T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 1 Name: Coral Reefs (5 pts) Section: Coral Biology Coral is a type of marine animal that lives in large colonies (groups of the same kind of animal). Each animal in the colony is a polyp that resembles a small sea anemone the size of an ant. Each polyp has tentacles with stinging cells, which they use to suspension feed or filter feed (to catch small plankton as they drift by). The coral animals live in a symbiotic relationship (a partnership) with tiny algae called zooxanthellae. The algae live inside the polyps’ tentacles, and provide oxygen, food, and a beneficial pH to the coral polyp, while the coral polyp provides protection, carbon dioxide, and nutrients to the zooxanthellae – and helps them get more sunlight. In this manner, the two organisms can help each other survive in the nutrient-poor tropical oceans. Note: The corals obtain nutrients by eating plankton. Instead of excreting nutrients in their wastes like most animals, the corals give the nutrients to their zooxanthellae instead. This allows the zooxanthellae to carry out more photosynthesis which is important for the corals, because the corals get most of their food from the zooxanthellae, not by eating plankton. 1. (a) Which is the animal, coral polyps or zooxanthellae? By the way, corals belong to the phylum Cnidaria, the same phylum as jellyfish. (b) Which is the algae, coral polyps or zooxanthellae? In a very real sense, corals and sea anemone are upside- down jellyfish stuck on the 2. What do zooxanthellae give to their coral polyp? bottom of the ocean. 3. What do coral polyps give their zooxanthellae? 4. How do corals get most of their food? by filter feeding plankton? by eating their zooxanthellae? given it by their zooxanthellae? 5. How do coral polyps get nutrients? T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 2 Coral Reefs The polyps build coral reefs by secreting a calcium carbonate (CaCO3) skeleton below them, a little more each year. Generation after generation of coral grow on each spot, slowly building up the reef (about ½ inch (12 mm) per year). If you cut into a piece of coral, you can see the growth bands. (They look like tree rings.) Corals grow the reef below them to get closer to the surface for the ocean where there is more sunlight. The more sunlight that the corals’ zooxanthellae get, the more photosynthesis the zooxanthellae can do, and thus the more food that the zooxanthellae will have to share with the corals. Corals remove carbon dioxide (CO2) from the ocean when they create their calcium carbonate skeletons: 2+ + Ca + H2O + CO2 → CaCO3 + 2H Since this reduces the amount of carbon dioxide in the ocean, the ocean has more “room” to take in more carbon dioxide from the atmosphere. Since carbon dioxide is a greenhouse gas, removing carbon dioxide from the atmosphere cools the Earth. (Greenhouse gases warm the atmosphere by preventing “heat” from the leaving the Earth. They absorb infrared radiation from the Earth, keeping its energy from flying off into space.) Corals more-or-less “permanently” remove carbon dioxide from the atmosphere by building their reefs. Even when waves erode the reef, the carbon stays locked in the resulting sediments. The sediments often bury the coral polyps and other organisms (such as foraminifera) that live on the reef before they can decompose. If the entire reef sinks down and is buried inside the Earth, this organic debris may be transformed into oil over millions of years. By the way, corals are not they only organisms in coral reef ecosystems who have symbiotic algae, make calcium carbonate shells, and help build the reef. Other organisms contribute too. Also, not all corals build calcium carbonate structures. Non-reef-building corals are often called soft corals. 6. What substance are coral reefs made out of? T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 3 7. When corals create calcium carbonate do they add or remove carbon dioxide from the environment? Does this warm or cool the Earth? 