Calcareous spherules produced by intracellular symbiotic bacteria protect the sponge Hemimycale columella from predation better than secondary metabolites

Benthic sessile organisms in general, and sponges in particular, have developed an array of defense mechanisms to survive in crowded, resource and/or space-limited environments. Indeed, various defense mechanisms may converge in sponges to accomplish a defensive function in an additive or synergetic way, or to operate at different times during the sponge's life cycle. Moreover, sponges harbor highly diverse microbial communities that contribute in several ways to the host's success. Although some symbiotic bacteria produce chemical compounds that protect the sponge from predation, the possible deterrent function exerted by the calcareous coat of a sponge's endosymbiotic bacterium has not, to date, been explored. Hemimycale columella is an Atlanto-Mediterranean sponge, which produces bioactive metabolites and has been reported to host an intracellular bacterium with a calcite envelope. Calcibacteria accumulate in high densities at the sponge periphery, forming a kind of sub-ectosomal cortex. They have been suggested to provide the sponge with several benefits, one of which is protection from predators. In this study, we assess the relative contribution of the endosymbiotic calcibacteria and bioactive compounds produced by H. columella to defend the sponge against sympatric predators. Deterrence experiments have revealed that the sponge combines >1 defense mechanism to dissuade a large array of potential predators; this represents an example of the evolutionary fixation of redundant mechanisms of defense. The chemicals deterred Paracentrotus lividus, Chromis chromis, Oblada melanura, and Diplodus vulgaris, but not Parablennius incognitus and Coris julis, while the spherules of the symbiotic calcibacteria significantly deterred all predators assayed.