Chapter 15

1. Coral Reef Communities Chapter 15

2. Key Concepts • Coral reefs are primarily found in tropical clear water, usually at depths of 60 meters or less. • The three major types of coral reefs are fringing reefs, barrier reefs, and atolls. • Both physical and biological factors determine the distribution of organisms on a reef.

3. Key Concepts • Stony corals are responsible for the large colonial masses that make up the bulk of a coral reef. • Reef-forming corals rely on symbiotic dinoflagellates called “zooxanthellae” to supply nutrients and to produce an environment suitable for formation of the coral skeleton. • Coral reefs are constantly forming and breaking down.

4. Key Concepts • The most important primary producers on coral reefs are symbiotic zooxanthellae and turf algae. • Coral reefs are oases of high productivity in nutrient-poor tropical seas. Nutrients are stored in reef biomass and efficiently recycled.

5. Key Concepts • Inhabitants of coral reefs display many adaptations that help them to avoid predation or to be more efficient predators. • Coral reefs are huge, interactive complexes full of intricate interdependencies.

6. World of Coral Reefs • Coral reefs are highly productive, but occur in nutrient-poor waters • This is made possible by the symbiotic relationship between coral animals and zooxanthellae • These symbionts + algae form the basis of the community; other reef animals depend on these organisms

7. Coral Animals • Stony (true) corals deposit massive amounts of CaCO3 that compose most of the structure of coral reefs • Hermatypic—coral species that produce reefs, found in shallow, tropical waters

9. Coral Animals • Ahermatypic—corals that do not build reefs, which can grow in deeper water from the tropics to polar seas • most do not harbor zooxanthellae

13. Coral Animals • Coral colonies • large colonies of small coral polyps, each of which secretes a corallite • a planula larva settles and attaches • a polyp develops, and reproduces by budding to form a growing colony • polyps’ gastrovascular cavities remain interconnected • a thin, usually colorful epidermis overlies the colony surface

15. Coral Animals • Sexual reproduction in coral • mostly broadcast spawners—release both sperm and eggs into the surrounding seawater • some are brooders—broadcast sperm, but retain eggs in the gastrovascular cavity • spawning is usually synchronous among Pacific reef species, but nonsynchronous among Caribbean species

16. Coral Animals • Reproduction by fragmentation • some branching corals are fragile and tend to break during storms • if they survive the storm, fragments can attach and grow into new colonies • fragmentation is a common form of asexual reproduction for branching corals

17. Coral Animals • Coral nutrition • symbiotic zooxanthellae • supply 90% of nutritional needs of stony coral • zooxanthella provide glucose, glycerol and amino acids • coral polyp provides a suitable habitat and nutrients, absorbed directly through the animal’s tissues • zooxanthellae remove CO2 and produce O2 • need of zooxanthellae for sunlight limits depths to which stony corals can grow

19. Coral Animals • Coral nutrition (continued) • corals as predators • small animals paralyzed by the nematocysts are passed into the digestive cavity

21. Coral Animals • Coral nutrition (continued) • other sources of nutrition • corals can feed off bacteria living in their tissues, which feed on dissolved organic matter directly from the water • mesenteric filaments (coiled tubes attached to the gut wall) can be extruded from the mouth to digest and absorb food outside the body

22. Reef Formation • Involves both constructive and destructive phases • Bioerosion—the destructive phase of reef formation • boring clams or sponges attack exposed surfaces on the undersides of large corals • the coral stand weakens, then topples in a storm or ocean surge • debris smothers boring organisms, cracks are filled with CaCO2 sediments, and coralline algae cement it together

26. Coral Reef Types • Fringing reefs border islands or continental landmasses

28. Coral Reef Types • Barrier reefs are similar to fringing reefs but separated from the landmass and fringing reef by lagoons or deepwater channels

31. Coral Reef Types • Atolls, usually elliptical, arise out of deep water and have a centrally-located lagoon

33. Coral Reef Types • In addition, patch reefs can occur within lagoons associated with atolls and barrier reefs • Darwin’s theory of atoll formation: • corals colonize shallow areas around newly-formed volcanic islands to form a fringing reef • the island sinks and erodes, and a barrier reef is formed about the island • the island sinks completely, leaving an atoll

38. Reef Structure • Reef front or forereef—portion of the reef that rises from the lower depths of the ocean to a level just at or just below the surface of the water, on the seaward side • drop-off—a steep reef-front that forms a vertical wall • spur-and-groove formation or buttress zone—finger-like projections of the reef front that protrude seaward; disperses wave energy and helps prevent damage

39. Reef Structure • Reef crest—the highest point on the reef and the part that receives the full impact of wave energy • where wave impact is very strong, it may consist of an algal ridge of encrusting coralline algae, lacking other organisms, and penetrated by surge channels—grooves of the buttress zone • Reef flat or back reef—portion behind the reef crest

44. Coral Reef Distribution • Major factors influencing distribution: • temperature – corals do best at 23-25o C • light availability – photosynthetic zooxanthellae need light • sediment accumulation – can reduce light and clog feeding structures • salinity • wave action – moderate wave action brings in oxygenated seawater, removes sediment that could smother coral polyps • duration of air exposure – can be deadly

46. Comparison of Atlantic and Indo-Pacific Reefs • Pacific reefs are older and have a greater depth of reef carbonates • Buttress zone is deeper on Atlantic reefs and coral growth may extend to 100 m down • Pacific coral growth rarely exceeds 60 m • Algal ridges more common in the Pacific because of wind and waves

47. Comparison of Atlantic and Indo-Pacific Reefs • Hydrozoan Millipora complanata (fire coral) is dominant on Atlantic reefs • Similar species never dominate in the Pacific • Gorgonians more abundant in the Atlantic • Soft corals (subclass Alcyonaria) more abundant in the Pacific • Atlantic corals nocturnal; Pacific corals diurnal

48. Comparison of Atlantic and Indo-Pacific Reefs • Atlantic corals often reproduce by fragmentation; Pacific corals by sexual reproduction • Coral diversity is far greater in the Indo-Pacific than the Atlantic • Greater sponge biomass in the Atlantic • Pacific has giant clams and sea stars that prey on corals

49. Caribbean reef

50. Pacific reef