Corals & Coral Reefs

1. Corals & Coral Reefs

2. “The Forests of the Sea” • Fringe 1/6th of the world’s coastlines • Largest reef is the Great Barrier Reef in Australia (2000km or 1200mi) • Member of the Cnidariaphylum • Only have a polyp stage • Have stinging tentacles and a soft body column, but they also have hard skeletons of calcium carbonate

3. Coral Growth • Start out life as a tiny planktonic larva, floating through the water • When it finds a hard substrate it attaches itself and begins secreting a hard skeleton towards the base • Coral Parts: • Exoskeleton: chalky white skeleton on that is located on the outside • Calyx: the cup-shaped depression directly beneath the polyp; the polyp can retreat into it • Polyp: the soft tissues on top of the calyx; often very brightly colored

4. Body column Tentacles

5. Solitary v. Colony • Solitary Corals • A single polyp • Some secrete a skeleton that produces a vertical column • Some produce a skeleton that grows outward (like the Mushroom Coral) • Few species are solitary corals

6. Polyp Mouth Skeleton

7. Solitary v. Colonial cont’d • Colonial Corals • Often described as “coral heads” • Some grow by budding (the new polyps will form around the original) • Some reproduce sexually • The new polyps begin secreting skeletal matter and adding to the thickness, diameter, and mass of the coral skeleton • All polyps are connected by a thin, often brightly colored layer of surface tissue • Classified as Branching or Encrusting corals

9. Branching v. Encrusting Corals • Fig 4-1 • Branching Corals • Formed by polyps that produce buds at the tips of small branches • Forms a complex, branching shape • More delicate than the encrusting type • Encrusting Corals • Formed by polyps that only bud at the edges; forms a “crust” over the base

10. Branching Coral Encrusting coral

11. MutualisticSymbionts • Zooxanthellae • A single-celled algae that is commonly found in the tissues of most cnidarians • 3 major purposes • Zooxanthellae photosynthesize, producing oxygen for the coral • The coral uses the O2 and produces CO2 and other wastes, which the algal cells use to photosynthesize and nourish themselves • Zooxanthellae also stimulate skeletal secretion – they enhance the ability to extract calcium carbonate (CaCO3) from the water *Reef-building corals (generally) only grow and thrive where there’s enough sunlight for the algae to photosynthesize

12. Reef Formation • Reef – a massive deposit of coral skeletons • Step 1 – A coral larva floats until it finds a hard substrate • Step 2 – The polyp begins to secrete a skeleton under the thin surface of soft tissue • Step 3 – The corals bud or sexually reproduce to form “buddies” to help with the growth of the coral head • Repeat steps 1-3 several million times…  • Reefs can be massive! • Ex: On the coral islands of Bikini and Enewetak, the researchers drilled through more than 2 km of skeletal matter before they hit the volcanic base!

13. Reef Formation cont’d • Reef Parts • Reef Flat • The flat upper surface of the reef • This is the part that is mostly exposed during low tide • Tides limit the height the reef grows • Prolonged exposure to air causes the coral to die, producing the skeletons that compose the flat • Few corals actually live on the flat • Reef Face • The outer, seaward surface of the reef • The reef grows fastest on this side due to the currents that bring food particles to the living polyps • As this part grows, the reef flat is expanded • The corals on the shallower outer edges are exposed to high wave energy and the abrasive force of moving sand particles • A few hardy corals and coralline algae grow on the face

14. Reef Formation cont’d • Coralline Algae • Marine algae that produce large amounts calcium carbonate that forms robust skeletons • Better adapted wave action than coral • They make up about 90% of the shallow outer reef face • Their skeletons resemble coral skeletons, but they don’t have calyces • Algal Reef • The part of the reef mostly made up of coralline algae • Encrusting coralline algae cement parts of the reef together; making it very resistant to heavy wave action

15. Reef Formation cont’d • Think of the reef face as being divided into sections • Shallow Outer Reef Face • Mostly coralline algae because of the heavy wave action • Midwater Outer Reef Face • This is where coral growth is excellent • There can be as many as 200 species of corals competing with each other • Deepwater Outer Reef Face (depths greater than 30m) • Coral growth limited by lack of light

16. Coral Reef Evolution • Think back to how reefs form… (look @ Fig4-7) • Fringing Reef • The early stage of the reef • It is small and close to shore • Barrier Reef • The reef has grown as the island has started subsiding • Lagoon • A region of (usually shallow) water between the reef and the island • Coral Atoll • Formed when the island sinks below the surface of the ocean and the coral reef grows over it • This is all that can be seen at the surface of the water • Motu(s) • Mounds of coral rubble on the reef face • Formed when something like a storm breaks off a large chunk of reef and deposits it on top of the face