Despite their appearance, corals are not underwater plants, but they are marine animals that house plants in their skeletal tissues. All corals belong to the taxonomic class Anthozoa within the phylum Cnidaria, the same phylum that encompasses jellyfish and sea anemones.
Most reef-building corals today are colonial and hard-bodied, meaning they are composed of identical, individual animals called polyps that precipitate a skeleton of calcium carbonate from seawater. These hard-bodied scleractinian corals are responsible for the formation of coral reef systems across the planet. Coral reefs typically exist in warm, shallow, tropical seas between 30°N and 30°S. As a foundation species, corals house 25% of all marine biodiversity and are critical to the oceanic biosphere because they provide shelter and food for organisms ranging from algae to sea stars to sharks. Coral reefs are the most biodiverse and perhaps the most important ecosystems of our blue planet.
Corals can appear in a variety of different colors, and that color comes from the zooxanthellae algae that reside in the skeletal tissue. These algae have a symbiotic relationship with the coral, providing food to corals in the form of carbohydrates produced from photosynthesis. In turn, corals provide shelter to the algae. This relationship is key for coral survival, for without the symbiotic algae, the coral would be susceptible to starvation, disease, and an ultimately death.
Coral reefs reduce the impact of shoreline waves and erosion produced by tropical storms and tsunamis and thereby prevent loss of life and infrastructure. Many commercial fish that are used for human consumption rely on reefs to sustain their populations, and many island populations rely on fish as a major source of protein. Coral reefs are also one of nature’s most captivating sights, bringing in billions of dollars of tourism revenue for economies across the globe.
Scleractinian corals first appeared in the fossil record during the middle of the Triassic Period between 247.2 and 237 million years ago. The Triassic Period is part of the Mesozoic Era, or the Age of Dinosaurs. The true origin of scleractinian corals remains unknown, however research suggests that they may have evolved from rugosan corals, but this hypothesis is still being investigated. After the Triassic, scleractinians continued to evolve and diversify into major reef builders until the Cretaceous - Paleogene (K - Pg) extinction that occurred 66 million years ago. That extinction decimated coral populations but the modern corals we see today extend from the lineages that survived the extinction. Since then, scleractinians have diversified into the hundreds of coral species we observe in today’s oceans.
While Indiana has fossils of rugosan and tabulate corals from the Paleozoic Era, scleractinian coral fossils are not found in Indiana because there are no Mesozoic or Cenozoic rocks in the entire state. This lack of scleractinian corals may be due to glacial movements from the Ice Age that eroded younger Mesozoic and Cenozoic rocks and only left older Paleozoic rocks containing rugosan and tabulate coral fossils.
The Indiana University Paleontology Collection houses many scleractinian corals that are a part of the Stotter Collection. Roscoe O. Stotter, IU Class of 1912 who graduated with a law degree, donated his collection of molluscan shells and corals to IU in 1975. Mr. Stotter started his interest in collecting in the 1940s when he found a shell on a beach in La Paz, Mexico. From there he collected and identified more than 10,000 shells in his lifetime, travelling across the globe as a shell hunter. He donated his collection to Indiana University in the hopes of it being preserved and seen by others. Here we honor his wishes and showcase some of the many unique and incredible specimens he collected over roughly 30 years. Within his collection we have more than 100 scleractinian coral specimens. These specimens range in size from a couple of inches to almost two feet in length. Many of the corals are from the Indo-Pacific and Caribbean regions. All the specimens are modern, and thus none are fossilized. Most of them are colonial, with a few exceptions being solitary.
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no audioThere is an ongoing loss of corals across the globe. Corals are most threatened around Southeast Asia, with approximately 95% of all reefs rated from low to very high risk according to the Reefs at Risk Integrated Local Threat Index (Burke et al., 2012). In addition, coral reefs are being severely bleached five times as often as in 1980. Corals were lost at an extremely high rate during the 1998 bleaching event, when in only one year 8% of all corals were lost.
The greatest threats to coral reef ecosystems are anthropogenic-induced climate change, marine pollution, overfishing, and coastal development. Other threats such as disease outbreaks and marine debris also jeopardize the future of corals. Anthropogenic climate change has influenced storm intensity, duration and frequency, increased ocean sea surface temperatures, and increased ocean acidification.
By the year 2050, it is estimated that approximately 95% of current coral reefs are going to have high amounts of thermal stress from increasing ocean temperatures, increasing the risk of mass bleaching events to dangerously high levels. By the mid-century, ongoing ocean acidification will greatly impact corals with only 15% of reefs potentially having sufficient aragonite (a form of calcium carbonate) to sustain further scleractinian coral growth. As aragonite is the key mineral that forms the scleractinian’s skeleton, increased ocean acidification will produce weaker skeletons as the acidification can dissolve calcium carbonate.
Coral reefs across the planet are at a critical point in time where damage will soon become irreversible. Conservation and restoration of these reefs is imperative to ensure coral reefs continue to exist on this planet in the future. There are a plethora of conservation and restoration efforts being done to help corals. Some of these efforts are at the governmental level including reducing greenhouse gas emissions, increasing the size of marine protected areas, passing legislation to ban certain harmful chemicals and products, and transitioning to reusable energy sources in many industries for sustainability. Other more direct efforts include building artificial reefs, planting coral reef nurseries, or artificially selecting and breeding heat-tolerant corals. In addition, educating the public to raise awareness about the threats that corals face can spark curiosity and encourage change. The more individuals that know how corals are being impacted by anthropogenic changes, the larger the impact we can have on coral restoration.
For more information on corals, a documentary on Netflix called Chasing Coral shows how divers, scientists, and photographers captured the disappearance of coral reefs and mounted an epic underwater campaign to save them.
Undergraduate Co-authors: Abigail Smith and Matthew Sullivan
Graduate Author: Lorena Jevnikar
CBRC Research Assistant: C.J. Salcido
CBRC Director: Claudia C. Johnson
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