1. Chapter 14
Coral Reefs
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

2. Corals
Corals are cnidarians, which are characterized by the presence of stinging cells (nematocysts) on their tentacles
The coral polyp secretes a calcium carbonate skeleton
Most reef-building corals are colonial: interconnected polyps that develop asexually from a single polyp

3. Basic Types of Corals Hermatypic corals Reef builders
Contain zooxanthellae, symbiotic photosynthetic dinoflagellates
Tropical in distribution: warm, shallow water
Ahermatypic corals
Do not build reefs
May not contain zooxanthellae
Can be found in cold, deep water (see Box 14.2, “Deep-Water Coral Communities”)

4. Coral Nutrition
The coral zooxanthellae produce organic matter through photosynthesis
Some of this organic matter is passed to the coral
Zooxanthellae help in the deposition of the calcium carbonate skeleton
Coral polyps also ingest plankton and other small prey aided by the nematocysts in their tentacles

6. Coral Nutrition
Corals can also feed using extensions of the gut wall called mesenterial filaments
The filaments secrete digestive enzymes
The coral can extend the filaments, allowing the coral to feed and digest food outside the body

7. Conditions for Growth of Reef-Building Corals
Hard substrate for settlement of first polyp of a colony
Optimal light for zooxanthellae
Narrow temperature range
Narrow salinity range
Low sediment load in water
Low pollution levels

9. Reproduction of Corals
Corals reproduce both sexually and asexually
Asexual reproduction allows for the growth of a colony of polyps from a single polyp
Sexual reproduction results in the release of planktonic planula larvae for dispersal into new locations
The mass spawning of planulae often occurs at the same time in some coral reefs (see Box 14.1, “Coral Reproduction”)

11. Temperature Requirements
Reef-building (hermatypic) corals tolerate a low temperature range compared with most marine organisms: limited by cold currents
Corals can only reproduce and grow if water temperatures are over 20˚C (68˚F)
Higher water temperatures are preferred for optimal growth
Temperatures above 30-35˚C (86-95˚F) causes coral stress
Coral stress can lead to bleaching, an expulsion of zooxanthellae from the coral polyp

12. Coral Stress and Bleaching
[insert Fig (Ch. 18) in 10th ed.]
- The expulsion of zooxanthellae is termed bleaching because the normally colored coral is bleached white after expulsion
- Zooxanthellae give the corals their distinctive color

13. Coral Stress and Bleaching
Other events besides high temperatures can cause coral bleaching
Poor water quality, increased sediment in water column, wave stress, or disease can also cause bleaching
If corals remain bleached for too long, it can result in death of the coral
Widespread bleaching often occurs as a result of events such as El Niño (see Chapter 18)

14. Other Requirements
Corals normally do not occur near areas where rivers deliver large amounts of freshwater because they have a narrow salinity tolerance
Low salinity can result in coral bleaching as well
Suspended sediments can settle on corals causing damage as well as reducing water clarity, which reduces the sunlight needed for photosynthesis by zooxanthellae
High wave action also increases the amount of suspended sediment

15. Coral Sensitivity to Pollution
Even low levels of pollution can kill coral polyps
High nutrient levels due to runoff from land can also allow seaweeds to take over (see “The Kane’ohe Bay Story,” Chapter 13)
Increased seaweed growth shade light-sensitive corals and their zooxanthellae

16. Types of Coral Growth Forms
Coral colonies can come in a variety of shapes and sizes
Taller, more branching colonies tend to be more common in shallower areas of the reef due to intense competition for space and light
Flatter forms tend to be more common in deeper areas of the reef, probably to help with capturing all possible light available.

