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

51. Trophic Structure of Coral Reefs

52. Tropical waters where reefs are found are usually poor in nutrients
Very little primary production or phytoplankton

54. How rich communities grow despite the lack of nutrients
Mutalistic relationships between organisms
Corals and the zooxanthellae
Zooxanthellae provide food and help make calcium carbonate skeleton
Corals provide a place to live and a steady supply of nutrients (N and P) and CO2
Nutrients are not released into the water - recycled

55. Sponges, sea squirts, giant clams and other reef invertebrates have symbiotic algae or bacteria and recycle nutrients just like the corals

56. Fish graze on seaweed and they excrete N, P and other nutrients as waste
These nutrients are quickly taken up by other algae

57. Ocean currents bring in additional nitrogen and phosphorus and other nutrients that are not produced on the reef
Some organisms (corals, bacteria and algae) absorb nutrients directly from the water
Water carries zooplankton a rich nutrient source to the wall of mouths

58. Nutrients in the zooplankton are passed on to the reef community
The production and efficient use of nutrients by coral reef communities result in high primary productivity
Many organisms eat corals or their products so the primary productivity of the zooxanthellae is even more important

59. Seaweeds are also important primary producers
A great many fishes, sea urchins, snails and other animals graze on these seaweeds

61. Coral Reef Communities

62. Competition Space is at a premium
Corals, seaweeds and many others need a hard place to anchor themselves
Reef is crowded and most of the available space is taken

64. How Corals compete for space
Fast growing ones tend to grow up ward and then branch out, cutting their neighbors off from light
Direct approach of attacking the neighbors – extrude filaments and digest away the tissue of other coral

65. Sweeper tentacles – specialized long tentacles that are loaded with nematocysts and sting neighboring colonies
Aggressive corals tend to be slow growing, massive types
Less aggressive corals are usually fast growing, upright and branching

66. Corals compete for space and light with each other but also with the seaweeds and sessile invertebrates
Soft corals are also important competitors for space on reefs
Lack the calcium carbonate skeleton and are able to grow faster than hard corals

68. Coral reef fishes are another group in which competition may be important
Many fish share similar diets for example many species graze on algae and many are carnivores

69. Two theories on what controls the structure of reef fish communities
1. Reef fish abundances are determined by how many larvae are available to settle out from the plankton

70. 2. There is an ample supply of larvae for most species and that reef fish communities are structured by competition among juveniles and adults after the larvae settle out

71. Grazing
Grazing on algae by herbivores is at least as important in coral reef ecosystems as is predation on corals
Surgeonfishes (Acanthurus), parrotfishes (Scarus, Sparisoma) and damselfishes (Pomacentrus, Dascyllus) graze intensively on reefs

72. Invertebrates like sea urchins (Diadema, Echinometra) are also important
Many seaweeds grow rapidly and have the potential to out compete and overgrow corals
Under natural conditions they are kept in check by grazers and to some extent nutrient limitation

73. In addition to controlling how much algae there is, grazers affect which particular types of seaweed live on the reef and where
Coralline algae are abundant because calcium carbonate discourages grazers
Noxious chemicals that are poisonous or taste bad discourage predators so theses seaweeds are usually abundant

74. Seaweeds that lack defenses are most heavily grazed upon and tend to be rare – however, they generally grow rapidly and are am important food source

75. Living Together
Among the vast number of species that live on coral reefs, many have evolved special symbiotic relationships
Mutualism between corals and their zooxanthellae is the essential feature of reef formation

76. Sea anemones, snails and giant clams (Tridacna) all harbor zooxanthellae
Anemonefishes (Amphiprion) have an interesting relationship with several kinds of sea anemones

78. Clownfish
The fish have a protective mucus that keeps the anemone from stinging them – the fish brood their eggs under the anemone
The fish protects the anemone

79. The End ….