Fungal disease resistance of Caribbean sea fan corals (Gorgonia spp.)

Abstract

An on-going, Caribbean-wide epizootic affecting sea fan corals (Gorgonia spp.) is caused by the fungus Aspergillus sydowii (Thom et Church). We examined the role of crude extracts in resistance of two species of sea fans, Gorgonia ventalina (L.) and G. flabellum (L.), against A. sydowii and a bacterial pathogen of fish, Listonella anguillarum (MacDonell et Colwell). Sea fans were collected in January 1997 from San Salvador, Bahamas, and in June 1997 and January 1998 from Alligator Reef, Florida Keys, USA. Crude extracts from both species were tested to determine concentrations inhibiting germination of A. sydowii spores. Crude extracts from both species inhibited spore germination at concentrations as low as 1.5 mg ml⁻¹; most samples were active at 5 to 10 mg ml⁻¹. These concentrations are within the range estimated in living tissue and were higher in healthy colonies suggesting their role in mediating disease susceptibility. We also detected within-colony gradients in antifungal activity, which varied with the disease state of the colony. In healthy sea fans, resistance was highest at colony edges and lowest in medial and central regions of the colony. Among sea fans with lesions in the colony center, resistance in tissue from proximal and medial regions was as high as tissue from the colony edge (i.e. distal region). The increase in antifungal activity suggests an inducible response by the coral host to the fungal pathogen. This response is most evident among sea fans with lesions in the colony center and not among colonies with lesions at the edge. Antibacterial activity of crude extracts against L. anguillarum was highest at the colony edge but did not vary with disease state or tissue location.