A black, purple or reddish, linear to annular band separating healthy coral tissue from a zone of recently-exposed white skeleton. The band width varies from about 0.1 mm to 5 centimeters or more in extreme cases. The length of the band can vary from a few cm to several meters, depending on the side of the colony. Often, the band encircles the entire perimeter of the coral in hemispherical colonies. The band overlays recently-killed tissue, and can be dislodged. Often, filaments are visible extending from the band into adjacent corallites.
BBD may initiate in a central previously denuded algal colonized lesion, or at the perimeter of the colony. Sites of physical injury, abrasion and algal/invertebrate overgrowth may serve as an entry point for the BBD. In severe infections, there may be more than one lesion per colony.
Infections usually start on an upper or side surface of a coral colony as a small dark pigmented patch 1 – 2 cm diameter. Infections do not initiate within the colony surface in areas that appear healthy; if the band initiates within the colony surface, it starts adjacent to a previously denuded or abraded area on the corallum.
The patch generally forms a ring, the circumference of which rapidly increases as the band migrates horizontally across the coral and denudes tissue in its path.
Linear tissue loss progresses at an average rate of 3 mm/day, but the band is capable of advancing up to 1 cm/day. Entire colonies of a small size may experience total mortality within a few days to weeks. Larger colonies typically lose portions of their tissue, with the disease advance declining or stopping at the onset of lower seasonal temperatures. Colonies may be reinfected in subsequent seasons.
In older infections and rapidly progressing advanced stages of the disease, a zone of stark white exposed skeleton may lie between the progressing band and older skeleton that has been colonized by algae.