White-band Disease - The Offline Version

About the same time that black-band disease (BBD) was discovered, Bill Gladfelter and colleagues noticed that tissue was slowly peeling off colonies of elkhorn and staghorn corals at Tague Bay, St. Croix, U.S. Virgin Islands. The loss of tissue resulted in a distinct band or line of bare white skeleton and, as a result, this disease was named white-band disease (WBD). Unlike in the case of BBD, despite intensive study, no consistent assemblage of microorganisms could be found at the junction of the sloughing tissue and the bare coral skeleton.

White-band disease on elkhorn coral, Acropora palmata in the Florida Keys Photo by J. Halas.
Appearance	Tissue peels or sloughs off the skeleton at a fairly uniform rate around the branch of the coral and progressing from the base of the branch towards the tip. On the staghorn coral (Acropora cervicornis), tissue loss may also occur in the middle of a branch. No consistent population of microorganisms has been found at the margin of the tissue loss. All that can be seen with the naked eye is the tissue peeling off the bare white skeleton with occasional small bits of tissue remaining on the exposed skeleton. The rate of tissue loss is several millimeters (1/8 to 1/4 inch) per day. The bare skeleton is eventually colonized by filamentous algae, but the band of bare white skeleton that remains visible can be 5 to 10 centimeters (2 to 3 inches) wide. The tissue remaining on the branch shows no signs of pronounced bleaching, although affected colonies might appear slightly lighter in color overall. However, a variant of WBD, termed WBD Type II, has been found on staghorn colonies in the Bahamas. In this disease, a margin of bleached tissue appears before the tissue is lost (Ritchie and Smith, 1995).
Close-up of white-band disease on elkhorn coral, in the U.S. Virgin Islands Photo by E.C. Peters.
Cause	The etiology of WBD remains unknown. Unusual aggregates of Gram-negative rod-shaped bacteria were found in the tissues of affected corals (WBD Type I). However, the role of this microorganism in the development of disease has not been determined. Further complicating the situation, some acroporids containing these bacteria appear healthy, and other colonies with sloughing tissues have no bacterial aggregates. In WBD Type II, bacteria of the genus Vibrio have been found in the surface mucus of the bleached margin. Tissue loss from acroporids can also be due to the grazing activities of predators, but their pattern of tissue removal differs (i.e., gastropods create scalloped incursions from the base of a branch or irregularly shaped patches; polychaete worms such as Hermodice remove tissue from the branch tips of staghorn coral).
Distribution	WBD Type I has been found on acroporid species around the Caribbean, the Philippines, the Great Barrier Reef, and the Red Sea. WBD Type II has only been found in the Bahamas to date.
Impact	The effects at Tague Bay were devastating. Within five years, approximately 50 percent of the shallow reef crest elkhorn corals and many of the deeper staghorn corals were losing tissue (Gladfelter, 1982). In another five years, up to 95 percent of the elkhorn corals had died. Many colonies of acroporids have succumbed to this disease in the Caribbean and the disease is still evident on many reefs (Bythell and Sheppard, 1993; Peters, 1993).