In addition to the commonly used chemical defense mechanism against predation, sessile organisms such as terrestrial plants, soft corals and seaweeds are known to have a physical defense mechanism comprising structural elements made of lignin, CaCO 3 , silica, etc. Most sponges have siliceous spicules that play a key role as skeletal elements. To date, there has been no evidence to show that these spicules also play a role in defense against predation. It is known that low nutritional value of a prey may make it less susceptible to predation. The siliceous spicules found in sponges are indigestible to predators and so sponges that produce large amounts of these spicules may be less susceptible to predation due to their low nutritional value. In the present study, we tested the physical defenses of 6 Red Sea sponge species and 6 Caribbean sponge species against the generalist Red Sea wrasse Thalassoma klunzingeri. Physical defense of the 6 species collected in the Red Sea was also tested using the Caribbean wrasse T. bifasciatum. The spicules of 4 out of the 6 Red Sea sponges deterred predation by T. klunzingeri. Two out of the 6 Caribbean sponges were found to deter predation by T. klunzingeri. In assays conducted in the Bahamas on the Caribbean wrasse T. bifasciatum, only 1 Red Sea sponge species, Suberites clavatus, was found to be physically defended by its spicules. A positive correlation was found between the size of the spicules and their ability to deter predation by T. klunzingeri. Only spicules larger than ~250 µm deterred predation. On the other hand, T. bifasciatum seemed to be deterred based on reduced nutritional quality resulting from high concentration of spicules in a sponge, irrespective of their size. The combination of Crella cyatophora spicules and crude chemical extract deterred predation to a greater extent than that observed for each defense mechanism separately. This finding most probably indicates an additive mechanism of defense used by this sponge species.KEY WORDS: Porifera · Physical defense · Spicules · Skeleton · Fish · Secondary metabolites Resale or republication not permitted without written consent of the publisher