Coral reef diseases have become more common as stresses on reefs increase. The causative organism is known only for a handful of these diseases, and those organisms were identified relatively recently. Most coral diseases are named for their appearance – white band disease, for example.
It is likely that the organisms causing these diseases have been around for a long time, but corals were not susceptible to their effects until recently. As corals have become under increasing pressure from human activity, they may have been weakened enough that they are now succumbing to infection by organisms they once were able to resist.
> Black Band Disease (BBD) <
One of the better understood coral diseases, BBD is characterized by a dark red to black band, up to 30 millimeters wide, moving slowly across the coral and leaving behind nothing but the coral skeleton. This “black band” is a bacterial mat composed of thee closely related species of cyanobacteria (also called blue-green algae), arranged in layers. The bottom layer secretes toxic sulfides, which suffocate the coral. Black band disease can destroy up to 2cm per day of coral.
BBD increases in frequency when temperatures are highest, in the late summer or early fall. Researchers have found that enclosing affected corals in a bag of antibiotics often can cure the infection, but this is expensive, time-consuming, and usually reserved only for particularly valuable coral colonies.
> White Band Disease (WBD) <
First identified in 1977, white band disease appears to affect only staghorn and elkhorn corals, and scientists believe it is the predominant cause in the decline of these two types of coral. The disease manifests as a uniform white band of peeling tissue. It can move at a rate of 5 mm per day.
Despite 20 years of research, the cause of white band disease has not yet been identified.
> White Plague <
White plague appears similar to White Band Disease, but affects different coral species. There are three known types of plague, but not much is known about any of them.
* Type I
White Plague Type I was first identified in 1977 in the Florida Keys and appears to affect 10 different species of coral. This disease spreads slowly, at most 3 mm/day, and can take three to four months to kill coral. The cause is still unknown.
* Type II
White Plague Type II also appeared first in the Florida Keys, this time in June 1995. It moves much quicker than Type I, at up to 2 cm/day and kills small colonies within days. Type II affects 33 coral species, many more than Type I, and has spread beyond its original territory into the southern and central Caribbean.
The cause of Plague Type II may be a previously unknown species of Sphingomonas. Scientists are unsure where it came from, whether it is related to Type I or why it is so much more virulent than Type I.
* Type III
White Plague Type III was first identified in 2000 and causes much greater tissue loss than the other two types. No cause has been identified.
> Red-band Disease (RBD) <
Like black-band disease, RBD consists of a band of cyanobacteria moving slowly across the coral, leaving behind dead tissue. However, the species of cyanobacteria are different in the two diseases. Experts have identified two types of RBD.
This type of RBD most closely resembles BBD, but the band is more reddish in color and the bacteria are not organized into layers.
RBD-2 has a different appearance than RBD-1, more like a loose net than a tight band.
> Dark-Spots Disease (DSD) <
This disease, characterized by dark purple, brown or grey markings, has been seen for many years but has only recently been studied and named. The discolored spots of DSD may be circular or irregular in shape and are scattered across the coral colony. Affected polyps appear smaller than unaffected polyps.
Although this disease affects multiple coral species, it appears most commonly on starlet and blushing star corals.
> Yellow Blotch Disease (YBD) <
Yellow blotch disease first appeared in the lower Florida Keys in 1994 and has since spread throughout the Caribbean. YBD starts as circular blotches or a narrow band of pale, translucent tissue. As the disease spreads, the original affected tissue dies and the damaged area slowly radiates outward from that point.
YBD affects only star and brain corals. It spreads slower than most other coral diseases, 5-11 centimeters per year, but may affect a colony for years. The cause of YBD is still unknown.
> Aspergillosis <
This disease, caused by the fungus Aspergillus sydowii, kills sea-fan coral in the Caribbean. The corals often try to protect themselves by encapsulating the fungus in purple galls (abnormal swelling of tissue).
Aspergillus is normally found on land, and is the cause of much moldy food. Some species apparently found an inviting home in the oceans some decades ago, but did not start killing coral until some time in the past 20 years. It is possible that the warmer sea surface temperatures have prompted this change.
> Coral Bleaching <
Coral bleaching occurs when coral polyps expel their symbiotic zooxanthellae. Coral tissues are translucent, and it is the zooxantellae that give them their color. The loss of these algae allows the white skeleton to show through, giving the coral a bleached appearance.
Bleaching occurs when corals are stressed. The most common cause is high sea surface temperatures, but water pollution and infections may also cause bleaching. Bleaching events occur in coral colonies worldwide.
Diseases of coral reefs have increased in number and frequency in the past few decades. We need to get a handle on this potential epidemic before it causes irreparable damage to the world’s most diverse ecosystems.