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

Abstract

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 pat- terns of within-colony variation of these 2 modes of defense. Lipophilic crude extract and sclerite con- centrations 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, Suber- gorgia 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 (Astro- gorgia 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 con- centrations, suggesting that other properties remaining after the structural integrity of the sclerites was destroyed optimize the calcite skeletal elements for a defensive role.