Tissue Loss Disease a threat to TCI reefs.

Story and Photos By Erin Bowman and Heidi Hertler, Ph.D.

In a time when climate change is wreaking havoc on coral reefs worldwide, the reefs of the Turks & Caicos Islands remain some of the most resilient and pristine in the Caribbean. They are home to more than 60 species of stony coral that have been building for millions of years, creating the intricate reef systems known today. As worldwide reef degradation goes, the Caribbean has been the unfortunate epicenter, with significantly more damage and less recovery than in reefs of the Pacific or Mediterranean. There are many theories as to why this may be the case and likely it is due to a wide variety of reasons.

Geographically, the Caribbean’s location makes it vulnerable to the tradewinds that blow across Africa towards the Atlantic, bringing dust and sand particles from the Sahara along with them. It is thought that these iron-rich particles carried to the Caribbean reefs increase the growth of algae, which bloom four times faster than on Indo-Pacific reefs. Algae compete with corals, which naturally grow slowly and struggle to outcompete these large blooms. On top of that, the Indo-Pacific is home to many more species of coral and fish, including vital herbivores like parrotfish which graze on algae and prevent it from taking over and smothering reefs.
Corals thrive in very specific living conditions; water that gets too hot or too cold, any change in nutrients or salinity or exposure to too many or too few UV rays all can cause a coral to be “stressed.” Once stressed, the microalgae living inside of coral polyps called zooxanthellae are expelled, stripping the coral of its color and main energy source. This is the phenomenon described as coral bleaching. Once bleached, a coral is not dead. If the unsuitable conditions that caused the coral to stress return to normal in a reasonable amount of time, the coral can regain its zooxanthellae and return to a healthy, unstressed state. If the condition remains imperfect, and does not improve, in a matter of weeks the coral will start to die, being unable to capture enough food for itself without its photosynthetic counterpart, zooxanthellae.