17. Types of Coral Growth Forms

18. Other Organisms that Contribute to Reef Building
Calcareous green algae and coralline algae (also produce calcium carbonate)
Soft corals such as sea whips and sea fans
Other cnidarians such as hydrozoans that also produce a calcium carbonate skeleton (fire coral [Millepora])
Sponges (help cement coral rubble into a substrate)
Bryozoans

19. Types of Coral Reefs
Fringing Reefs
Barrier reefs
Atolls

20. Fringing Reefs Simplest and most common form of reefs
Develop as narrow strips along the shore
Consist of an inner reef flat of varying width and an outer reef slope
Growth of corals is typically more intense on the outer reef slope
Growth is slower on the reef flat due to runoff from land, which includes sediment, freshwater, and pollution

22. Barrier Reefs
Barrier reefs also develop along coasts, often farther from shore
A deep lagoon is found between the shore and reef
Waves often wash sediment onto the back reef slope or reef flat, which causes reduced coral growth
Enough sediment may accrue to form small islands on the reef: keys or cays
Ex.: Great Barrier Reef, Australia; Belize off Central America

23. [Insert Fig. 14.18 of 10th ed. here]

25. Atolls An atoll is a circular reef surrounding a central lagoon
Sand cays and islands may be part of the atoll
Width varies from less than one mile to over 20 miles
The inner slope has a more gradual slope than the outer slope; the lagoon normally has a depth of 200 feet or less
The two largest atolls are in the Maldives (Indian Ocean) and the Marshall Islands (Pacific)

26. [insert Fig of 10th ed.

27. Atoll Formation
Atolls begin developing when a volcanic island is formed: a hypothesis first presented by Charles Darwin, now supported by a wide body of evidence
The coral begins as a fringing reef around the newly formed island
Over time, weathering erodes the island
The island eventually disappears below the water surface, leaving only the outer band of coral surrounding a lagoon: the round fringing reef becomes an atoll

28. Coral Reef Ecology
Coral reefs are among the most biodiverse and productive ecosystems on Earth
The coral reef ecosystem is very complex and limited by many physical (light, depth, nutrients, wave action) and biological (competition, predation, grazing) factors
The warm, tropical water where reefs develop is low in nutrients (phosphates, nitrates) due to stratification of the water column and distance from rivers and other sources of nutrients

29. Coral Reef Ecology
Due to low nutrient content, the water surrounding reefs has a low primary production (poor in phytoplankton)
The food web is basically based around the primary production of zooxanthellae in corals and in a few other invertebrates
The coral benefits the zooxanthellae by providing carbon dioxide and nitrogen and phosphorus in excretory products
The zooxanthellae provide organic molecules to the corals (and to all animals that feed on the corals)

30. Coral Reef Ecology
Cyanobacteria (photosynthetic bacteria) present on the reef fix nitrogen into nitrogen compounds (including nitrates)
Seagrasses, mangroves, and seaweeds can also contribute to primary production in the form of detritus
Nitrogen and phosphorus waste products are efficiently recycled, not only be corals and zooxanthellae but by other reef dwellers

31. Typical Coral Reef Food Web

32. Many Organisms Feed Directly on Coral (mucus, eggs, planulae larvae)

33. Competition on the Reef
There is tough competition for space and light on the reef
Some corals grow fast and upright, others grow slow but massive to out-compete other organisms
Corals may feed directly or sting one another if they contact each other
Many coral reef inhabitants compete for space and/or deter predation by producing toxic or foul-tasting chemicals, common in soft corals and sponges

34. Competition on the Reef
Other animals must also compete for limited resources
The many species of fishes, molluscs, echinoderms, crustaceans, and members of other groups normally do this by feeding in slightly different foods or areas or in slightly different ways and/or times of the day, sometimes by reproducing at different times of the year
Each species has its own, well-defined ecological niche

35. Coral Predation, Grazers
Both are important in structuring of coral reef communities
Examples of coral predators: fishes (butterflyfishes), sea stars (crown-of-thorns [Acanthaster]) [see “The Crown-of-Thorns Sea Star”, Ch. 14)
Grazers (fishes, sea urchins) keep seaweeds from taking over coral surfaces

36. [Insert Fig in 10th ed.]