4.4. Effects of habitat depth
A rapid reduction in metabolic rates with increasing habitat depth of the pelagic fishes and cephalopods has been established based on die comprehensive data by Torres et al. (1979) and Seibel et al. (1997), respectively. These depth-related changes in metabolic rates are consistent with in situ observations from submersibles that deeperliving fishes and cephalopods are inactive and often neutrally buoyant (or "lethargic"; Vecchione and Roper, 1991 and literatures therein) Their feeding strategies of these deeper-living organisms, so called "sit-and-wait“ behavior, appear advantageous in these of energy saving for life in the food-poor deep sea, where encounter rates with prey are extremely low. The bodies of many deeper-living fishes are characterized by higher water content, less muscle (or protein or N) and a less ossified skeleton (or ash) than observed in shallow-living counterparts (Denton and Marshall, 1958' Childress and Nygaard, l973).With the exception of the higher water content (Fig.1 in Childress et al., 2008). comparable body composition data of fishes are not available for deeper-living cephalopods. Notably. these body composition features of the deeper-living fishes and cephalopods are not the only cause of the reduction in metabolic rates with increasing depth. as Significant reductions still occur in respiration with habitat depth regardless of body mass unit (WM. DM C or N) examined in the fishes and cephalopods in the present study (Table 2).