Despite the unfortunate state of Caribbean reefs as a whole, the TCI reefs have somehow managed to remain in a comparatively healthy state. While various bleaching epidemics have left their impact, TCI reefs show much less bleaching than those in Florida or other parts of the Caribbean. This could in part be due to the deeper nature of these reefs, where the UV light reaching the corals is less harsh.
There is also much less human impact in the TCI than elsewhere in the Caribbean. In the Caribbean, 22 countries have larger populations than the TCI, which has just over 36,000 residents as of 2019. South Florida, including the counties of Miami-Dade and Monroe, where much reef degradation has been seen, is home to over 2 million people, so there is significantly less routine exposure to human impacts at TCI reefs.
Water temperature surrounding the TCI remains quite stable throughout the year. At its coldest, the water is about 26ºC (79º F) and at its hottest, it is 29ºC (84ºF). This is very different from the waters of South Florida, in which the temperature fluctuates throughout the year from 21ºC (69ºF) to 31ºC (87ºF).
The School for Field Studies Center for Marine Resource Studies (SFS CMRS) on South Caicos has been diving and conducting research on the surrounding reefs for the past 30 years. Studies have varied over the years but largely revolve around the topic of reef health and monitoring. Now in 2019, for the first time in 30 years, researchers at SFS have noticed a change in the reefs they know so well. Suddenly, at multiple sites, corals were dying; sometimes just one area on an individual coral and other times entire coral heads. It was noticed immediately that these corals weren’t just bleached, they no longer showed the fuzzy outline indicating polyps still living within the coral and only the white skeleton remained.
It was quickly determined that a disease of some sort must be causing this sudden death of previously healthy corals. Coral diseases have been studied for years, and in many cases, researchers have been able to identify the causes, transmission factors, and solutions to such diseases. Whatever was damaging the reefs of South Caicos however, didn’t look or behave like any well-known coral diseases. Over the past few months, the researchers at SFS had been hearing about a new disease that is causing major problems on the Florida Reef Tract, and elsewhere in the Caribbean.
First reported in Florida in 2014, Stony Coral Tissue Loss Disease (SCTLD) is killing corals faster and across more species than any other coral diseases known. Since 2014, it has traveled from northern Miami–Dade County all the way down the Florida Keys and into the Caribbean in places like Jamaica, St. Maarten, and now the TCI. The method by which the disease is transmitted from coral to coral is unknown, making it extremely difficult to study or replicate so that methods of putting a stop to it can be tested. Over half of the stony coral species found on the Florida Reef Tract are affected by SCTLD, though there are many factors that influence the likelihood that a coral will contract the disease (such as the location of the coral and time of the year.) Many researchers throughout Florida and the Caribbean are actively trying to pinpoint the indicators of the disease and the rate at which it progresses, though it has proven to be a difficult feat as these attributes may vary by species affected.
In what is widely being considered an epidemic, the reefs that are already deteriorating now are forced to face this new and deadly disease.
One quality that makes SCTLD so lethal is that it functions from the inside of the coral to the outside. Typical first signs of SCTLD on a coral are bands of discoloration that gradually begin to turn white and move across a colony, often in a circular pattern radiating out or a line moving from one side to the other. By the time any signs show up on the outside of a coral though, the disease has already infected the colony and begun to do its damage on the inside. Due to the necrotic nature of the disease, as the coral polyps die, the tissue begins to deteriorate rapidly and slide off the colony. Once all living tissue is gone, the coral is left with only the skeleton, which will eventually be overgrown by algae.
Since the discovery of the disease off of South Caicos, SFS researchers and students have worked together to gather data on affected corals. Knowing the local reefs gives researchers a better perspective as to how much of the reef is being affected. Identifying differences between SCTLD in the TCI and elsewhere can help to narrow down characteristics of the disease. Any similarities or differences in affected species, location, or depth may be useful in identifying the pathogen or transmission vectors of the disease.
The good news for TCI reefs comes back to their impressive resilience in the face of the devastation that the rest of the Caribbean is facing. Compared to conditions in Florida, SCTLD here has not even made a dent in our reefs. The fact alone that it took a full five years from the original outbreak to reach TCI reefs has spared us from facing the same damage. The coral heads here that have contracted the disease are isolated from one another, often with one or more other corals in between that remain unaffected. This is largely in contrast to Florida’s reefs, where many groups of coral heads can be seen having died from the disease, though this does not necessarily indicate that all corals within a group contracted the disease one right after the other in those cases.
The work that researchers have been doing in Florida since the first outbreak will be incredibly useful in the fight against SCTLD here, in that because of their work we essentially have a head start on the disease and know more about it than we would if this was a nameless, unheard of condition. As of now, SFS is the only team in the TCI working to gather data on SCTLD, but as word gets out and more people learn about the possible destruction this is going to cause our reefs, we are optimistic that other capable hands will join the fight. Research will not only help us to minimize the effect of SCTLD on our pristine reefs, but it can also empower us to prevent reefs elsewhere in the world from having to experience the alarming loss seen throughout the Caribbean.

Sources
ARC Centre of Excellence in Coral Reef Studies. “Coral reefs in the Indo-Pacific Ocean naturally tougher than Caribbean reefs.” ScienceDaily, 12 July 2012.

Florida Keys National Marine Sanctuary. (2018). Florida Reef Tract Coral Disease Outbreak: Disease. National Marine Sanctuaries, NOAA.

Kinane, S. (2019) Florida’s devastating coral disease has spread to the Caribbean: scientist. 88.5WMNF Florida Public Radio.

Riggs, B. (2006) Coral Bleaching: Bad news and (a little) good news for Turks & Caicos Reefs. Times of the Islands.

The Museum Turks and Caicos (2019) “The Living Reef.” Turks and Caicos Islands, British West Indies.

Turks and Caicos Reef Fund (2019) “Coral Bleaching.” Turks and Caicos Islands, British West Indies.

Weinberg, E. (2018) Scientists work together to solve a coral disease mystery in the Florida Keys National Marine Sanctuary. National Ocean Service, NOAA, Department of Commerce.