The extreme environmental conditions of deep-sea habitats have important implications not only for the size structure of deep-sea communities but also for the analysis of factors controlling the pattern of biodiversity. Abyssal macrofaunal and megafaunal species diversities are depressed (Rex 1981). For one important macrofaunal taxon-mollusks-the abyssal fauna is composed largely of sparsely occupied range extensions for a subset of bathyal species with larval dispersal (Rex et al. 2005). There is little evidence of abyssal endemism (McClain et al. in press) except at chemosynthetically-driven ecosystems such as hydrothermal vents (Desbruyères et al. 2006), cold seeps (Sibuet and Olu 1998), or whale carcasses (Smith and Baco 2003). Rex et al. (2005) hypothesized that the bathyal and abyssal zones may function as a source-sink system in which abyssal populations of many macrofaunal species are not reproductively self sustaining but represent a balance between chronic local extinction from inverse density dependence and immigration from bathyal sources. Extremely low abundance and biomass may also constrain the potential for evolutionary diversication in larger size groups (Etter et al. 2005). Even if speciation is successful in terms of producing reproductively isolated populations, food levels in the abyss may be insufcient to support subsequent coexistence. Patterns of standing stock suggest that there is considerably more ecological and evolutionary opportunity for adaptive radiation in the bathyal zone (200 to 4000 m) than in the abyss (>4000 m). The dramatic decrease in total community standing stock and the ascendancy of smaller organisms with depth has important implications for deep-sea biodiversity. The bathyal zone (200 to 4000 m) affords more ecological and evolutionary opportunity in the form of energy availability for larger organisms, and consequently supports higher macrofaunal and megafaunal species diversity than the abyss (>4000 m) (Figure 25.1).