White band disease is a condition characterized by the rapid loss of tissue in certain coral species. First documented in the 1970s, it has become a significant cause of coral mortality, particularly within the Caribbean. The disease is named for the distinct band of white, exposed skeleton that appears on affected colonies.
Visual Characteristics of the Disease
The primary sign of white band disease is the sharp, uniform line of exposed white coral skeleton that separates healthy, pigmented tissue from areas where tissue has recently died. This band can range from a few millimeters to over 10 centimeters wide and progresses from the base of a coral’s branches toward the tips. After the living tissue is lost, the bare skeleton often becomes covered by filamentous algae.
This disease specifically targets acroporid corals, which are known for their complex, branching structures. The two most affected species are Elkhorn coral (Acropora palmata) and Staghorn coral (Acropora cervicornis). These species were once dominant on Caribbean reefs, and their susceptibility has led to their classification as threatened under the U.S. Endangered Species Act.
Pathogenesis and Transmission
While the definitive cause of white band disease remains under investigation, research strongly suggests it is infectious and caused by a communicable agent. Bacterial pathogens are the primary suspects, and studies have associated the bacterium Vibrio charchariae with the condition, though a consortium of different microbes may be involved. Bacterial aggregates have been observed in the tissues of some diseased corals, but their inconsistent presence makes a definitive identification challenging.
The transmission of the disease is thought to occur through direct contact between infected and healthy corals, as well as through water circulation carrying the pathogens. The progression of tissue loss is rapid, with the disease advancing at rates from 1 millimeter to as much as 3 centimeters per day. The disease typically starts at the base of a coral colony or in the middle of a branch, moving upward and outward.
Impacts on Coral Reef Ecosystems
The widespread loss of Elkhorn and Staghorn corals due to white band disease has had cascading effects on reef ecosystems. These branching corals are foundational species, creating the complex three-dimensional framework that defines a healthy reef. Their architectural complexity provides shelter, nurseries, and foraging grounds for a vast array of fish and invertebrates. The loss of these corals leads to a collapse of this habitat structure.
As the reef framework disappears, so does the biodiversity it supports. Fish populations decline due to the loss of protective spaces for juvenile fish and the reduction of food sources. This decline in biodiversity diminishes the overall resilience of the reef, making it more vulnerable to other stressors. The degradation of the physical reef structure reduces its capacity to act as a natural barrier, leaving coastlines more exposed to the erosive power of storm surges and wave action. The disease has killed up to 95% of Acropora corals in some areas, such as the Florida Keys, fundamentally restructuring these marine environments.
Current Research and Management Strategies
Current efforts to combat white band disease focus on understanding its pathology and mitigating its spread. Researchers are working to definitively identify the pathogen or pathogens responsible and investigate how environmental factors, such as rising water temperatures, might influence the severity and frequency of outbreaks. Management strategies often involve direct intervention on a local scale.
One technique includes applying antibiotic pastes directly onto the lesions of infected corals to halt the progression of the disease. Another approach is a form of coral restoration known as coral gardening. This involves rescuing healthy fragments from diseased colonies and growing them in underwater nurseries, with the goal of eventually outplanting them back onto the reef to help restore the coral population.