![Elkhorn_coral](https://upload.wikimedia.org/wikipedia/commons/5/5f/Elkhorn_coral.jpg)
Stony coral tissue loss disease (SCTLD) is a
disease of corals that first appeared off the southeast coast of Florida in 2014. It originally was described as
white plague disease. By 2019 it had spread along the
Florida Keys
The Florida Keys are a coral cay archipelago located off the southern coast of Florida, forming the southernmost part of the continental United States. They begin at the southeastern coast of the Florida peninsula, about south of Miami, and e ...
and had appeared elsewhere in the
Caribbean Sea
The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...
. The disease destroys the soft tissue of at least 22 species of reef-building corals, killing them within weeks or months of becoming infected. The causal agent is unknown but is suspected to be a
bacterium
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, although more recent studies suggest a
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
. The degree of susceptibility of a coral, the symptoms, and the rate of progression of the disease vary between species.
Due to its rapid spread, high mortality rate, and lack of subsidence, it has been regarded as the deadliest
coral disease Coral diseases, comprising the diseases that affect corals, injure the living tissues and often result in the death of part or the whole of the colony. These diseases have been occurring more frequently in the twenty-first century as conditions beco ...
ever recorded, with wide-ranging implications for the biodiversity of Caribbean coral reefs.
History
The disease was first detected in late 2014 when a few corals off
Virginia Key
Virginia Key is an barrier island in Miami, Florida. It is located in Biscayne Bay south of Brickell and north of Key Biscayne and is accessible from the mainland via the Rickenbacker Causeway.
The island is mainly occupied by the Virginia Key ...
, in
Miami-Dade County, Florida
Miami-Dade County is a County (United States), county located in the southeastern part of the U.S. state of Florida. The county had a population of 2,701,767 as of the 2020 United States census, 2020 census, making it the most populous county in ...
, were affected. By the following year, the disease had spread and extended from
Pompano Beach
Pompano Beach ( ) is a city in Broward County, Florida, United States. It is located along the coast of the Atlantic Ocean, just north of Fort Lauderdale. The nearby Hillsboro Inlet forms part of the Atlantic Intracoastal Waterway. As of the 2020 ...
to
Biscayne National Park
Biscayne National Park is an American national park located south of Miami, Florida in Miami-Dade County, Florida, Miami-Dade County. The park preserves Biscayne Bay and its offshore barrier reefs. Ninety-five percent of the park is water, and t ...
. In 2016 it reached
Palm Beach County, Florida
Palm Beach County is a county located in the southeastern part of Florida and lies directly north of Broward County and Miami-Dade County. The county had a population of 1,492,191 as of the 2020 census, making it the third-most populous county ...
, and the upper
Florida Keys
The Florida Keys are a coral cay archipelago located off the southern coast of Florida, forming the southernmost part of the continental United States. They begin at the southeastern coast of the Florida peninsula, about south of Miami, and e ...
, and the next year the lower Florida Keys. By August 2018, the disease had spread over of sea, and almost half the species of coral growing in the Florida Reef Tract had been affected.
By January 2019 it had been reported from
Jamaica
Jamaica (; ) is an island country situated in the Caribbean Sea. Spanning in area, it is the third-largest island of the Greater Antilles and the Caribbean (after Cuba and Hispaniola). Jamaica lies about south of Cuba, and west of His ...
, the Mexican Caribbean,
Saint Maarten
Sint Maarten () is a constituent country of the Kingdom of the Netherlands in the Caribbean. With a population of 41,486 as of January 2019 on an area of , it encompasses the southern 44% of the divided island of Saint Martin, while the northe ...
, and
Saint Thomas, U.S. Virgin Islands
Saint Thomas ( da, Sankt Thomas) is one of the Virgin Islands in the Caribbean Sea which, together with Saint John, Water Island, Hassel Island, and Saint Croix, form a county-equivalent and constituent district of the United States Virgin ...
.
[
A 2021 study suggested that the spread of the disease is likely facilitated by ]ballast water
Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to vo ...
from ships, due to several cases of the disease crossing currents in the opposite direction that they would flow.
Signs
The signs of this disease are somewhat variable, and it may be difficult to distinguish from other coral diseases. SCTLD manifests as rapidly expanding lesions on the coral. Sometimes the infection exhibits a series of blotches that radiate outwards and coalesce. Small corals may be killed within weeks while larger individuals may survive for months or even a year or two.[
]
Research
This is a particularly deadly coral disease with mortality rates varying between 66% and 100%. Although the cause of the disease has not yet been established, evidence suggests that a bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
l pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
is being transmitted either by contact or by means of water movement. Not all corals are susceptible to the disease, with staghorn coral
The staghorn coral (''Acropora cervicornis'') is a branching, stony coral with cylindrical branches ranging from a few centimetres to over two metres in length and height. It occurs in back reef and fore reef environments from depth. The upper ...
