The Basics about Coral

When I was younger I remember vacationing in tropical places such as Mexico and the Dominican Republic and picking up stony rock material and asking my mom what it was. She replied saying “it is a piece of coral” (now, understood as a coral skeleton).  I grew up only understanding what coral looked like from vacations and movies. However, I had no idea about the basics of coral, let alone if coral was a plant or animal. My intent for this post is to help people understand what coral is and some of the basic information that surrounds them. This background knowledge will help us understand the bigger picture of coral reefs.

What is a coral?

“A little bit algae, a little bit rock, and a lot animal.” Coral is a marine invertebrate that is composed of a simple structure known as a  polyp. The polyp has one opening which is the coral’s mouth that is surrounded by a ring of tentacles. These tentacles have stinging cells called cnidocytes, used for food and protection. These cells fire tiny toxic barbs called nematocysts, when they are touched to ward off predators. Inside the polyp are reproductive and digestive tissues. Coral can live as an individual polyp, but many corals come together to create colonies.

“Coral Polyp” by Wikimedia Commons under CC by 2.0

Coral come in many different shapes and colors. These are not the only aspects that vary in coral. Their sex lives vary as well. Some species have distinct male or female polyps while other coral can alternate gender. For some coral that only have single-gender polyps certain events (ie. full moon) trigger the polyps to release massive amounts of sperm and eggs that flood into the seas. This results in many fertilized eggs that can become a polyp once they settle on a surface and begin to grow. Other species can even reproduce asexually, budding new polyps that grow either on an older polyp or drift around before landing on a new surface.

Types of Coral

When people think about coral they often think about the stony calcified coral they get at a gift shop during a vacation to a tropical island. However, these stony corals are only one type of coral. There are also deep water corals that live in dark water and soft corals.

• Soft Corals

Soft Corals are often resemble plants and trees. They are soft and bendable so they do not have stony skeletons. For protection and support they grow wood-like cores for stability. This type of coral is referred to as ahermatypes (non-reef building corals).

“Goniopora djiboutiensis (Soft coral)” by Wikimedia Commons under CC by 2.0

• Hard Corals

Hard Corals are the fundamental part of building the coral reef. Hard corals create skeletons out of calcium carbonate (limestone), that will eventually become rock. Since hard corals can build reefs they are known as hermatypes. These corals have a symbiotic relationship with tiny algae called  zooxanthellae (pronounced zo-zan-THEL-ee).

“Chromis viridis on purple Acropora” by Brian Gratwicke under CC by 2.0

• Deep Water Corals 

Deep Water Corals can be found up to 6,000m below the oceans surface. These corals live in icy cold water with little to no sunlight. Since these corals live without sunlight they contain no Zooxanthellae. This means these corals must obtain their energy and nutrients else where. They do this by trapping tiny organisms in passing currents. You would think no coral would be able to survive yet scientists have identified nearly as many deep sea corals as shallow water species.

Symbiotic Relationship between Corals and Zooxanthellae

Most corals contain photosynthetic algae, called zooxanthellae, that live in their tissues. The coral and algae have a symbiotic mutualistic relationship. What this means is that both organisms benefit from each other. The coral provide the algae with a protected environment as well as place for photosynthesis to occur. On the other hand the algae produce oxygen and help the coral remove waste. The most important part is that the zooxanthellae  provide the corals with nutrients (ie. glucose, glycerol and amino acids). The corals can they use these products to further make food. It is this relationship that allows corals to grow fast enough and build the reefs. In addition to providing nutrients,  zooxanthellae are responsible for the unique and colorful stony corals we see.

What do Corals Need to Survive?

Sunlight: Just like plants corals need to grow in shallow water where sunlight can reach them. Corals depend on the zooxanthellae (algae) that grow inside them for oxygen. Since the zooxanthellae need sunlight to survive so do corals due to the symbiotic relationship they possess (Note this does not pertain to deep water corals).

Nutrients: Zooxanthellae will provide some nutrients to the corals, but not all corals have these organisms living inside them. Corals also capture food by using their stinging tentacles to catch tiny organisms floating by.

Clear water: Corals need clear water that will allow the sunlight to come through.

Warm water temperature: Typically corals require warm water conditions to survive. However like we discussed above deep water corals are able to withstand cold water.  Different corals living in different regions can survive temperature fluctuations. Corals generally live in water temperatures of 68–90° F. 

Clean water: Corals are sensitive to pollution and sediments. Sediment can create cloudy water and be deposited on corals, blocking out the sun light with is crucial for their growth and nutrients. Wastewater put into the ocean can cause seaweeds to overgrow the reef.

Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water.