The rise and fall of a six-year coral-fungal epizootic.

Drivers of disease cycles are poorly understood in marine ecosystems in spite of increasing outbreaks. We monitored a newly emerged fungal epizootic (aspergillosis) affecting sea fan corals (Gorgonia ventalina L.) in the Florida Keys to evaluate causes of its rise and fall over 6 years. Since August 1997, aspergillosis has nearly eradicated large sea fans at some sites. However, sea fan densities have remained relatively constant due to episodic recruitment replacing large fans with small. Recruitment itself was affected by infection and occurred only when prevalence of disease was low. This impact on recruitment occurred because the largest, potentially most fecund colonies had the highest prevalence of disease, and the pathogen significantly suppressed reproduction of infected fans. Moreover, high mortality among adults resulted in a demographic shift to smaller colonies. The most dramatic impact of aspergillosis was the Keys-wide loss of >50% of sea fan tissue from complete and partial mortality. Aspergillosis prevalence has declined steadily over the last 6 years, and we consider the following hypotheses for decline of the epizootic: change in environment, change in pathogen input, and increase in host resistance. We conclude that increasing host resistance is the most likely driver of the decline. However, a change in any of a number of factors, for example, recruitment of naive hosts, rate of pathogen input, or environmental conditions (water quality and temperature), is likely to promote reemergence of the epizootic.