In various ecosystems, predators indirectly affect resources via intermediate prey. This is realized by reducing density of the prey (density-mediated indirect interactions; DMIIs) or by changing their behavior, life history, or morphology (trait-mediated indirect interactions; TMIIs). Although features of predators (e.g., hunting modes) and resources (e.g., quantity) in such indirect interactions are well documented, little attention has been paid to the prey’s features (e.g., density). To address this issue, we focused on a three-level marine food chain involving the carnivore snail Thais clavigera, its prey limpet Siphonaria sirius, and the algae Lithoderma sp. and Ulva spp. The limpets have home ranges in which most rock surface is covered with the cyanobacterium Lithoderma sp. We experimentally manipulated the density of limpets to make low- and high-density plots, and measured strengths of DMIIs and TMIIs in each condition. In low-density plots, Lithoderma sp. was replaced by Ulva spp. through both DMIIs and TMIIs. On the other hand, in high-density plots, only weak DMIIs were detected. These results suggest that a feature of the prey, such as its density, is a key determinant of the strengths of DMIIs and TMIIs.