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Chapter 15 Coral Reef Communities © 2006 Thomson-Brooks Cole 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. © 2006 Thomson-Brooks Cole 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. © 2006 Thomson-Brooks Cole 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. © 2006 Thomson-Brooks Cole 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. © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Animals • Coral nutrition (continued) – corals as predators • small animals paralyzed by the nematocysts are passed into the digestive cavity © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Reef Types • Fringing reefs border islands or continental landmasses © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Reef Types • Barrier reefs are similar to fringing reefs but separated from the landmass and fringing reef by lagoons or deepwater channels © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Reef Types • Atolls, usually elliptical, arise out of deep water and have a centrallylocated lagoon © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Caribbean reef Pacific reef © 2006 Thomson-Brooks Cole Reef Productivity • Source of nutrients – land runoff for reefs close to land – source for atolls unclear – possible explanations: • nutrients accumulated over time are efficiently recycled • reef bacteria and filter feeders capitalize on nutrients from dissolved/particulate organic matter – nutrients brought from other communities © 2006 Thomson-Brooks Cole Reef Productivity • Reef photosynthesis – photosynthetic organisms: zooxanthellae, benthic algae, turf algae, sand algae, phytoplankton, seagrasses – more dense than tropical ocean, with greater biomass than reef animals – associations of producers with other organisms assist in efficient recycling – turf algae process the most organic carbon © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Reef Productivity • Reef succession – ratio of primary production to community respiration = P-R ratio • P = gross photosynthesis • C = community respiration – P-R ratio used to measure state of development of a biological community © 2006 Thomson-Brooks Cole Reef Productivity • Reef succession (continued) – P-R ratio > 1 = primary production exceeds respiratory needs • biomass increases, excess biomass available for growth or harvesting – P-R ratio = 1 = steady state • little biomass remains available for growth – P-R ratios for coral reefs are typically close to 1 • high productivity balanced by high respiration © 2006 Thomson-Brooks Cole Reef Productivity • Reef succession (continued) – increases in productivity are often the result of eutrophication • eutrophication—nutrient enrichment – eutrophication typically manifested as a dramatic proliferation of algae • if grazing doesn’t increase, algae can grow over and smother corals © 2006 Thomson-Brooks Cole Coral Reef Ecology • Coral provides: – foundation for reef food webs – shelter for resident organisms • Reefs form a complex 3-dimensional habitat for many beautiful and strange creatures © 2006 Thomson-Brooks Cole Coral Reef Community • Sponges and cnidarians – sessile organisms, though anemones can move if necessary – filter feed; anemones also paralyze and consume small fishes and crustaceans • Annelids – sessile filter feeders include featherduster and Christmas tree worms – fireworms are mobile predators – palolo worms burrow through and weaken coral and usually deposit feed © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Reef Community • Crustaceans – shrimps, crabs and lobsters – vary from parasites to active hunters • Molluscs – gastropods eat algae from coral surfaces – giant clams are filter feeders, but also host symbiotic zooxanthellae – octopus and squid are active predators © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Coral Reef Community • Echinoderms – feather stars, sea urchins, brittle stars, sea stars, and sea cucumbers – filter feed, scavenge, or eat sediment • Reef fishes – most prominent and diverse inhabitant – diverse food sources, including detritus, algae, sponges, coral, invertebrates, other fish © 2006 Thomson-Brooks Cole Species Interactions on Coral Reefs • Competition among corals – fast-growing, branching corals grow over slower-growing, encrusting or massive corals and deny them light – slower-growing corals extend stinging filaments from their digestive cavity to kill faster-growing corals – fast-growing corals can also sting and kill using long sweeper tentacles with powerful nematocysts © 2006 Thomson-Brooks Cole Species Interactions on Coral Reefs • Competition among corals (continued) – slower-growing corals are more tolerant of shade, and can grow at greater depths – as a result… • fast-growing, branching corals on many reefs dominate upper, shallower portions • larger, slower-growing corals dominate deeper portions © 2006 Thomson-Brooks Cole Species Interactions on Coral Reefs • Competition between corals and other reef organisms – sponges, soft corals and algae can overgrow stony corals and