Disease Transmission of Aspergillosis in Sea Fans: Inferring Process from Spatial Pattern

Despite recent high impacts of disease in the ocean, quantitative studies of diseases in natural marine populations lag far behind terrestrial systems. Transmission processes are poorly known in relatively open marine systems. We studied infection of sea fans Gorgonia ventalina by the fungus Aspergillus sydowii. To assess detectability and mechanisms of secondary transmission of the fungus, we analyzed the spatial distribution of disease among the fans, using Ripley's K as a measure of disease aggregation. Coral populations and disease were mapped at three reefs in the Florida Keys. Where disease prevalence was low, the disease was distributed randomly among sea fans. This is consistent with disease transmission by input of infectious fungal material from terrestrial sources only. However, where disease prevalence was high, the disease was significantly aggregated among sea fans at the 2-8 m scale, consistent with secondary disease transmission. Such transmission could take place by physical contact between neighboring fans, or infected fans might shed fungal material into the water column, and other fans become infected by these fomites. Our results suggest that water-borne transmission occurs, but secondary transmission by physical contact is also likely. We cannot falsify the hypothesis that small-scale local environmental conditions also contribute to disease aggregation.