Figure 2.24 Relationship between percent utilization and ventilation volume in Gnathophausia ingens utilizing the values given in Figure 2.23.

      Source: Figure 5 from Childress (1971), Biol. Bull. 141: 115. Reprinted with permission from the Marine Biological Laboratory, Woods Hole, MA.

      No other oxygen‐minimum‐layer species has been examined as well as G. ingens. Taken together, the several studies on the species’ respiratory physiology paint a complete picture of how it is possible to survive at vanishingly low oxygen concentrations. Nonetheless, a few pieces of the puzzle have been collected in other taxa to suggest that elements of the suite have been employed by other species to achieve the same end. The most important characteristic to look for is a P_c at or below the lowest PO₂ encountered in nature. That characteristic has been observed in many of the Crustacea living in the oxygen minimum in the California borderland (8 mm Hg, Childress 1975; Childress and Seibel 1998). It has also been seen in at least one crustacean dwelling in the Eastern Tropical Pacific where the oxygen minimum layer is as low as 3 mm Hg O₂: the galatheid red crab Pleuroncodes planipes with a P_c of 3 mm Hg (Quetin and Childress 1976).

      Once an individual reaches its P_c, it responds behaviorally and metabolically. Since metabolism scales positively with activity level, activity is minimized, precipitously dropping metabolic demand for ATP. Any ATP deficit resulting from the inability to meet its needs aerobically must be made up by anaerobic glycolysis. The hypoxia‐induced drop in activity resulting in lowered ATP demand is termed metabolic suppression (Seibel 2011; Seibel et al. 2016) and is not confined to pelagic fauna. It is the first weapon any species can wield to reduce the demand for ATP and is exploited by intertidal species, such as bivalves, during low tide exposure (Hochachka and Somero 1984; Hochachka and Guppy 1987).

Pelagic cephalopods dwelling in the California oxygen minimum also exhibit low P_cs (3–7 mm Hg, Seibel et al. 1999). Data were collected from the vampire squid Vampyroteuthis infernalis, a fulltime resident of the California borderland’s oxygen minimum zone, on two characteristics of the “Gnathophausia suite”: gill diffusion capacity and blood pigment efficiency (Table 2.4). The diffusion capacity, DGO₂, and oxygen affinity, P₅₀, of Vampyroteuthis shown in Table 2.4 are indicative of a highly efficient gas‐exchange surface and a blood pigment capable of binding oxygen at extremely low concentrations. Both are quite close to those of Gnathophausia, suggesting that, with respect to these two important physiological characteristics, the species are employing a common strategy. Data from the other cephalopod species in Table 2.4 show substantially lower gill diffusion capacity and oxygen affinity in the species inhabiting more normoxic environments.

Vampyroteuthis has a much lower metabolic rate than Gnathophausia. On a mass‐specific basis, the vampire squid’s respiratory rate is a little less than 10% that of G. ingens. On one hand, Vampyroteuthis’ lower rate makes the job of the respiratory system a little easier because the absolute amount of oxygen required per unit time is less. On the other, because of the physics of diffusion it makes little difference, the oxygen gradient between environment and organism is still a tiny one. What might be expected, a priori, in Vampyroteuthis would be a lower capability for removing oxygen in quantity. Adaptations such as very high gill surface area and a highly developed circulatory system, important for delivering oxygen in quantity to satisfy tissue demands, would likely be less in evidence. For at least one of those characteristics, gill surface area, this appears to be the case; Vampyroteuthis has only a moderate gill surface area relative to other cephalopods (Madan and Wells 1996; Seibel et al. 1999).

Table 2.4 Metabolism (VO₂ = ml O₂ kg⁻¹ min⁻¹), gill diffusion capacity (DGO₂ = ml O₂ kg⁻¹ kPa⁻¹ min⁻¹), blood‐water oxygen gradient (∆P_g = VO₂/(DGO₂; in kPa) and hemocyanin‐oxygen affinity (P₅₀ = PO₂ in kPa at 50% hemocyanin‐oxygen saturation) of Vampyroteuthis ingernalis in comparison to other cephalopods. Data for the lophogastrid crustacean, Gnathophausia ingens, are also shown.

      Source: Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Experimental Biology Online, Vampire Blood: respiratory physiology of the vampire squid (Cephalopoda: Vampyromorpha) in relation to the oxygen minimum layer, Seibel et al. (1999).