Calcified sclerites are common in many benthic marine invertebrates, and despite their widespread occurrence, little is known about their ecological roles. Previous studies suggested that the sclerite composition of coral colonies may be altered in response to environmental cues such as predation and water motion. Furthermore, larger sclerites are thought to be more effective than small ones in deterring predators, while small sclerites may provide greater stiffness and resistance to deformation. The present study compared the length of the sclerites of the sea pansy Renilla muelleri from three depths in Guanabara Bay in southeastern Brazil. Our results show that sclerites are larger in deep-water specimens than in those from shallow water. Field assays were conducted in which sclerites from sea pansies at three depths were incorporated into artificial foods and offered to a natural assemblage of fish. These assays demonstrate that sclerites from R. muelleri from all three depths significantly reduced consumption by generalist carnivorous fishes. We conclude that R. muelleri uses skeletal elements not only to give the body its form but also as a defense against biotic threats.