(''Acropora cervicornis'') and elkhorn coral
Elkhorn coral (''Acropora palmata'') is an important reef-building coral in the Caribbean. The species has a complex structure with many branches which resemble that of elk antlers; hence, the common name. The branching structure creates habita ...
(''Acropora palmata''), both of which are critically endangered, appearing to be immune.[
A 2021 study by the ]Florida Fish and Wildlife Conservation Commission
The Florida Fish and Wildlife Conservation Commission (FWC) is a Florida government agency founded in 1999 and headquartered in Tallahassee. It manages and regulates the state's fish and wildlife resources, and enforces related laws. Officers ar ...
and U.S. Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...
involving electron microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
of infected corals suggests that SCTLD may not be caused a bacterial agent as previously thought, but rather a virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
that lethally affects the zooxanthellae
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...
of corals and leads to host cell death. In diseased corals, no pathologies were noted in the coral host cells aside from lysis
Lysis ( ) is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ''lysate''. In molecular bio ...
, whereas zooxanthellae displayed chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
pathologies and potential single-stranded RNA viruses
''Orthornavirae'' is a kingdom of viruses that have genomes made of ribonucleic acid (RNA), those genomes encoding an RNA-dependent RNA polymerase (RdRp). The RdRp is used to transcribe the viral RNA genome into messenger RNA (mRNA) and to repli ...
similar to those found in plant viruses
Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathogenic to higher plants.
M ...
. This may explain why amoxicillin
Amoxicillin is an antibiotic medication used to treat a number of bacterial infections. These include middle ear infection, strep throat, pneumonia, skin infections, and urinary tract infections among others. It is taken by mouth, or less c ...
paste, the only known effective treatment method, suppresses the growth of new lesions but does not prevent new lesions from forming. Given that amoxicillin is intended for bacteria, its apparent suppression of SCTLD lesions may be an unintended side-effect that affects the virus rather than actual suppression of a bacterial agent.
Susceptibility
The first species on a reef to be infected are usually the meandroid corals such as pillar coral
Pillar coral (''Dendrogyra cylindrus'') is a hard coral (order Scleractinia) found in the western Atlantic Ocean and the Caribbean Sea. It is the only species in the monotypic genus ''Dendrogyra''. It is a digitate coral -that is, it resembles fi ...
(''Dendrogyra cylindrus''), elliptical star coral (''Dichocoenia stokes''), smooth flower coral (''Eusmilia fastigiata''), and maze corals (''Meandrina'' spp.). Other susceptible species include boulder brain coral (''Colpophyllia natans''), grooved brain coral (''Diploria labyrinthiformis''), maze coral (''Meandrina meandrites''), symmetrical brain coral (''Pseudodiploria strigosa'') and knobby brain coral (''Pseudodiploria clivosa''). The genetically distinct Florida population of pillar coral has been essentially extirpated in the wild, and now primarily survives only in captivity.
Limited lab experiments suggest that Indo-Pacific
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
coral species may also be susceptible to this disease as many of them are of the same groups present in the Caribbean, which could have severe consequences if the disease happened to reach the Indo-Pacific via the Panama Canal
The Panama Canal ( es, Canal de Panamá, link=no) is an artificial waterway in Panama that connects the Atlantic Ocean with the Pacific Ocean and divides North and South America. The canal cuts across the Isthmus of Panama and is a conduit ...
. In contrast, other studies suggest that if SCTLD is of viral origins, the zooxanthellae clades
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, t ...
inhabiting the Caribbean may be uniquely susceptible to it, in contrast to the Indo-Pacific, which is dominated by a different clade of zooxanthellae and has not seen major coral diseases.
Conservation
Due to the wide-ranging effects of the disease, many of the susceptible coral species have had fragments harvested from the wild, which have then been taken into captivity across the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
for the purpose of conservation, captive breeding
Captive breeding, also known as captive propagation, is the process of plants or animals in controlled environments, such as wildlife reserves, zoos, botanic gardens, and other conservation facilities. It is sometimes employed to help species that ...
and selective breeding
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant mal ...
, until either conditions improve or the captive corals develop resilience to the disease.
References
{{corals
Coral diseases