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Investigating a Disease

First Reports of Coral Disease

Coral habitats in South Florida have been declining since around the 1970s, due to factors that include coral bleaching, disease outbreaks, hurricanes and cold-snaps. These environmental stressors have decreased the resiliency of our reef system and depressed the natural restorative processes of the reef tract.

In 2014, Stony Coral Tissue Loss Disease (SCTLD) was first detected off Miami-Dade County and has spread north to Martin county and is currently southwest of Key West. We are currently entering the fifth year of this disease in Florida and its impact is unprecedented with greater than 20 coral species affected. It is highly lethal with the majority susceptible colonies (reef building corals – boulder and star corals) affected.

Symptoms of Disease

Tissue loss progresses as a band or line from the base of the colony to the margins or as a series of irregular blotches that radiate outward and often coalesce. Small colonies often die within a few weeks to months, while infections can persist on larger colonies for several seasons.

Corals are relatively simple animals that exhibit signs of stress through changes in pigmentation and loss of tissue. Because a coral consists of only three layers of tissue, it is often difficult to distinguish this tissue-loss disease from other diseases such as white plague. Disease signs can also vary among coral species and within individual corals, ranging from a distinctive white band to a series of irregular blotches that erupt across the colony surface and progressively increase in size.

In some species lesions begin as areas of discolored tissue. As tissue dies, the skeleton becomes exposed and the tissue loss area expands as death advances. Over time, algae or other microorganisms colonize the exposed skeleton (a).

The lesions can originate as focal (one spot) (a,c,d) or multifocal (b,e) lesions that eventually coalesce. Lesions can have sharp (c) and/or irregular (d) borders of living coral tissue.

In other species a "bleaching band" forms ahead of the tissue loss (e).

Five different examples of lesions from coral disease

Coral Tissue Sampling

Specific corals were targeted for histopathology which allows researchers to assess the changes in tissue caused by disease using electron microscopy and molecular samples. To collect samples staff removed tissue from both healthy and affected corals using a 1-inch diameter stainless steel corer that has been sterilized, making sure to collect healthy specimens first to avoid contamination and to use new protective gloves on each colony. After drilling, the holes are plugged with epoxy to reduce negative impacts to the coral.

Three images showing steps of collecting a sample from a healthy coral

Coral Tissue Processing

The samples are then brought back to the laboratory where they are photographed, and observations are made about the condition of the diseased corals and any external parasites are recorded.

Samples are then decalcified to remove the hard, calcareous skeleton so that the remaining soft tissues can be processed. After several weeks, tissues are ready for histology, a process used to cut very thin slices of tissue, which are then stained on slides. Different stains are used to visualize specific cells or pathogens (e.g., bacteria).

Photo: Diseased Montastraea cavernosa

Close up view of coral polyp

Light and Electron Microscopy

The slides are then examined under a light microscope to see if there are any pathogens, pathologies or unusual cell damage.

Tissues are also prepared for transmission electron microscopy. The electron microscope can be used magnify coral cells up to 100,000 times, and scientists can look inside the cells to see if there are any viruses or other microbes present.

Man and woman sitting in front of computer and microscope

Healthy Polyps and Diseased Coral

The photo to the right is a close-up view of the stages of this disease. Within the box are healthy polyps next to the disease border. The exposed white skeleton of a recently dead coral and a green band of algae, where mortality occurred at least a few weeks prior are both present. The center of the lesion is older and overgrown with turf algae and other organisms.

Close up view of polyp with healthy and diseased areas

Ongoing Investigation

This ongoing event is one of the largest coral disease outbreaks in recent decades. Stony coral tissue loss disease has been documented in more than 20 coral species, with outbreaks spreading north and to the south of the epicenter off Miami.

FWC researchers and partners are still trying to determine the cause of the unknown disease.

Researchers continue to process and evaluate samples collected during 2016-2019, as well as working with partners to continue monitoring the disease situation.