Corals Are Underwater Architects

At first glance, some corals (called stony corals) look a lot like rocks. However, if you look really closely, you may see circles of tentacles, swaying with the water currents. Each circle of tentacles is a tiny animal called a polyp. As stony corals grow, they slowly add one, two, or a few polyps at a time to their community, each one an exact copy of the last. Eventually, hundreds or even thousands of individual animals will live in harmony as a coral colony.

As stony corals add to their colony, the corals begin to take unique shapes. Some corals look like the horns of elk or deer, due to their extensive and intricate branches. Other corals create magnificent castles constructed of tall pillars. Some corals look like dinner plates, and others create massive mounds and boulders! Many of these shapes have grooves, ridges, and holes—structures that serve as perfect homes for fish and other sea creatures. As hundreds and thousands of coral colonies grow together, they create one of the most beautiful and diverse ecosystems in the world: coral reefs. Because of the role that stony corals play as architects and creators, they are often called reef builders.

Corals also come in many colors, including green, brown, yellow, red, and even purple! However, corals do not get to pick which color they would like to be. Rather, much of their color comes from the presence of another organism living inside of their tissues: symbiotic microalgae. Symbiotic microalgae are teeny-tiny cells that do more than provide corals with their colorful pizzazz. Like plants, these tiny algae perform photosynthesis, allowing them to create and share much of the food that corals need to survive. These tiny cells are grateful for the corals as well, because corals give them both a safe place to live and the nutrients that they need to carry out photosynthesis. This kind of relationship, in which two organisms live in close harmony with one another, is called symbiosis. This is why the tiny algal cells are called symbiotic microalgae!

Not only do corals play irreplaceable roles in marine ecosystems, but they are also really important in the lives of humans. Coral reefs provide food for billions of people around the world, protect the coasts from tropical storms, are tourism magnets, produce life-saving medicines, and are a fundamental part of many cultures. Plus, coral colonies can live for a very, very long time. In fact, some coral colonies can continue to grow for up to 5,000 years! This is longer than any other animal on Earth. Corals are amazing!

Corals Are in Trouble

Corals are in serious trouble due to climate change and disease, and coral colonies are dying faster than ever before. When corals die, they leave their skeletons behind. Though dead corals can continue to provide homes for reef creatures for a little while, over time these skeletons start to erode due to water currents and wave pressure. As coral skeletons break down, the homes of the many organisms living on the reef disappear.

In the Caribbean, a new disease is relentlessly infecting and killing corals like never before. The disease is known as stony coral tissue loss disease (SCTLD), nicknamed “skittle-D.” Of the 45 known species of reef-building corals in the Caribbean, almost half can catch this disease. Once infected, a coral colony that took hundreds of years to grow can die within a couple of weeks or months (Figure 1). However, coral species catch the disease at different rates. Species that catch the disease quickly once it has arrived at a reef are considered highly susceptible to SCTLD. Some of the most susceptible stony corals include brain, pillar, star, and starlet corals. SCTLD can exist on a reef for multiple years, and species with lower susceptibility are often infected later on.

Although the exact cause of SCTLD is still unknown, researchers believe that it is probably caused by either a bacterium, virus, or combination of the two.

The Rapid Spread of SCTLD Throughout the Caribbean Sea

SCTLD was first spotted off the coast of Florida in 2014. Scientists have since discovered the disease off the coasts of 20 more countries in the Caribbean, reaching from Mexico to Honduras to St. Lucia (Figure 2).

SCTLD spreads alarmingly quickly through the water. With the vast majority of new outbreaks occurring within the last few years (between 2019 and 2021), SCTLD seems to be picking up speed. For example, in the Bahamas, SCTLD has been estimated to travel at a rate of up to 50 meters (55 yards) per day [1]. Like COVID-19, SCTLD is now considered an underwater pandemic because of its quick and shocking spread across the Caribbean region.

However, SCTLD can be spread in more ways than just by water currents. For example, SCTLD was first spotted in the Bahamas in late 2019, in the area surrounding a major shipping port in Freeport, Grand Bahama. Just a couple of months later, in early 2020, the disease was identified on the reefs near a shipping port in another part of the Bahamas—Nassau, New Providence. The separation of 200 kilometers between these outbreaks—and the lack of disease around the islands between them—led researchers to believe that the disease arrived by way of commercial shipping vessels [1]. When commercial ships travel from one place to another, they take on large amounts of ocean water, known as ballast water, to make them more stable while they are moving. When the ships arrive at their destinations, they release their ballast water. While it may seem harmless, the release of ballast water containing diseases like SCTLD can cause outbreaks in new locations.

