FEEDING DETERRENCE IN SPONGES - THE ROLE OF TOXICITY, PHYSICAL DEFENSES, ENERGETIC CONTENTS, AND LIFE-HISTORY STAGE

The toxic and deterrent properties of two sympatric Mediterranean spon ges, Crambe crambe and Dysidea avara, were studied at three stages of their life cycle: larvae, rhagons (functional recruits), and adults. W e surveyed potential predators of these stages in the field, and selec ted the benthic fish Parablennius incognitus for predation tests on th e larvae and rhagons, and the sea urchin Paracentrotus lividus for gra zing experiments on artificial food containing adult sponge material. Larvae and one-week-old rhagons of C. crambe were readily eaten by the fish, while two-week-old rhagons were not. Larvae and rhagons of D. a vara were not eaten by fish, and even larvae stained bright red to mak e them more conspicuous were rejected. Artificial food containing fres h C. crambe material, natural doses of its crude extract, or extracted sponge material were not grazed by P. lividus. All these preparations were grazed at the same rate as the controls in the case of D. avara. A deterrent effect was obtained in food containing double the natural concentration of avarol, the main active metabolite of D. avara. The energetic content of C. crambe is significantly higher than that of D. avara. Thus, adult stages of the energy-rich species are better prote cted from predation than those of the energy-poor species, by both phy sical structures and chemical molecules. It is concluded that contrast ing defensive strategies can be displayed at different stages of the l ife cycle. No dichotomy was found between chemical and physical (spicu les, tough organic structures) defenses: the species defended chemical ly as an adult (C. crambe) also featured physical defenses, while in t he less chemically deterrent species (D. avara) structural, tough mate rials were much more scarce. The need to seek suitable test organisms at each life-history stage to understand the role and importance of de fense mechanisms in the species' survival is stressed.