Reference: Biol. Bull. 182: 23 l-240. (April, 1992) Antipredator Defenses in Tropical Pacific Soft Corals (Coelenterata: Alcyonacea). I. Sclerites as Defenses Against Generalist Carnivorous Fishes KATHRYN L. VAN ALSTYNE’, CHAD R. WYLIE, VALERIE J. PAUL, AND KAREN MEYER University of Guam Marine Laboratory, UOG Station, Mangilao, Guam 96923 Abstract. Calcified sclerites are common in many in- vertebrate species and are frequently used as taxonomic indicators; however, little is known about the function of sclerites. To determine whether sclerites could function as antipredator defenses, we conducted field assays in which sclerites from the Indo-Pacific soft corals Sir&aria maxima, S. polydactyla, and S. sp. were incorporated into an artificial diet and offered to a natural assemblage of fishes in the field. Sclerites from both the tips and bases of all three species of Sir&aria reduced feeding by a nat- ural assemblage of generalist carnivorous fishes off Guam by 27-44%; however, sclerites from the bases of the col- onies were 18-5 1% more deterrent than tip sclerites. The greater effectiveness of sclerites from the bases of the col- onies was largely attributable to their high concentrations. Sclerites in the tips of the colonies occurred in mean con- centrations from 24 to 58% by dry weight and were gen- erally less than 0.5 mm in length. Sclerites in the bases of the colonies were larger and occurred in average concen- trations of 82-88%. Sinularia sclerites were increasingly effective as feeding deterrents with increasing concentra- tion at concentrations less than 30-50% by dry weight. The effectiveness of sclerites as deterrents leveled off at higher concentrations. Sclerite morphology was also im- portant in determining antipredator activity. Although sclerites can play a role in predator deterrence, they can also function in the structural support of colonies. Thus, the sizes, shapes, and abundances of sclerites at different locations within colonies will be determined by their Received 11 July 199 1; accepted 23 January 1992. ’ Present Address Department of Biology, Kenyon College, Gambier, Ohio 43022. functions at particular locations as well as constraints upon their use or production. Introduction Soft corals are frequently a conspicuous component of shallow, Indo-Pacific tropical reef communities despite the abundance of carnivorous fishes. For example, on Guam, soft corals have been reported to provide -95% of the total living animal cover on some reefs (Wylie and Paul, 1989). On New Guinean reefs, soft corals constitute approximately 50% of the living cover between depths of 0 and 5 m (Tursch and Tursch, 1982). The persistence of soft corals and gorgonians in areas with high levels of predation has previously been attributed to their production of predator-deterrent secondary me- tabolites (Co11 et al., 1983; LaBarre et al., 1986; Pawlik et al., 1987; Wylie and Paul, 1989); but, soft corals and gorgonians also produce sclerites or spicules that could potentially serve as antipredator defenses (Harvell and Suchanek, 1987; Sammarco et al., 1987; Harvell et al., 1988; Harvell and Fenical, 1989). Mineral-hardened spio ules are common within a number of invertebrate groups including sponges, cnidarians, platyhelminth worms, mollusks, echinoderms, and ascidians (Kingsley, 1984). However, despite the widespread occurrence of sclerites within marine invertebrates, little is known about the function of these structures, in particular, their abiity to deter feeding by potential predators. Only in recent studies has the role of sclerites as anti- predator defenses been explored. When incorporated into an artificial diet, sclerites of the gorgonian sea whip Pseu- dopterogorgoria acerosa deterred feeding by carnivorous fishes in field assays in Belize (Harvell et al., 1988). Scler- 231