Emerging infections on the reefs

Natural History, Dec-Jan, 1997 by Andrew W. Bruckner, Robin J. Bruckner

Overlying the immense skeletons that we know as corals is a thin, fragile layer of tissue made up of having coral polyps and their symbiotic algae. Like our skin, the "epidermis" of a reef is easily abraded, and injuries are susceptible to infection. Coral diseases have probably been around for as long as reefs have existed, but they have been recognized by scientists only in the past twenty-five years. Over the last decade, they seem to have intensified.

Black-band disease. The first scientific report of a coral disease came out of Belize in 1973, when Arnfried Antonius, of the University of Vienna, described an infection that moved like a line of fire over a coral head, leaving behind white, denuded skeleton. This dark band of filaments -- primarily cyanobacteria, along with sulfur-oxidizing and sulfur-reducing bacteria -- is known as black-band disease. Brain and star corals are most frequently affected. Curiously, this disease can disappear for a year or more, then mysteriously reinfect the same coral colony. Once it invades, it tends to be chronic, slowly killing off corals year after year.

White-band disease. In 1982, William Gladfelter, of Fairleigh Dickinson University's West Indies Laboratory, published a startling report on a disease that was killing the elkhorn corals in the waters around Saint Croix. The slow-acting affliction, which destroyed up to 95 percent of the elkhorn coral at Tague Bay and Buck Island Monument over a ten-year period, causes coral tissues to peel off the skeleton, from base to branch tips. Esther Peters and her colleagues at the U S. Environmental Protection Agency are investigating whether an unusual rod-shaped bacterium is the primary agent. A second form of the disease was discovered in the 1990s. The two types are responsible for the loss of more than half of the staghorn and elkhorn coral in the Caribbean and Florida during the last two decades; they are still a threat.

Yellow-band disease, also known as yellow-blotch disease, was first described by Craig Quirolo, of Reef Relief, in 1994. The disease attacks boulder star coral -- a major framework builder in the Caribbean, where some colonies are hundreds of years old. It may at first resemble bleaching, the syndrome in which corals become pallid because they have lost their symbiotic algae. Lesions start as narrow, light yellow bands or as round patches; then the section of pallid tissue slowly spreads outward, killing the coral, which becomes overgrown with filamentous algae. The cause has not yet been determined, but bacterial pathogens are being investigated.

Patchy necrosis appear on elkhorn coral branches as one or more irregular patches of denuded skeleton. The patches spread outward as the tissue quickly dies, further exposing the skeleton. We first observed patchy necrosis in 1994 on elkhorn coral thickets off western Puerto Rico and have since found the disease on nearby Mona Island.

White pox, identified in 1996 off the Florida Keys by James Porter, of the University of Georgia, looks like a white rash, or as though bleach has splattered on an otherwise healthy stand. Porter noted that a vast quantity of the elkhorn coral on several reefs had been destroyed. if white pox spreads throughout the Florida Keys reef system at the rate he has estimated, it may jeopardize the remaining elkhorn thickets.

White plague, a coral killer first recognized in 1977, reappeared in 1995 in a more deadly form. On the Florida Keys it has attacked elliptical star coral, a close relative of pillar coral. White plague invades coral tissue at the base of the formation; exposed patches, injuries, and depressions within the colony may also serve as a point of entry. We identified an outbreak of white plague, primarily affecting brain corals, in Puerto Rico during 1996, less than a month after Hurricane Hortense hit the area. Laurie Richardson, of Florida International University; Garriet Smith, of the University of South Carolina; and Kim Richie of the University of North Carolina found white plague affecting eighteen reef-building species in Florida and the Virgin Islands. The disease may be bacterial in origin.

Rapid wasting disease, described in 1997 by James Cervino and Thomas Goreau, of the global Coral Reef Alliance, and Kalli deMeyer, director of the Marine Park Bonaire, appears to be destroying boulder star and brain corals, two major reef building species found in Bonaire, Curacao, Aruba, and elsewhere in the Caribbean. Irregular white lesions appear on the corals' upper surface, and as the disease advances, the top layer of skeleton erodes, forming a distinct boundary between tissue-stripped, damaged skeleton and normal tissue. While some researchers attribute the lesions to a fungal pathogen, we and other observers have linked them to repeated bites by stoplight parrotfish.

Andrew W. Bruckner is a doctoral candidate in the Department of Marine Sciences at the University of Puerto Rico. Robin J. Bruckner, his wife, is his research partner. They have been examining the effects of coral diseases on reefs throughout the Caribbean since 1991.