Colony versus population variation in susceptibility and resistance to dark spot syndrome in the Caribbean coral Siderastrea siderea

Scleractinian corals appear to be increasingly susceptible to pathogenic diseases, yet it is poorly understood why certain individuals, populations or species are more susceptible to diseases than others. Clearly an understanding of mechanisms of disease resistance in corals is essential to our understanding of patterns of disease incidence and virulence; this work must begin by examining the colony and population levels of organization. The Caribbean coral Siderastrea siderea exhibits variability in susceptibility to dark spot syndrome (DSS), a disease of unknown origin that can result in tissue necrosis. On the reef scale, variability in DSS prevalence in S. siderea occurred through time, but was not correlated with site, seawater temperature or depth. We monitored colonies of S. siderea affected by DSS, as well as their nearest neighbor controls, for 2 years in the Bahamas and found a marked decline in extent of DSS infection in October of both years. A preliminary survey of antimicrobial activity in S. siderea indicated selective activity against certain ecologically relevant bacteria. To assess whether changes in chemical defenses were responsible for the observed temporal variability in DSS prevalence, we sampled S. siderea for qualitative and quantitative analysis of chemical variability between resistant and susceptible colonies of S. siderea. These data suggest that phenotypic plasticity in antimicrobial activity may impact microbial settlement and/or survival.