Fig. 3. Minimum depth of occurrence (MDO), defined as the depth below which 90% of the individuals of a given species are captured (Childress, 1995), explains more than 90% of the variation in the normalization constants (b₀) derived from oxygen consumption (solid circles, b₀=79.1MDO^-0.9; r²=0.92) and citrate synthase activity (open circles, b₀=141.2MDO^-0.80; r²=0.94) scaling relationships in cephalopod families (Table 1). The family Octopodidae is benthic, allowing similar lifestyles, and hence similar metabolic rates, across a depth gradient as described in the text and elsewhere (Seibel and Childress, 2000). Octopodids are exluded from the analysis presented in this figure. Individuals of some cranchid species can be found occasionally in near-surface waters despite their low metabolic rates, an ability facilitated by their extreme transparency and associated relief from visual predation (Seibel et al., 1997). However, the majority of specimens in this family are collected below 300 m (MDO>300 m).