Coral bleaching and disease: contributors to 1998 mass mortality in Briareum asbestinum (Octocorallia, Gorgonacea)

Abstract

High sea surface temperature associated with the recent El Niño was responsible for widespread coral bleaching and mortality around the globe in 1998. In addition to mortality caused by temperature and bleaching associated stresses, some of the coral mortality could be due to the outbreak of diseases among already weakened hosts. One possible example of this is the October 1998 epizootic affecting Briareum asbestinum in the Florida Keys, USA. At Carysfort, Sand Key and Western Dry Rocks, between 75 and 90% of B. asbestinum colonies were bleached with prevalence of necroses on bleached colonies ranging from 18 to 70%. Between October 1998 and January 1999, 18 to 91% of colonies on seven 25 × 2 m transects died (mean=68%). In addition, at Carysfort Reef, 65% of necrotic colonies that were tagged in October 1998 were dead by January of the following year. A grafting experiment revealed that lesion-causing infections were transmissible: lesions occurred on 50% of recipient colonies treated with diseased grafts whereas none of the grafts with healthy tissue resulted in disease. Preliminary work to isolate a causative agent yielded a cyanobacterium Scytonema sp., although work to confirm its role in the mass mortality is still on-going. By January 1999, when surviving colonies had regained their color and many lesions had healed, the cause of the Briareum asbestinum mass mortality or even whether a mass mortality had occurred, would have been difficult to ascertain. By any measure, this was a significant epizootic that would have gone undetected or been attributed to bleaching stress in the absence of our evaluation of the role of an infectious disease.