After arriving in the Bahamas, SCTLD has likely spread between islands and reefs via local currents and smaller boats. Once present on a reef, SCTLD can spread quickly from coral to coral through the water column, through the movement of sediments across the sea floor, and through physical contact by underwater animals and divers [1].

What Does SCTLD Mean for the Future of Coral Reefs?

Due to SCTLD, Caribbean corals are dying faster than they have ever before. In a matter of months, reefs are transforming from coral-dominated havens to algae-covered graveyards. This rapid decrease in live coral is creating an imbalance between the rate that the remaining stony corals can build new reef and the rate that ocean currents are eroding the coral skeletons that are left behind. This imbalance will affect the structure of Caribbean coral reefs in the coming years, as water pressure erases cracks, crevices, and other critical habitat—flattening the reef as a whole [2]. Sea creatures are already starting to lose their homes, and before long, humans will begin to notice the effects as well—especially when it comes to reduced fishing, tourism and coastal protection.

We Can Stop the Spread, Together

Despite this bad news, we have not lost hope! By working together, we can stop the spread of SCTLD and reverse the decline of Caribbean coral reefs. One way to slow the spread of SCTLD is through antibiotic treatments, similar to medicines the doctor gives you when you are sick! Trained scuba divers will put an antibiotic paste right on the line separating living coral tissue from the newly dead skeleton. For many coral species, this will hopefully stop SCTLD in its tracks, preventing it from spreading to the rest of the coral colony [3]. However, treated corals can get reinfected with SCTLD, so the same corals and reefs must be continually monitored over time, and more antibiotic paste must be applied if necessary.

Additionally, all boaters, divers, fishermen, and ocean-lovers can help stop the spread of SCTLD. The first way to contribute is by learning how to identify the disease. If you think you see SCTLD while snorkeling, fishing, or diving in the Caribbean, snap a picture, record the location, and report it to a local conservation organization. This helps us monitor the spread between reefs within a country and between countries in the Caribbean.

Next, do your best not to spread SCTLD from infected reefs to healthy reefs. If you are going scuba diving, fishing, or snorkeling in the Caribbean, it is important to disinfect your gear before traveling to a new site (Figure 3). Boat drivers should be careful to not carry any water between reefs—they should make sure to pump out any bilge water at reefs where SCTLD is present, and to disinfect any remaining bilge water before releasing it into open water. Bilge water can be disinfected by adding 1 cup of natural detergent (for example Seventh Generation, Earthbound Elements, Tru Earth, or another natural detergent that contains sodium percarbonate) for every 20 liters (approximately 5 gallons) of water and letting it soak for 10 min. It is critical that commercial ships also take the proper steps to disinfect their ballast water.

With your help, we can slow the spread of SCTLD and discover its root cause, eventually stopping the spread altogether and saving the world’s coral reefs!

Glossary

Stony Corals: ↑ A marine animal that builds a hard skeleton out of calcium carbonate, leading to the creation of entire coral reefs! Corals are Cnidarians and are closely related to anemones and jellyfish.

Polyps: ↑ Small animals that can live by themselves or in community with many others. For example, a single coral colony is made of hundreds or thousands of polyps.

Symbiotic Microalgae: ↑ Tiny algae that live within the tissues of stony corals, providing the corals with up to 90% of the food that they need to survive.

Symbiosis: ↑ A close relationship between two different organisms, which often helps one or both of the organisms involved.

Susceptible: ↑ Able to be infected with a disease or illness.

Pandemic: ↑ An outbreak of disease that begins in one place and spreads across other countries and/or continents.

Ballast Water: ↑ A large amount of water that is taken up by and stored within ships. The added weight increases stability and maneuverability during travel.

Antibiotics: ↑ A medicine that fights infections caused by bacteria. They work by either attacking and killing the bacteria or by preventing the bacteria from growing and multiplying further.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors would like to thank Natalia Hurtado (PIMS & Cape Eleuthera Institute) and Will Greene (PIMS) for their image and photomosaic contributions, and Craig Dahlgren for his guidance and support (PIMS). The authors would also like to acknowledge TagLab, a software created by the Visual Computing Lab at the Institute of Information Science and Technologies (ISTI) and the National Research Council of Italy (CNR). Financial support was provided by the Bahamas Ministry of Tourism, Bahamas Protected Areas Fund, Builders Initiative, Devereux Ocean Foundation, and Disney Conservation Fund.