8. Which of the following statements are true? (Circle the true statements.) a. “Corals are not the only animals with symbiotic algae inside them.” b. “Many different kinds of reef animals have calcium carbonate shells.” c. “All corals build reefs made out of calcium carbonate.” Corals, Water Temperature, Salinity, and Sunlight The primary factor controlling where corals grow is water temperature. Reef-building corals grow best in warm, tropical oceans with temperatures between 23oC and 25oC. Among other things, it is easier for corals to grow their calcium-carbonate exoskeleton in warm water. Dissolved calcium carbonate will actually precipitate (“solidify”) in very warm water, and calcium carbonate dissolves in cold, carbon-dioxide-rich water. As noted previously, corals put layers of calcium carbonate below them so that they can get closer to the surface of the ocean. Coral like clear, shallow water because their zooxanthellae (their symbiotic algae) must have enough sunlight to undergo photosynthesis. This is important to the corals, because the algae provide the corals with a large proportion of their food, over 90% in some cases! The ideal depths are between 5 and 10 meters (16 to 33 feet), although coral can be found from the low tide level to a depth of about 100 meters (328 feet). Growing some calcium carbonate beneath them each year helps corals get closer to the Sun, a little bit at a time. Salinity can also affect coral growth; they need water with a salinity between 27 to 40‰. Like all animals, corals need food and oxygen, which are not abundant in calm, tropical waters. They get used up. As a result, corals grow best on the reef crest (the edge of the reef) where currents and breaking waves coming in from the open sea can bring in new supplies of food and oxygen. As noted earlier, the corals’ food (plankton) are mainly useful, because the plankton’s bodies contain nutrients. Corals give these nutrients to their zooxanthellae, so that the zooxanthellae can make more food for them. Waves also erode the reef, breaking down the calcium carbonate reef into sand, some of which is pushed up onto the reef or nearby islands, creating beautiful white sandy beaches. Corals constantly need to regrow the reef to balance out the damage done by waves. In addition, sand and mud can be stirred up by waves and currents, making the water murky and blocking sunlight. T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 4 The sand can also make it hard for corals to grow. Corals need a solid bottom (solid rock, not mud or sand) to hold onto firmly, so they do not get ripped off the bottom and carried away or turned upside down by strong waves and tidal currents. 9. Do corals like warm water or cold water? Why? 10. Why do corals want to get sunlight? How do they get sunlight? 11. Do corals like fresh water or salty water? 12. Do corals grow best on a rocky bottom or a sandy bottom? 13. Do corals like clear water or turbid (murky), dark water? Why? 14. In what ways is calm water good for corals? 15. In what ways are waves (and the associated water motion) good for corals? 16. Where does the beautiful white sand on the beaches along or near coral reefs come from? What is it made out of? T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 5 Spatial Distribution of Corals Examine the map below showing where coral reefs are found (the little red squares). Corals are more common on the western side of an ocean than the eastern side of an ocean because of the difference in water temperature. Warm water helps corals grow layers of calcium carbonate below them, allowing them to get closer to the Sun. As we learned in earlier Units (like 9A-2), the western side of an ocean at the Equator is warmer than the eastern side of an ocean. Once the cold water of eastern boundary currents reaches the tropics, it turns to the west and spends a long time traveling across the ocean, soaking up more and more sunlight and getting warmer and warmer during its journey. Once the water reaches the western side of an ocean and the east coast of a continent, it has become quite warm. This warm water is then taken towards the Poles by western boundary currents. 17. Where are there more coral reefs, on the western sides of oceans or on the eastern sides of oceans? (Examine the Atlantic Ocean and Pacific Ocean where the difference is clearer.) 18. Do corals like warm water or cold water? Why? 19. Which side of the ocean has warmer water, the western side of the ocean or the eastern side of the ocean? Why is the water warmer on this side of the ocean? T. James Noyes, El Camino College Coral Reefs Unit (10D) – page 6 Coral Bleaching Water can become too warm for corals. Under these circumstances, the coral polyps kick out their zooxanthellae. Coral polyps are typically transparent which helps the zooxanthellae in their bodies get sunlight. It is their zooxanthellae that give corals the brilliant colors that we associate with coral reefs. So, we say that corals which have lost their zooxanthellae are bleached: they appear white, because all that is left are the transparent polyps and the white, calcium-carbonate reef. In the long term, coral polyps that have thrown away their zooxanthellae are in trouble, because they have lost their main source of food.