals like pesticides and industrial wastes can harm them • Nutrients too can be harmful to reef growth

31. Most coral reefs grow in water that is naturally low in nutrients • Seaweeds do not grow when nutrients are low • If nutrients are added seaweeds grow and they can shade and choke out the slow-growing corals

33. Kinds of Coral Reefs

34. Coral reefs are usually divided into three main categories • Fringing Reef • Barrier Reef • Atolls

35. Fringing Reefs • Are the simplest and most common kind of reef • Develop near shore throughout the tropics • Need a hard surface for settlement • Grow in a narrow band or fringe along the shore • Occur close to land – vulnerable to sediment, freshwater runoff and dredging

39. Barrier Reefs • Lie along the coast • Occur considerably far from shore • Separated from shore by a relatively deep lagoon

43. Atolls • Ring of reef and often islands or sand cays surrounding a central lagoon • Vast majority are located in the Indo-West Pacific Region • Can be found far from land, rising from the depths of thousands of meters or more

44. Since there in no land around there are few problems with river-borne silt and freshwater runoff

47. Formation of an Atoll • Darwin described • Atolls start when a deep-sea volcano erupts to build a volcanic island • Corals soon colonize the shores of the new island and a fringing reef develops • Eventually the volcano sinks and disappears but the corals continue to grow up

49. The Ecology of Coral Reefs

50. Richest and complex of all marine ecosystems • Thousands of species