smother them – algae outcompete corals at shallow depths unless grazers control the algae growth • Other competitive interactions – hydrozoans overgrow gorgonians – fast-growing colonial invertebrates on coral surfaces overgrow many species © 2006 Thomson-Brooks Cole Species Interactions on Coral Reefs • Effect of grazing – grazing of larger, fleshier seaweeds permits competitively inferior filamentous forms or coralline algae to persist – herbivory decreases with depth – damselfish form territories where they exclude grazers and permit abundant algal growth • provides habitat for small invertebrates • overgrows corals; fast-growing, branching corals are most successful near damselfish © 2006 Thomson-Brooks Cole Species Interactions on Coral Reefs • Effect of predation – predation of sponges, soft corals and gorgonians provides space for competitively inferior reef corals – species that feed on fast-growing coral assist slower-growing species to remain – corallivores seldom destroy reefs – small invertebrates are almost all well hidden or camouflaged, indicating the prevalence of predation in the reef © 2006 Thomson-Brooks Cole Coral Reef Ecology • Coral reefs - marine habitats with greatest diversity/abundance of fishes • Seems to defy competitive exclusion principle, which suggests that no 2 species can occupy the same niche – 60-70% of reef fishes are general carnivores – about 15% are coral algae grazers or omnivorous © 2006 Thomson-Brooks Cole Coral Reef Ecology • Hypotheses proposed to explain this: – competition model—factors such as time of day or night, size of prey, position in the water column, etc. provide each species with a unique niche (hence, no competition) – predation disturbance model—assumes competition, but suggests that the effect of predation or other causes of death keep populations low enough to prevent competitive exclusion © 2006 Thomson-Brooks Cole Coral Reef Ecology • Hypotheses proposed to explain this: – lottery model—assumes competition occurs, but suggests that chance determines which species of larvae setting from the plankton colonize a particular area of the reef – resource limitation model—suggest that available larvae are limited and that limitation prevents fish population from ever reaching the carrying capacity of the habitat © 2006 Thomson-Brooks Cole Threats to Coral Reefs • Effect of physical changes on the health of coral reefs – hurricanes and typhoons topple and remove coral formations – El Niño Southern Oscillation (ENSO) • changes winds, ocean currents, temperatures, rainfall and atmospheric pressure over large areas of tropical and subtropical areas • can cause massive storms © 2006 Thomson-Brooks Cole Threats to Coral Reefs • Coral bleaching – a phenomenon by which corals expel their symbiotic zooxanthellae – most often associated with warming of the ocean water by ENSO or global warming – if the stress is not too severe, corals may regain zooxanthellae and recover – if the stress is prolonged, corals may fail to regain zooxanthellae and die © 2006 Thomson-Brooks Cole Threats to Coral Reefs • Coral diseases – black band disease—a distinct dark band of bacteria migrates across the living coral tissue, leaving behind a bare white skeleton © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Threats to Coral Reefs • Coral diseases – white pox—characterized by white lesions and caused by Serratia marcescens – other coral diseases: • white band disease • white plague • yellow blotch disease © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Threats to Coral Reefs • Human impact on coral reefs – overfishing may occur – human-sewage bacteria cause white pox – nutrient-rich runoff (eutrophication) increases algal growth, which covers and smothers corals • e.g. Kane’ohe Bay in Hawaii © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Evolutionary Adaptations of Reef Dwellers • Protective body covering – tough, defensive exteriors help animals avoid predation, but can limit mobility and growth • Protective behaviors – producing a poisonous coating of mucus – burying the body in sand to hide – inflating to appear larger – hiding at night when nocturnal predators are active © 2006 Thomson-Brooks Cole Evolutionary Adaptations of Reef Dwellers • Role of color in reef organisms – color for concealment and protection • countershading • disruptive coloration • camouflage (bright colors in reef environment) © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole Evolutionary Adaptations of Reef Dwellers • Role of color in reef organisms – other types of camouflage • body shape – warning coloration – other roles of color • defending territories • mating rituals © 2006 Thomson-Brooks Cole Evolutionary Adaptations of Reef Dwellers • Symbiotic relationships on coral reefs – cleaning symbioses • cleaner wrasses, gobies, etc. feed on parasites of larger fishes • cleaning organisms set up a cleaning station – Other symbiotic relationships • clownfishes and anemones • conchfish and the queen conch • gobies and snapping shrimp • crustaceans and anemones © 2006 Thomson-Brooks Cole