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II. Acropora Palmata
Acropora palmata, more commonly known as Elkhorn coral, is the sole coral species to be targeted by white pox disease. This massive, branching structures characteristic of the coral were described by James Porter, the research team leader who discovered the cause of white pox, as “the giant redwoods of the reef” (Porter 1). Throughout documented history of the Caribbean reef systems, A. palmata has been the most abundant reef building coral, but has recently been decimated by widespread outbreaks of white pox disease. Accounting for around twenty-five percent of the coral cover of many reefs throughout the Florida Keys and other Caribbean reefs in the mid-nineties, this coral species fills many important ecological niches. A keystone species, Elkhorn coral maintains much of the structural and functional integrity of many Caribbean reefs, creating the foundation for the highly complex structure of the shallow-water reefs found off of many coasts throughout the Florida Keys, as well as other coastal areas. The coral also provides food and shelter within its broad branches for other reef inhabitants. It provides further protection for the reef itself – of which it is an integral part – by acting as a buffer between the nearby land and the far-reaching ocean (Sutherland et al 289).
A. palmata could be found frequently throughout the Florida Keys and in other Caribbean reefs in the past. The number of colonies left have shrunk drastically since studies were started in 1996, but it can still be found, though less frequently, in the reefs it helped to build. This particular type of coral reproduces by fragmentation, a process by which a healthy coral piece breaks off from the main colony, and can grow into a new colony (Davidson 44). This process requires healthy coral colonies from which the small pieces can break off, reducing the capacity for reproduction within colonies stricken by white pox, and other coral diseases. Though other reproductive strategies, such as budding, may enable coral populations to rebound from widespread decimations from natural disturbances, fragmentation does not, as entire colonies are often killed off by disease, leaving few colonies capable of reproducing (Sutherland et al 294-5).
III. White Pox Disease
White pox disease is caused by the bacterial pathogen, Serratia marcescens. Coral afflicted with the disease can be identified by the trademark irregularly-shaped patches of bare white coral skeleton, devoid of any colorful coral tissue (McCarty et al). Tissue loss occurs at an average rate of two square centimeters per day, but has been observed at rates upwards of 10.5 square centimeters; the disease progresses quickly, and can easily jump from one A. palmata colony to a neighboring colony (Porter et al). The irregular white patches appear to spread, and are later filled with filamentous algae, which can eventually inhabit the entire coral skeleton (McCarty et al). Because of the rapid spread of the disease, it can quickly decimate and kill an individual coral colony; compounded with the contagiousness and virulence of the bacteria, nearby colonies are especially susceptible and can quickly become infected as well. This allows for the quick spread of the disease throughout individual reefs, which have seen as much as a 97% decrease over the course of a short eight years, as observed in the Eastern Dry Rocks of the Florida Keys National Marine Sanctuary (Sutherland et al 292).
The disease is not to be confused with white band disease or bleaching, though both also cause the coral tissue to die off and leave the white coral skeleton. A. palmata may be affected by all three diseases, but there are characteristic differences in the patterns that the different diseases follow. White band disease begins at the base of the coral structure and moves upwards, through the branches, in a concentric ring. Divers can observe a recent white band of bleached coral tissue along the edge of the remaining coral skeleton, which is then followed by a line of necrosis. White pox disease, in contrast, is evident in specimens with characteristic abnormally shaped white patches, though such patches can merge to create complete tissue loss or kill the entire coral colony (Sutherland et al 289-90).
S. marcescens, the pathogen that causes white pox disease, is a rod-shaped, gram negative bacterium commonly found in the intestines of humans, insects, and other animals, as well as being present in different types of soil and plants. Despite its residence inside humans, it can be pathogenic, causing both water-borne and hospital infections, often manifested in urinary tract infections, wound infections, pneumonia, and bacteremia. Though the mechanism by which S. marcescens infects humans is well understood, researchers have yet to discover the exact method by which the bacterium infects the Elkhorn coral (Sutherland et al 290-91). What is known is that the bacterium attacks the coral by consuming the thin layer of living tissue on the exterior of the coral’s limestone skeleton (Porter). This lack of information about the mechanism of infection has created another hurdle for researchers to jump over as they investigate how to halt the spread of white pox disease.
IV. Ongoing Research and Theories
Because S. marcescens is commonly found in human intestines, and thus, human fecal matter, it has been recently stipulated that the increased concentrations of the bacterium in the waters of Caribbean coral reefs may be related to increased amounts of untreated fecal matter being discharged into the ocean. The bacterium is not known to be common to normal, healthy ocean waters (Harder), as it has often only been found and documented in the past in sewage-polluted estuaries (Sutherland et al 291). It has proven to be dangerous to other lentic and marine populations, including white perch (Morone americanus) in the United States. Researchers have predicted a strong link between increased sewage dumping and the increased prevalence of white pox disease throughout the Caribbean, as coastal populations of S. marcescens bloom and greater amounts of fecal matter have to be disposed of somewhere, often in the ocean. Swelling human populations along the coast are constantly putting increased pressure on reefs to sustain themselves in damaged environments, as waters are filled with excesses of nutrients and sewage, and organisms are constantly removed from reefs for human consumption or entertainment.
The additions to the water come with bacteria and other pathogens, organisms that can easily attack the corals and compromise the natural immune defenses corals have. The pathogens tamper with the delicate symbiosis between the coral itself and the zooaxthellae that live within the coral’s structure, leaving the coral defenseless against future attackers. Such pathogens can also interrupt the surface mucous membrane that many corals depend on for protection, resulting in increased susceptibility to diseases caused by these new organisms (Harvell et al).
White pox disease has also been shown to be most virulent, killing the greatest number of coral colonies, at the end of the summer when water temperatures hit their peak. This is consistent with studies of other coral diseases, which has lead researchers to conclude that different corals’ immune responses and defenses against such diseases may be compromised with even a degree of increase in water temperatures. This conclusion is particularly alarming in a time when global warming is a world-wide phenomenon of concern, but is not being actively combated to the degree necessary to change the trend in water temperatures (Raymundo).
Harder, Ben. “Raw Human Waste Killing Off Coral Reefs?”. National Geographic News. [http://news.nationalgeographic.com/news/2002/06/0627_020627_coral.html]. 27 June 2002.
Harvell, Drew, Smith, Garriet, Azam, Farooq, Jordan, Eric, Raymundo, Laurie, Rosenberg, Eugene, Weil, Ernesto, and Willis, Better. “Distribution, causes and impacts of coral disease worldwide”. published by Coral Reef Targeted Research and Capacity Building for Management. 2006.
Porter, JW, Dustan P, Japp W, Patterson KL, Kosmynin V, Meier OW, Patterson ME, and Parsons M. “Patterns of spread of coral disease in the Florida Keys”. Hydrobiologia 460:1-24. 2001.
Raymundo, Laurie J. “Coral Disease as an Emerging Management Issue”. International Coral Reef Initiative, General Meeting. Koror, Palau, 31 October – 2 November 2005. Agenda Item 11.2.
Sutherland, Kathryn Patterson and Ritchie, Kim B. “White Pox Disease of the Caribbean Elkhorn Coral, Acropora Palmata”. 2005.
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