Ships contribute to spread of deadly coral disease 

By Danielle Moloney 

Edited by Jasmine Haskell


A recent study by researchers at the University of Miami has confirmed that ships are facilitating the spread of a deadly coral reef disease in Florida and the Caribbean. Corals are notorious for their fragility- they thrive best under precise conditions including certain water temperature, turbidity (murkiness of the water column), and nutrient levels (preferring lower nutrient waters as corals usually get their nutrients from symbiotic zooxanthellae living  in their polyps). When changes affect their environment, it can be difficult for them to thrive and even survive. Disease outbreaks have been increasing in frequency over recent years across coral reefs, and now we know that ships can contribute to the problem in Florida and the Caribbean-  regions infamous for diseased corals. 

What’s making the coral sick?
In 2014, a mysterious phenomenon was observed off the coast of Miami, Florida, where corals became sick with lesions and often died as a result. This was the first known occurrence of what became known as stony coral tissue loss disease (SCTLD). Although less than a decade has passed since the original disease outbreak, SCTLD can now be found across all Florida reefs and many Caribbean reefs, including those in Belize, Jamaica, the US Virgin Islands, and more. The exact cause of SCTLD is still a topic of rigorous study, but is believed to be caused by a bacterial pathogen. Certain types of corals and coral species can be more susceptible to infection and death from the disease but overall SCTLD results in high rates of mortality across species. 

Method of transmission

Usually, SCTLD spreads rapidly over reefs due to direct contact between diseased and healthy corals (when the corals are physically touching one another). There has also been evidence suggesting that contact with the pathogen in infected water can cause disease as well. New research led by Michael S. Studivan has confirmed another vector: ships. Scientists who study marine invasive species will recognize this phenomenon- boats are very common spreaders of non-native species as organisms attach themselves to the hull of the ship (also known as biofouling) when it travels from place to place, or through unintentional storage in the ship’s ballast water. Ballast water is a storage of fresh or saltwater in the hull of a boat during voyages. This water can be taken in or discharged in different locations. It provides stability, reduces stress on the hull, and has other sea-faring benefits. However, it does not come without costs, such as the spread of non-native species, or in this instance, the spread of disease. Furthermore, large vessels are not the only culprits, and even small or recreational boats contribute to the unintentional spread of species and pathogens. 

Studivan’s team tested whether ballast water could be contributing to SCTLD spread by exposing healthy corals to three types of water: disease exposed, disease exposed and UV treated, and non-disease exposed (Figure 1). Ballast water is usually regulated by local, state, or federal agencies because it is already known as a potential vector for non-native species or disease spread. UV light is sometimes used to treat ballast water before it is re-released into local waters, as UV light exposure kills organisms and viruses in the water. A second experiment assessed whether holding the ballast water for 120 hours after SCTLD exposure would change the rate of disease transmission or progression (Figure 2). 

Figure 1. A diagram depicting experimental setup for disease exposure and UV treatment. On the left, the researchers collected coral colonies showing SCTLD lesions and used it to create their disease-exposed water. They also include their UV light treatment. On the right, the figure shows non-disease exposed water contact with coral fragments as an experimental control. (Figure courtesy of Studivan et. al 2022).
Figure 2. A diagram showing variation in the timeline of exposure to experimental water treatments: direct exposure (while diseased corals were in ballast water), a delay of 24 hours post disease exposure, or a delay of 120 hours. (Figure courtesy of Studivan et. al 2022). 

What did they find? 

Results indicated that SCTLD was much more likely to occur from direct coral to coral contact as opposed to ballast water exposure. They also found that ultimately, UV treatment of ballast water did not significantly decrease the risk of developing disease lesions after exposure as opposed to exposure to untreated infected water. Interestingly, the timeline experiment found that holding ballast water for 120 hours increased the likelihood of developing disease lesions after water exposure, suggesting that pathogens may concentrate over time and thereby become more potent. 


The findings noted here warrant further research into how ballast water risks can be mitigated to decrease the spread of SCTLD. More information on ballast water hold time should be obtained to address long voyages and how the concentration effect of the SCTLD pathogen in ballast water may be reduced. Additionally, it is imperative to determine a more effective means of treating ballast water to kill SCTLD pathogens, as UV light treatment did not significantly reduce disease transmission. Although SCTLD has been prominent in Florida and the Caribbean, it has the potential to spread further. Since shipping is a confirmed vector of the disease, other areas with reefs across the globe should consider how to prevent disease spread via boating. 

Read the full study from Studivan here

Please contact the author with any questions:

Feature Image: Photo courtesy of Audley Travel. 

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