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 sponges, Crambe crambe and Dysidea avara, were studied at three stages of their life cycle: larvae, rhagons (functional recruits), and adults. We surveyed potential predators of these stages in the field, and selected the benthic fish Parablennius incognitus for predation tests on the larvae and rhagons, and the sea urchin Paracentrotus lividus for grazing 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. avara were not eaten by fish, and even larvae stained bright red to make them more conspicuous were rejected. Artificial food containing fresh 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 protected from predation than those of the energy-poor species, by both physical structures and chemical molecules. It is concluded that contrasting defensive strategies can be displayed at different stages of the life cycle. No dichotomy was found between chemical and physical (spicules, tough organic structures) defenses: the species defended chemically as an adult (C. crambe) also featured physical defenses, while in the less chemically deterrent species (D. avara) structural, tough materials were much more scarce. The need to seek suitable test organisms at each life-history stage to understand the role and importance of defense mechanisms in the species' survival is stressed.