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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 © 2006 Thomson-Brooks Cole © 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. © 2006 Thomson-Brooks Cole Coral Animals • Stony (true) corals deposit massive amounts of CaCO3 that compose most of the structure of coral reefs © 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 © 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 – 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 © 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 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 • 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 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 © 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 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 © 2006 Thomson-Brooks Cole © 2006 Thomson-Brooks Cole © 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 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