Antipredator Defenses in Tropical Pacific Soft Corals (Coelenterata: Alcyonacea). I. Sclerites as Defenses against Generalist Carnivorous Fishes

Calcified sclerites are common in many invertebrate 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 Sinularia 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 Sinularia reduced feeding by a natural assemblage of generalist carnivorous fishes off Guam by 27-44%; however, sclerites from the bases of the colonies were 18-51% more deterrent than tip sclerites. The greater effectiveness of sclerites from the bases of the colonies was largely attributable to their high concentrations. Sclerites in the tips of the colonies occurred in mean concentrations from 24 to 58% by dry weight and were generally less than 0.5 mm in length. Sclerites in the bases of the colonies were larger and occurred in average concentrations of 82-88%. Sinularia sclerites were increasingly effective as feeding deterrents with increasing concentration at concentrations less than 30-50% by dry weight. The effectiveness of sclerites as deterrents leveled off at higher concentrations. Sclerite morphology was also important 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 functions at particular locations as well as constraints upon their use or production.