Megafauna are composed by large benthic organisms. These organisms are visible without the help of microscopes or other specic tools, and can be investigated using remote systems (video, images, etc.) or after their collection and retrieval on board. Despite their size, megafaunal organisms are still largely unexplored in deep-sea ecosystems. Megafauna are present at all depths down to the deepest trenches, but their abundance decreases signicantly with depth (Figure 19.1). The slope of the regression of megafaunal abundance versus depth suggests a rate of decrease higher than that for the other faunal components inhabiting the deep-sea oor. Multiple comparisons show that slopes of two combinations-bacteria versus metazoan meiofauna, and macrofauna versus megafauna-do not differ. Within the 200-6000 m depth range reported by Rex et al. (2006), there are sequences of signicant difference in the overall abundance of bacteria > meiofauna > macrofauna > megafauna.