What is a coral?

Corals are small invertebrate animals that create the foundation of an ecosystem that supports thousands of other organisms, including humans. Corals are in a group of organisms called Cnidarians, and are closely related to jellyfish, anemones, and hydroids. An individual coral polyp has multiple tentacles surrounding a central mouth, and a sac-like digestive tract. At its base and sides, the coral animal secretes a hard calcium carbonate skeleton to support and protect its soft body tissues (Figure 1). Multiple coral polyps that are living together in the same skeleton form what is known as a coral colony (Figure 2). In contrast to small individual polyps, coral colonies can reach up to several meters in diameter. When these colonies grow in the same location, the resulting ecosystems, coral reefs (Figure 3), support some the highest biodiversity on the planet.

Figure 1. A diagram of a hard coral polyp (Scleractinia) anatomy.
Figure by Constance Sartor.
Figure 2. A coral colony comprised of hundreds of nearly identical

coral polyps (Diploastrea heliopora). Photo by A. McInnis.
Figure 3. A coral reef flat community comprised of different

coral colonies and species.
Coral reefs are also home to many large animals like this blacktip reef shark, Carcharhinus melanopterus. Photo by A. McInnis.

Although anatomically simple, corals have a complex natural history that allows them to form some of the most significant physical structures on earth. The ability of corals to grow and survive in nutrient-poor tropical waters is attributed to a delicate symbiosis developed with a unicellular, photosynthetic alga (zooxanthellae). These algae live inside the coral tissue and use light to photosynthesize. To benefit their coral host, zooxanthellae pass nutrients, such as sugars and other metabolites, to the coral polyp. Most of the corals’ food comes from this symbiosis, while the other small fraction comes from capturing planktonic particles using their tentacles. However, sometimes when corals are stressed, most often due to excessive heat, they will spit out their symbiotic partners. If corals lose their zooxanthellae, in a process known as coral bleaching, they often are no longer able to survive.

Unlike many land vertebrates, corals can reproduce both sexually and asexually. They reproduce sexually by releasing gametes (egg and sperm) into the water. When these gametes meet, fertilization occurs and a mobile coral larva will develop. These coral larvae will later settle in a new area on the seabed where they remain for the rest of their lives. Asexual reproduction happens in two ways. One way is through fragmentation, which most often occurs due to a physical disturbance such as wave action from storms. If the fragment falls into a suitable area, it will reattach to the substrate and the colony will continue to grow. The second asexual reproduction mode, most common for coral colony growth, is the symmetrical division or ‘budding’ of an existing polyp into two separate polyps (Figure 4). These various methods of reproduction are used by scientists to help restore coral reefs.

Figure 4. Intratentacular coral budding, a form of sexual reproduction,
where an individual polyp divides into two separate individuals.
Photo by L. Raymundo, used with permission.

What is a coral reef?

When scientists refer to a coral reef, they mean an ecosystem dominated by hard corals from the order Scleractinia. Over many years, the accumulation of coral skeletons (mainly secretions of calcium carbonate, or limestone) creates massive reef structures, some of which can even be seen from space. These hard corals can come in a wide variety of shapes, sizes, and colors. For example, some corals from the genus Acropora can form long branching colonies (Figure 5), while others from the genus Porites form large dome-shaped colonies that can grow larger than a car (Figure 6). These different growth forms make diverse habitats where other organisms can live, which is why corals are considered the ecosystem engineers of the coral reef ecosystem.

Figure 5. A branching coral growth form, Acropora pulchra,
also known as a staghorn coral. Photo by A. McInnis.
Figure 6. A hydrozoan fire coral Millepora platyphylla (left)
and right a massive Porites lobata colony. Photo by A. McInnis.

Coral reefs only cover about 1 percent of the seafloor but, still, they support the most biodiversity in the ocean. They are mainly distributed in warm tropical waters within 30 degrees north and south latitude of the equator and thrive in clean, clear, nutrient-poor saltwater. Most coral reefs are located at depths of between 1 and 30 meters. This is because they need light for their symbiotic zooxanthellae. However, some corals can be found as deep as 90 meters, forming delicate mesophotic (meaning they need only a moderate amount of light) reef communities.

Importance of coral reefs for Guam

Guam has extensive and vibrant coral reef ecosystems. It has approximately 100 kilometers of shallow reef systems and additional deep reef area. The biodiversity of Guam’s coral reefs is the highest of any US state or territory. Through Guam and the Northern Mariana Islands, there are approximately 377 species of hard corals (Scleractinia), 199 species of soft corals (Alcyonacea), and 26 species of other corals (Hydrozoa).

Coral reefs benefit small islands in many ways, both directly and indirectly. For Guam, coral reef-associated fisheries are an essential source of food and are valued around $4 million per year. Coral reefs also offer many ecosystem services to communities, such as protecting coastlines and houses from storm waves. On Guam, it is estimated that, each year, coral reefs prevent $8 million worth of damage to local infrastructure by weakening wave energy from storms. And tourism, a major income source for Guam, is mainly based on Guam’s beautiful reef ecosystems and beaches that reefs help create. Each year Guam receives approximately $95 million from tourists and tourism-based activities associated with coral reefs. Overall Guam’s reefs are valued around $127 million annually.

About the author

Andrew McInnis wa a graduate student at the University of Guam Marine Laboratory. He was interested in a wide range of topics, including ecology, fisheries management, and socioeconomic factors affecting marine environments, particularly coral reefs.

Editor’s note

This author was a beginning graduate student in 2019 taking a course in scientific writing at the University of Guam. This article was assigned to provide the student with practice in communicating science to non-scientists. The student chose the topic which is related either to his thesis project or work experience. The instructor in the course is Dr. Laurie Raymundo, a UOG Marine Laboratory faculty member.

For further reading

Burdick, David. “Home Page.” Guam Reef Life, 2017.

Porter, Val, Trina Leberer, Mike Gawel, Jay Gutierrez, David Burdick, Victor Torres, and Evangeline Lujan. Status of the Coral Reef Ecosystems of Guam. Technical Report No. 113. Mangilao: Marine Laboratory, University of Guam, 2005.

van Beukering, Pieter, Wolfgang Haider, Margo Longland, Herman Cesar, Joel Sablan, Sonia Shjegstad, Ben Beardmore, Yi Liu, and Grace Omega Garces. The Economic Value of Guam’s Coral Reefs. Technical Report No. 116. Mangilao: Marine Laboratory, University of Guam, 2007.