Co-occurrence of chemical and structural defenses in the gorgonian corals of Guam

Chemical and structural defense mechanisms have been reported to co-occur in algae and some soft corals. This study addressed the co-occurrence of chemical and structural defense mechanisms in 7 of the common gorgonian species found around Guam, specifically focusing on patterns of within-colony variation of these 2 modes of defense. Lipophilic crude extract and sclerite concentrations were compared among the base, mid-axis and tip of multiple colonies of each species. Crude extract concentrations differed significantly among parts of Annella mollis, A. reticulata, Subergorgia suberosa, and Viminella sp. colonies. Sclerite concentrations differed significantly within colonies of Astrogorgia sp. and Villogorgia sp. Crude extracts and sclerites from the mid-axes and tips of the colonies were assayed in the field against natural assemblages of reef fishes. Extracts from the tips of 6 of the 7 species were unpalatable to fishes. Of these 6 species, extracts from the mid-axes of A. mollis, A. reticulata and S. suberosa , all of the family Subergorgiidae, deterred fish feeding. Chemical and structural defenses co-occurred in representatives of the families Plexauriidae (Astrogorgia sp. and Villogorgia sp.) and Subergorgiidae (A. mollis and S. suberosa). Only 1 gorgonian coral, Viminella sp., appeared to rely primarily upon structural defenses rather than chemical defenses. To determine if size and shape were factors associated with sclerite structural defenses, a second set of feeding assays was conducted with powdered and whole sclerites from the unpalatable species. Fishes avoided the powdered sclerites from 4 of the 5 species when offered at natural concentrations, suggesting that other properties remaining after the structural integrity of the sclerites was destroyed optimize the calcite skeletal elements for a defensive role.