Size structure and geographic variation in chemical resistance of sea fan corals Gorgonia ventalina to a fungal pathogen

Despite the intense impact of diseases on tropical marine communities, little is known about mechanisms of resistance to disease or the role of disease as a selective factor in these communities. Spatial variation in chemical resistance of Gorgonia ventalina (sea fan coral) to the fungus Aspergillus sydowii was investigated by sampling G. ventalina populations in the Florida Keys, USA, and San Salvador, Bahamas. A. sydowii is the causative agent in aspergillosis, a fungal disease affecting G. ventalina throughout the Caribbean. Using field census and experimental inoculation, this study examines if sea fan chemical extracts provide resistance to disease caused by A. sydowii and the potential role of disease as a selective force in shaping the chemical defenses of sea fan populations. Chemical resistance against A. sydowii was assessed with a fungal growth assay. Higher anti-fungal activity of small versus large fans observed in field sampling and increased disease severity in mature versus young fans in inoculation studies support the hypothesis that resistance decreases with increasing fan age. This result is also consistent with patterns predicted by the optimal defense theory and growth-differentiation balance hypotheses in plant studies. Anti-fungal activity of large sea fans increased upon inoculation, consistent with the hypothesis of inducible resistance. Anti-fungal activity of sea fan extracts from the Florida Keys and Bahamas did vary significantly with site and was negatively correlated with fan height and extract concentration (mg extract per g dry coral weight). Anti-fungal activity did not correlate significantly with disease status in a multiple regression model. The mean anti-fungal activity of sea fans varied significantly among Florida Keys but not among San Salvador sites, nor between Florida Keys and San Salvador sites. Disease pressure was not correlated with anti-fungal activity across sites, as would be expected under a simple resistance-selection model. A simple mean resistance-selection model may not be appropriate in relatively open marine systems in which environmental induction may obscure natural selection. However, the coefficient of variation in anti-fungal activity among sites is negatively correlated with disease pressure, consistent with the hypothesis that A. sydowii imposes detectable directional selection and eliminates susceptible individuals from sites with high disease pressure.