Coral threats and diseases

Direct threats to Caribbean Sea ecosystems include an alarming near extinction of the critically important black-spined grazing sea urchin (Diadema antillarum) from an unknown pathogen in a very short period of time (1983-1984). Coral bleaching, which is the expulsion of symbiotic algae from coral tissue due to rising water temperatures, is leaving vast expanses of coral reef vulnerable to pathogenic microbes and viruses (1).The release of a non-native tropical (aquarium) fish of the Indo-Pacific lionfish (Pterois volitans), a voracious predator species with no natural enemies, is wreaking havoc among fish species and recent, sporadic “blooms” of large drifting mats of seaweed (Sargassum spp.) are releasing nutrients and chemical components that darken huge stretches of water and decrease light penetration. The prohibitively high cost of cleanup is making it impossible for small tourism-dependent nations to sustain themselves (2). 

Lethal outbreaks of coral disease have only recently been detected by the scientific community. They seem to come in all shapes, sizes and colors, but knowledge of their causes, paths of transmission  and outbreak triggers is still very limited (3).  The swift spread of coral disease, first observed during the 1970’s affecting Acropora spp, was the white band disease. It is estimated that almost 80% of the populations of elkhorn coral (A.palmata) and staghorn coral (A.cervicornis) were lost during this event. Other diseases include the Black Band Disease (BBD), which forms a dark ring-like band of cyanobacteria that kills coral tissue as it migrates. Increased temperature also improves the mobility of the bacteria, broadening the damage, killing in months colonies of coral that took more than a 100 years to grow. BBD was initially found in the western Caribbean, but has spread to other tropical waters worldwide, also affecting sea whips and fans.  The yellow band syndrome, which affects the genus Orbicella (lobed start coral,is a slow spreading disease on a rather slow growing coral. (4; 1).

As of 2014, a new and highly lethal disease, called stony coral tissue loss disease (SCTLD) has affected many reef-building coral. The most characteristic symptom is that sick colonies display multiple lesions that cause the rapid expulsion of the tissue, leaving only bright white skeletons. Highly susceptible species include diverse brain corals (eg Pseudodiploria strigosa) and pillar corals (eg Dendrogyra cylindrus), among others (3).  

The increase and prevalence of these diseases is so widespread it is feared these reefs may disappear. High sea temperatures and other environmental factors act as stressors and affect the severity and dynamics of infectious diseases by increasing host susceptibility and pathogen virulence (4).

References

1) Reyes-Bonilla H.; Jordán-Dahlgren E. (2017) Caribbean Coral Reefs: Past, Present, and Insights into the Future. In: Rossi S., Bramanti L., Gori A., Orejas C. (eds) Marine Animal Forests. Springer, Cham

2) Schell, JM; Goodwin, Ds; & Siuda, AN. (2015). Recent Sargassum inundation events in the Caribbean: shipboard observations reveal dominance of a previously rare form. Oceanography, 28(3), 8-11.

3) Alvarez, L; Estrada,N; Perez, E; Molina, A; y González, FJ. (2019). A rapid spread of the stony coral tissue loss disease outbreak in the Mexican Caribbean. PeerJ 7:e8069 

4) Goldberg, WM. (2013). The biology of reefs and reef organisms. University of Chicago Press. 6: 114-121.