Decline in Pelagic Cephalopod Metabolism With Habitat Depth Reflects Differences in Locomotory Efficiency

The metabolic rates of 33 species of pelagic cephalopods from California and Hawaii were measured and correlated with minimum depth of occurrence. Mean metabolic rates ranged from 0.07 {mu}mol O2g-1 h-1 for the deep-living vampire squid, Vampyroteuthis infernalis, to 8.79 {mu}mol O2 g-1 h-1 for Gonatus onyx, a vertically migrating squid. An individual of V. infernalis, which lives within the oxygen minimum layer off California, had the lowest mass-specific metabolic rate ever measured for a cephalopod (0.02 {mu}mol O2g-1 h-1, 1050 g wet weight). For species collected in sufficient quantity and size range, metabolism was related to body size. Critical partial pressures of oxygen (Pc) were determined for Hawaiian and Californian cephalopods. Pc values for Hawaiian animals were considerably higher than for those taken off California, a trend that corresponds to the higher levels of environmental oxygen in the Hawaiian waters. Buffering capacity ({beta}) of mantle muscle, assayed in eight cephalopod species, was used to estimate the capacity for glycolytic energy production. Mean {beta} ranged from 1.43 slykes for a bathypelagic octopod, Japetella heathi, to 77.08 slykes for an epipelagic squid. Sthenoteuthis oualaniensis. Significant declines with increasing depth of occurrence were observed for both metabolism and {beta}. The decline in metabolic parameters with depth is interpreted as a decreased reliance on locomotory abilities for predator/prey interactions in the light-limited deep sea. The decline in metabolism with depth observed for pelagic cephalopods was significantly steeper than that previously observed for either pelagic fishes or crustaceans. We suggest that since strong locomotory abilities are not a priority in the deep sea, deeper-living cephalopods may rely more heavily on means of locomotion that are more efficient than jet propulsion via mantle contractions--means such as fin swimming or medusoid swimming utilizing the arms and extensive webbing present in many deep-living species. The greater efficiency of deeper-living cephalopods may be responsible for the observation that the decline in metabolic rates with depth is more pronounced for pelagic cephalopods than for fishes or crustaceans.