Reference: Biol. Bull. 192: 262-278. (April, 1997) Decline in Pelagic Cephalopod Metabolism With Habitat Depth Reflects Differences in Locomotory Efficiency BRAD A. SEIBEL, ERIK V. THUESEN 1 , JAMES J. CHILDRESS, AND LAURA A. GORODEZKY 2 Oceanic Biology Group, Marine Science Institute, University of California. Santa Barbara. California V3106 Abstract. 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 ^mol O : g~' h~' for the deep-living vampire squid, I 'ampyroteuthis infer-nalis, to 8.79 /imol O 2 g ' h ' for Gonatus onyx, a verti-cally migrating squid. An individual of I', infernalis, which lives within the oxygen minimum layer off Cali-fornia, had the lowest mass-specific metabolic rate ever measured for a cephalopod (0.02 ^molO : g"' h~', 1050g wet weight). For species collected in sufficient quantity and size range, metabolism was related to body size. Crit-ical partial pressures of oxygen (P c ) were determined for Hawaiian and Californian cephalopods. P t 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 (ft) of mantle muscle, assayed in eight cephalopod species, was used to estimate the ca-pacity for glycolytic energy production. Mean ft ranged from 1.43 slykes for a bathypelagic octopod, Japetella heathi, to 77.08 slykes for an epipelagic squid, Stlieno-teuthis oualaniensis. Significant declines with increasing depth of occurrence were observed for both metabolism and ft. The decline in metabolic parameters with depth Received 31 January 1996; accepted 18 November 1996. ' Present address: The Fvergreen State College. Olympia. WA 98505. -Present address: Channel Islands National Marine Sanctuary, 1 13 Harbor Way, Santa Barbara, CA 93 109-23 1 . Abbreviations: ft, buffering capacity; MDO. minimum depth of oc-currence; MDdO, minimum depth of daytime occurrence: />c>2, partial pressure of dissolved oxygen; P f , critical oxygen partial pressure. is interpreted as a decreased reliance on locomotory abil-ities 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 crus-taceans. We suggest that since strong locomotory abili-ties are not a priority in the deep sea, deeper-living ceph-alopods 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 effi-ciency 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. Introduction Cephalopods are morphologically diverse, visually ori-enting predatory molluscs. The five groups of extant cephalopods squids (Teuthoidea), cuttlefishes (Sepi-oidea), octopuses (Octopoda), vampire squids (Vampyr-omorpha), and the chambered Nautilus (Nautiloidea) are easily distinguished by morphological characteristics, among which are locomotory adaptations to their habi-tat (Roper et al.. 1984). Locomotory differences are also reflected in an animal's physiological properties. Previ-ous physiological studies on cephalopods have primarily focused on the more commercially important squids, and on the shallow-water octopuses and cuttlefishes (Grieshaber and Ga'de, 1976; Baldwin. 1982; O'Dor, 1982; O'Dor and Webber, 1986: Portner et al., 1993). 262
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