Coral Reefs (1992) 11:155-159 Coral Reels 9 Springer-Verlag ~ 992 ~ ~ ~ Chemical and structural defenses in the sea fan Gorgonia ventalina: effects against generalist and specialist predators Kathryn L. Van Alstyne * and Valerie J. Paul University of Guam Marine Laboratory, UOG Station, Mangilao, Guam 96923, USA Accepted 26 April 1992 Abstract. Cyphomagibbosum is an ovulid predatory gas- tropod that specializes on gorgonians, many of which contain secondary metabolites and calcified sclerites that serve as antipredator defenses. In field and shipboard feeding assays, we examined the role of gorgonian crude extracts and sclerites as feeding deterrents to generalist predators and to C.gibbosum. Crude extracts and sclerites were isolated from Gorgonia ventalina, a Carib- bean sea fan on which C.gibbosum feeds, and incorpo- rated into a carrageenan-based artificial diet. In ship- board feeding experiments, artificial diet containing G. ventalina crude extracts was consumed 49% less by C. gibbosum, than artificial diet lacking extracts. The ad- dition of G. ventalina sclerites to the diet also reduced feeding by Cyphoma by about one half. The addition of extracts to the artificial diet reduced feeding by natural assemblages of tropical fishes at Cross Harbor, Great Abaco Island, Bahamas by 87%; sclerites reduced feed- ing by fishes by 95%. Gorgonia ventalina extracts were composed of at least a dozen nonpolar terpenoids. Frac- tions containing these compounds were feeding deter- rents towards fishes in the field. Unlike many terrestrial oligophagous specialists, C.gibbosum is not immune to the defenses produced by its prey. Introduction Dietary specialists tend to be less susceptible to the effects of chemical and structural defenses produced by their prey than predators that feed upon a broad range of spe- cies. Numerous examples of monophagous insects that are resistant to their host's defenses have been docu- mented (Harborne 1982; Strong et al. 1984), and many * Present address: Department of Biology, Kenyon College, Gam- bier, Ohio 43022, USA Correspondence to: V.J. Paul monophagous marine herbivores are also not deterred by the secondary metabolites produced by the seaweeds they feed upon, even though these compounds have strong de- terrent effects on generalist herbivores (Paul et al. 1987; Hay et al. 1987, 1988; Paul and Van Alstyne 1988). Although it is a carnivore, the ovulid gastropod Cy- phoma gibbosum shares several similarities with oligo- phagous herbivores: (1) its diet is limited to a single tax- onomic group, the gorgonian corals (Harvell and Sucha- nek 1987), (2) it feeds upon species that are entirely sessile throughout their adult life, and (3) its prey produce both chemical and structural defenses that effectively deter generalist predators (Pawlik et al. 1987; Harvell et al. 1988). Thus, Cyphoma might also be expected to be im- mune to the defenses of its gorgonian prey. The lack of predation on gorgonians by generalist predators has generally been attributed to their use of secondary metabolites as chemical defenses. Octocorals produce a variety of secondary metabolites, the majority of which are terpenes; however, they also can contain ni- trogenous compounds, fatty acids, and compounds of mixed biogenesis (reviews: Tursch et al. 1978; Sammarco and Coil 1988). Many of these secondary metabolites act as feeding deterrents towards generalist predators, such as the lipid-soluble crude extracts of Caribbean gorgon- ians that deter feeding by the generalist wrasse Thalas- soma bifasciatum (Pawlik et al. 1987; Harvell et al. 1988) and assemblages of fishes (Harvell et al. 1988), and me- tabolites from Sinularia spp. that deter feeding by gener- alist fishes on Guam (Wylie and Paul 1989; Van Alstyne et al. 1992). Alcyonarians als0 contain calcified sclerites that serve as structural defenses towards predators. For example, sclerites from the Caribbean gorgonians Pseudopterogor- gia acerosa (Harvell et al. 1988) and Leptogorgia virgulata (Gerhart et al. 1988), and the Pacific soft corals Sinularia spp. (Van Alstyne et al. 1992) have all been shown to deter feeding by fishes. In this paper, we test sclerites and crude extracts as feeding deterrents towards Cyphomagibbosum and a gen- eralist tropical reef fish assemblage in the Bahamas to de-