Specificity of prey defenses against predators influences habitat distribution patterns, prey community composition, and species diversity, and can illuminate evolutionary arms races, canalization of biochemical pathways, and mechanisms of speciation. A strong phylogenetic signal in specificity of defenses against consumers, as has been discovered for defenses of terrestrial plants and marine algae, might be predicted in sponges and other chemically defended sessile animals. But the comprehensive data set required for analysis of phylogenetic patterns, and clear distinction from patterns caused by other factors, has not existed for any group of sessile animals. Aiming to remedy this lack, 94 species of tropical marine sponges, the most common and ubiquitous throughout the Caribbean, were made available in the field to 7 species of spongivores in a total of 4493 in situ trials. The sponge species represent 4 distinct habitats (coral reef exposed surfaces, coral reef cryptic spaces, mangrove prop roots, and seagrass meadows), as well as a variety of growth forms, associations with symbionts, and 13 of the currently recognized taxonomic orders of demosponges. The predators represent one echinoderm and three teleost families, and degrees of spongivory range from rare and opportunistic sponge consumption to diets focused on sponges.
Results/Conclusions
Most sponge species (84%, 79/94 species) were consumed by at least one predator, but predator species disagreed on whether or not particular sponge species were palatable in more than half (219/396) of pairwise comparisons. Specificity is asymmetric for predators and their sponge prey, and although closely related predators consumed similar sets of sponges, phylogeny did not predict defenses for sponges, contrasting with conclusions for plants and algae. Habitat did, however, predict patterns of defense specificity for sponges; in particular, sponges that did not share habitat with a predator were significantly less likely to be defended against that predator. The resulting strict habitat boundaries, enforced by predation, can set the stage for ecological speciation, which in turn may help to explain both global and within-region patterns of species diversity in tropical sponges that stand out as different from other groups, such as corals and reef fishes. Theory of defense syndromes and strategies, currently based on far better studied terrestrial plants and marine algae, requires augmentation to include sessile animals.