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Chapter 14

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Chapter 14

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  1. Chapter 14 Coral Reefs

  2. Corals • Corals are _____, a phylum of animals characterized by the presence of _____ on their tentacles • These _____ are used to capture prey primarily, but can also be defensive • The soft _____ is enclosed in a hard shell of calcium carbonate created by the coral.

  3. Anatomy of a Coral Reef • Coral reef polyps are interconnected by thin layers of tissue • They share a nervous system and digestive connection – this is due to the fact that the entire colony came from one original polyp that reproduced asexually by _____to produce the colony

  4. Anatomy of a Coral Reef • Each polyp continues to lay down new CaCO3 beneath the polyp body so the entire coral colony continues to grow upward • Each polyp is normally 1 to 3 mm in width • However, the collection of polyps together can collectively grow to extremely large sizes and weigh several tons

  5. Coral Nutrition • The zooxanthellae housed inside the coral produce _____ through photosynthesis • Some of this _____ is passed to the coral • This mutualism aids the coral and helps the entire reef grow faster • In fact, if zooxanthellae are supplied with enough light, they can feed the coral completely

  6. Coral Nutrition • Of course, coral also possess ____ that contain nematocysts to sting potential prey that come within reach of the tentacles • These cells allow the coral to feed on small organisms in the water, mostly zooplankton • Some corals also produce sheets of mucous to capture zooxanthellae or detritus

  7. Coral Nutrition • Coral can also feed using extensions of the gut wall called mesenterial filaments • These filaments secrete digestive enzymes • The coral can extend the filaments with its enzymes allowing the coral to feed and digest food outside the body

  8. Conditions Required for Reef Growth • Hard substrate • Light so in relatively shallow water of photic zone • Narrow temperature range= 68-86 degrees F • Stenohaline staying around 35 ppt • Low sediment load in water • Low pollution

  9. Reproduction and Growth of New Coral • Corals reproduce both sexually and asexually • Asexual reproduction allows for the growth of buds from a single polyp • Sexual reproduction results in the creation of a planula larvae that is able to ride on the water currents to a new environment • These larvae will settle out of the currents to begin growth on existing reefs or other hard substrate

  10. Coral Stress and Bleaching • The expulsion of zooxanthellae is termed bleaching because the normally colored coral is bleach white after this expulsion • The zooxanthellae give the corals their distinctive color

  11. Coral Stress and Bleaching • High temperatures (> 86 degrees F) 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 extreme weather events such as hurricanes

  12. Coral Sensitivity to Pollution • Even low levels of pollution can kill coral polyps • High nutrient levels (eutrophication) can also allow algae levels to rise • Increased algal growth can shade light-sensitive corals and their zooxanthellae

  13. Types of Coral Growth Forms • Coral colonies can come in a variety of shapes and sizes • Taller, more branching 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; this probably assists with capturing all possible light available.

  14. Types of Coral Growth Forms

  15. Branched and massive respectively. (Photographs by Dr. Donald Keith)

  16. Foliaceous. Photograph by Dr. Donald Keith Brain, lettuce leaf, and staghorn would be a few of the corals seen in the Flower Gardens Coral Reef off the Galveston, Texas Coast

  17. Other Organisms that Contribute to Reef Structure Besides Stony Corals • Coralline algae (also produce calcium carbonate) • Soft corals such as sea whips and sea fans (gorgonians) (produce a soft protein) • Other cnidarians such as hydrozoans or anemones • Sponges

  18. Types of Coral Reefs • Fringing Reefs • Barrier reefs • Atolls

  19. Fringing Reefs • Simplest and most common form of reefs • These reefs develop near the shore in tropical waters • These reefs develop as narrow strips along the shore • The fringing reef shown in the next photograph encircles the island it has formed near

  20. Fringing Reefs • Fringing reefs consist of an inner reef flat and an outer reef slope • The reef flat is wide, gently sloping and may be exposed in places at low tide • The reef slope is much more steep and is not exposed to air • Growth is normally high in this area

  21. Barrier Reefs • Occur along shore also but separated by a lagoon. • Typical construction of a barrier reef: a back-reef slope, a reef flat and crest and a fore-reef slope

  22. Barrier Reefs • Waves often wash sediment onto the back reef slope or reef flat which causes reduced coral growth in this area • Enough sediment may accrue to form small islands on the reef • These islands are called keys in the US or cayes elsewhere. Florida has many keys.

  23. A rich back-reef slope of a Pacific barrier reef not covered by sand

  24. Great Barrier Reef, Australia • The Great Barrier Reef is considered the largest reef structure in the world with regards to total coral area • The length of the reef systems exceeds 1200 miles • The width varies from 10 – 200 miles • Many cayes exist along the reef system

  25. Atolls • An atoll is a circular reef structure surrounding a central lagoon • Sand cayes may be a part of the atoll structure • Width varies from less than one mile to over 20 miles

  26. Atoll Formation • Atolls form when a volcanic island is formed • The coral begins as a fringing reef around the newly formed island • Over time, weathering and geologic activity lowers the level of the island • Eventually, the island disappears below the water’s surface, leaving only the outer band of coral surrounding a lagoon

  27. Atolls • Atolls possess a reef flat and inner and outer reef slopes • The inner slope has a more gradual slope than the outer slope and 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)

  28. Coral Reef Ecology • Like any marine environment, the coral reef community is structured due to limiting resources, competition, and predation • The water column surrounding the reef is normally very low in nutrients • This is due to the position of reefs away from rivers and other sources of nutrients (remember corals form best away from possible sedimentation sources)

  29. Coral Reef Ecology • Due to this low nutrient content, the water surrounding the reef does not support the degree of primary production by phytoplankton seen in other marine communities • The food chain here is based around the primary production of zooxanthellae located in the body of the corals and other organisms • The coral benefits the zooxanthellae by providing carbon dioxide, nitrogen and phosphorus (by-products), and shelter • The zooxanthellae provide sugars to the corals (and any organism that feeds on the corals or their parts

  30. Coral Reef Ecology • Cyanobacteria present on the reef are able to fix nitrogen which is then available to other reef inhabitants who feed on the cyanobacteria • Seagrasses and algae can also feed reef inhabitants • Water currents may bring some phyto- and zooplankton to the reef community

  31. Typical Coral Reef Food Web

  32. Many Organisms Feed Directly on Corals or their mucus, eggs, & larvae

  33. Deterring Predation • Many coral reef inhabitants can deter predation by producing toxic or foul tasting chemicals – this is common in soft corals and sponges • These chemicals can be released into the water to kill organisms or limit growth surrounding these organisms • Other reef dwellers may produce hard structures to deter predation – sponges produce spicules of calcium carbonate or silica while soft corals produce calcium carbonate needles known as sclerites • Coralline algaes as well as other organisms grow in an encrusting form that is difficult to feed upon because of its thinness and hardness

  34. Competition on the Reef • There is high competition for space and light on the reef • To limit competition, some corals grow fast and upright to maximize their ability to reproduce quickly • Others grow slow but massive to out-compete other organisms in the long run • Corals may feed directly or sting one another if they contact each other • Shown at right is interspecific coral competition (and some coral death in the pink area)

  35. Competition on the Reef • Other organisms such as fish and crustaceans must also compete for limited resources • They normally do this by feeding in slightly different areas or in slightly different ways • Each has its own ecological niche