by Rachel G. Jordan
If the ocean kept a diary, what would it write? Perhaps coral reef ecosystems are the pages where a personal history is etched in the calcium carbonate skeletons, chronicling chemical, geological, and biological events. While some entries in this underwater diary document the quiet flow of tide through crystal clear visibility, others tell of storms stirring up sediment and shipwrecks leaving scars on the reef’s foundation. But lately, the diary records a recurring, villainous presence: disease.
Documenting Disease:
Most coral diseases are named logistically rather than creatively. Black band disease manifests as a black band, white plague results in white, plague-like splotches, and stony coral tissue loss disease (SCTLD) does precisely what its name describes— causing various stony coral species to rapidly lose their tissue through the development of stark white lesions where skin rapidly sloughs off skeleton.
Diseases are an expected and normal part of healthy ecosystems, but when disease happens at a disproportionate scale, disrupting the equilibrium of an ecosystem, they might be termed a disease “event.” These events tend to follow the same pattern, categorized in stages. First, there is an “outbreak,” where the disease initially appears and starts traveling through the ecosystem. Next, follows the “mid-epidemic” stage, when the disease appears to be most active and violent. Lastly, the disease becomes “endemic,” where its presence is normalized within the affected ecosystem and a new equilibrium is reached.
Disease events progress from a pre-epidemic stage to a final endemic stage.
Stage 1: Outbreak
In 2014, SCTLD was first observed off the coast of Miami, Florida. Marine researchers hypothesized that a pathogen living in underwater sediment was stirred up during routine port dredging. While SCTLD shares characteristics with white syndromes and alternate forms of tissue loss (such as the growth of a white lesion, where the center dies out at surrounding tissue peels), SCTLD is an extreme case, making headlines for its wide geographic range, ability to infect twenty-one different species, and for racking up an extremely high death toll.
A symmetrical brain coral (Pseudodiploria strigosa) infected with SCTLD rapidly loses tissue as a lesion progresses across its body. Photo courtesy of Brian Reckenbiel at Florida Fish and Wildlife Conservation Commission.
With such dramatic impacts, SCTLD’s manifestation and progression promptly raised alarm among marine scientists as they attempted to answer multiple questions at once: the cause of the disease, how it spread, and a cure.
Stage 2: Mid-epidemic
By 2020, SCTLD had spread along the coast of Florida and throughout the Caribbean. But a handful of patch reefs had remained as-yet untouched. As most infected locations entered the mid-epidemic stage, I was encountering SCTLD face to face for the first time at Dry Tortugas National Park.
Dry Tortugas is composed of 99% water and only 1% land, approximately 70 miles west from Key West, Florida. As a coral biologist for the park, my job was to facilitate field operations in response to SCTLD, placing me on the frontline of divers witnessing, combatting, and tracking the disease. SCTLD arrived at Dry Tortugas in early 2021 and within one year had swiftly transitioned the reefs to mid-epidemic status.
The spread of SCTLD along the Florida Reef occurred between 2014 and 2021. Photo courtesy of Florida Keys National Marine Sanctuary.
Several breakthroughs had occurred. The scientific consensus was that SCTLD was composed of a suite of microbes, including bacteria and viruses acting in tandem. Miami-based researchers discovered that ships were facilitating the disease’s spread, where microbes were unintentionally transported from one region to another through ballast water. It was also thought that recreational vessels might also be contributing to the spread.
Furthermore, a potential cure had been developed— that is, a new treatment for corals sick with SCTLD was actively being tested and proving more successful than the few available alternatives. The powdered antibiotic amoxicillin was mixed with a cocoa butter-based substance called Base2b to form a toothpaste-like goo that could be applied to the surface of sick corals. While the treatment didn’t stop SCTLD altogether, it did slow the advancement of lesions.
Dry Tortugas benefited tremendously from these valiant research achievements, gaining preventative knowledge and intervention insights. Unfortunately, it was not enough to stave off SCTLD permanently. Like other regions of the Florida Reef and Caribbean, the disease continued to progress, albeit more slowly, despite the treatment provided by coral biologists like myself.
A diver treats an infected coral with the amoxicillin and Base2b mixture, in hopes of preventing further spread of the lesion. Photo courtesy of Rachel G. Jordan and the National Park Service.
Stage 3: Endemic
The year is now 2025 and we have reached the point at which SCTLD is now being considered “normal” in Florida and Caribbean waters. What was once an exceptional circumstance has become yet another day on the reef. While the disease continues to occasionally infect new corals, the vast majority of stony corals that were susceptible to infection have already been lost. A new ecological equilibrium has been reached in Dry Tortugas and beyond, which requires study.
Researchers previously focused on intervention via the antibiotic treatment are now carefully documenting the long-term consequences of this disease event to better prepare for future outbreaks. Current research has pivoted to focus on the effectiveness of restoration practices as an intervention measure. Coral nurseries, both land-based and submerged in the underwater environment, are growing, thriving, and preparing their coral crops for outplanting to the reef—to take the place of their predecessors who were overcome by disease.
A New Entry:
How might the ocean perceive the SCTLD outbreak? Documented through the remnant skeletons scattered over reefs, perhaps it will be remembered for its violence to the coral community. Written alongside a long list of climate change-induced impacts, maybe SCTLD will be written off as yet another ecological warning siren hinting toward an unwanted future. Or maybe, etched somewhere into the ocean’s collective memory, there is an account of discovery and hope, where correspondence takes place between corals and people, and the reef remembers the many who endeavored to save it.
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