Background

What is a Coral Reef?

http://www.wired.com/images_blogs/wiredscience/2012/12/coral_reef.jpg

Coral reefs are made from other living things when they die. These reefs are very complex ecosystems that are needed for certain species to survive in general. Reefs are made almost entirely from coral polyps, and when they die their skeletons are left behind and the limestone they accumulate in their skeletons are then incorporated into the coral reef. There are several different types of coral like hard and soft corals. Hard corals are made of the coral polyps, and soft corals are made from other organisms and reefs aren't formed, and aren't as common. Most coral reefs are made entirely of hard coral, so that is the type of coral I want to focus on (Bellwood 2004).

How are they formed?

As mentioned before they are made by coral polyps. These invertebrates vary in size up to a foot in diameter. Coral reefs are formed almost entirely from these little animals, and each branch of a coral reef is a different colony of polyps. Their skeletons are calcium carbonate also known as limestone, and they feed at night.

http://nofishleft.files.wordpress.com/2010/09/coral_reef__7.jpg

They are found all over the world and cover about 110,000 square miles (Coral Reef Aliance 2014). Most coral reefs are about 5,000 to 10,000 years old (Coral Reef Aliance 2014). They grow the best in warmer climates, however they can grow in colder waters, just not as well. They grow in clear, shallow waters where the algae can still prefer photosynthesis. More reefs are about 60-90 feet under the waters surface (Coral Reef Aliance 2014). It takes a long time for reefs to form, because they only grow a couple inches each year. The growth rate of corals are dependent on the availability of resources, temperature, and other factors like salinity.

Coral reefs are large complex structures with many holes that are shared by many different species. Not all polyps build reefs, but the ones that do are the species I'll be talking about, because when they die they leave their skeletons behind and new polyps grow on top of them. They outer layer of coral has many polyps, but if you go further in we would find the skeletons of many older polyps. Other organisms that use shells like mollusks are also used to form the reef, although not to the same degree as polyps.

What are Coral Polyps?

Coral polyps are invertebrates that can vary in size up to a foot in diameter and be as small as an inch in diameter. They make their shells out of calcium carbonate, which is what the reef is made of when they die. Each polyp has a jelly body, and a mouth. They use the calcium carbonate from the seawater to build their skeleton. Coral polyps are nocturnal creatures, feeding only at night by exposing their tentacles (Coral Reef Alliance 2014).

http://www.enchantedlearning.com/cgifs/Coralpolyp_bw.GIF

Many Coral polyps live in symbiosis with algae called zooxanthellae. This algae lives inside the coral polyps and use sunlight to do photosynthesis to provide sugar and resources and the polyps give the algae somewhere safe to live and key resources needed for photosynthesis like carbon dioxide. They can also feed on zooplankton at night by sticking out their tentacles and are captured and eaten at night (Coral Reef Alliance 2014).

They reproduce in a variety of ways. Some species of polyps are hermaphrodites like Star and Brain corals. There are other coral like Elkhorn and Boulder corals that produce one sex colonies, which requires many other coral to be present. One colony produces sperm and another colony produces the eggs. Other species of coral just put eggs and sperm into the water and hope they reach their destinations. The larvae go to the surface where they stay for a couple weeks they fall down the ocean surface and attach to a hard surface, such as the skeletons of other polyps, forming the coral reefs (Burke 2010).

What does Coral need to survive?

http://www.oist.jp/sites/default/files/photos/20130712_Figure2_English.jpg

They need sunlight for their symbiotic algae, zooxanthellae, to be able to do photosynthesis. Coral need warm water temperature to be able to survive, they have a range they can survive, but they generally cannot survive in colder waters that are below 60 degrees. The last thing they need to survive is saltwater, they cannot live outside of a certain amount of salt. They also use salt waters in oceans to get the calcium carbonate for their shells among other nutrients (Burke 2010).

Different types of Reefs

http://www.naturefoundationsxm.org/images/coral/coral-reef-structures.jpg

There are five different types of reefs which are fringing reefs, barrier reefs, atolls, and patch reefs, and platform reefs (Environmental Protection 2009). Fringing reefs grow near the coast and are the most common reefs that are observed. Barrier reefs are deeper out, at their deepest point, forming barriers around certain points they cannot reach, where they get their name. Atolls are rings that are usually in the middle of a sea that used to be islands that have sunk into the sea, usually underwater volcanoes. Patch reefs are small reefs that form in unpredictable areas and can be located in a variety of areas. Platforms are fairly deep, and form little platforms on the water (Environmental Protection 2009).