Antipredator defenses in tropical Pacific soft corals (Coelenterata: Alcyonacea) II. The relative importance of chemical and structural defenses in three species of Sinularia

Abstract

Concentrations and types of antipredator defenses vary among tissues within colonies of many colonial marine invertebrates. However, experimental tests have rarely been conducted to assess the consequences of differences in defenses among tissues. We used field bioassays at Guam with generalist carnivorous fishes to compare feeding rates on artificial diets that contained physical and chemical defenses from three species of tropical soft corals, Sinularia maxima, S. polydactyla, and an undescribed species of Sinularia. Crude extracts and sclerite concentrations were added to the diets in concentrations similar to those found in two areas of the colonies, the polypcontaining tips and the sterile bases. By manipulating extract and sclerite concentrations in the diets, we determined that extracts were the major form of protection in the tips of the colonies of all three species. Sclerites from the tips had little impact on predator deterrence. In the bases of two species, S. polydactyla and Sinularia sp., both extracts and sclerites contributed to the deterrence of predators. However, in S. maxima, only base sclerites affected the foraging rates of fishes, the addition of base extracts had no effect. We also manipulated crude extract concentrations without sclerites present to determine whether differences in crude extracts concentrations between the bases and the tips of the colonies were responsible for the differences in the effectiveness of the extracts as predator deterrents. When tested at the same concentrations, tip and base extracts were equally effective deterrents. Field bioassays with flash coloumn fractions demonstrated that in all three species of soft corals, predator deterrence is due to the combined effect of several different compounds. Our results show that differences in the types and concentrations of antipredator defenses present in different locations within the colonies of benthic invertebrates affect the susceptibility of those tissues to predation.