Life in the Open Ocean. Joseph J. Torres

Figure 2.29 Metabolic rates of diverse marine species as a function of minimum habitat depth. (a) Pelagic groups with image‐forming eyes, including fish (closed circles), cephalopods (plus signs) and crustaceans (open squares); (b) pelagic taxa lacking image‐forming eyes, including chaetognaths (open circles) and medusae (closed circles).

      Source: Seibel and Drazen (2007), figure 1(a) and (b) (p. 3). Republished with the permission of The Royal Society (U.K.), from The Rate of Metabolism in Marine Animals: environmental constraints, ecological demands and energetic opportunities, B.A. Seibel and J. C. Drazen, Philosophical Transactions, Biological Sciences, volume 362, issue 1487, © 2007; permission conveyed through Copyright Clearance Center, Inc.

We have already noted that part of the decline in metabolic rate with depth is the result of a more watery structure in deeper‐living species, including a lower protein and, by implication, lower muscle content. Further studies revealed that there are also changes in the muscle itself. Activity of the important intermediary metabolic enzymes lactate dehydrogenase and citrate synthase declines with the depth of occurrence in both California and Antarctic pelagic fishes. In fact, the slopes are very similar to that observed in the decline of oxygen consumption rate with depth (Figure 2.30) (Childress and Somero 1979; Torres et al. 1979; Torres and Somero 1988; Childress and Thuesen 1995; Seibel and Drazen 2007). Thus, not only is there less muscle in deeper‐living species but the muscle that is present is less metabolically active.

Figure 2.30 Activities of the respiratory enzymes citrate synthase (aerobic; closed symbols) and lactate dehydrogenase (anaerobic; open symbols) in marine animals as a function of minimum depth of occurrence. (a) Pelagic fish; (b) pelagic cephalopods.

      Source: Seibel and Drazen (2007), figure 4 (p. 7). Republished with the permission of The Royal Society (U.K.), from The Rate of Metabolism in Marine Animals: environmental constraints, ecological demands and energetic opportunities, B.A. Seibel and J. C. Drazen, Philosophical Transactions, Biological Sciences, volume 362, issue 1487, © 2007; permission conveyed through Copyright Clearance Center, Inc.

Our last question on the decline in metabolism with depth is whether it is confined to pelagic species or whether it can be observed also in benthic and benthopelagic species. Keep in mind that the benthos and the pelagial are fundamentally quite different in their potential food availability. The pelagic realm does not retain the organic matter raining from the sunlit waters above, it’s “just passing through.” Large near‐neutrally‐buoyant particles may move through slowly but move through they do. In contrast, the benthos is the final resting place for the organic rain from the surface, however diminutive that rain may be. Even in non‐productive waters, the benthos is a predictably richer environment than the deep pelagic realm. Also, due to episodic events like food‐falls (those dead whales have to go somewhere), the richness of the environment can vary considerably in the horizontal plane. Thus for mobile species like crabs, shrimps, and fishes, the ability to move well can be very beneficial. Because benthic species like crabs have their weight supported by the sea floor, they do not need to worry about buoyancy. Similarly, though buoyancy is a concern for benthopelagic fishes, changes in their vertical profile are minimal and allow the use of a swim bladder. The advantage of being able to move quickly to a food‐fall might make it advantageous to retain a robust musculature.

      Does a change in metabolism with depth occur in all open‐ocean taxa? Data show that assumption to be both right and wrong, depending upon the taxa of interest. Benthic Crustacea show no change in metabolism with depth of occurrence outside of that predicted by the declining temperature with depth (Childress et al. 1990), a trend very different from that of their pelagic counterparts. Fishes are another matter. Benthic and benthopelagic fishes both show marked declines in metabolism with depth of occurrence (Smith and Brown 1983; Drazen and Seibel 2007), though the slopes for the trend are slightly less than those observed in pelagic species. As in pelagic fishes, the declines in oxygen consumption rate with depth are mirrored by similar declines in enzyme activities (Drazen and Seibel 2007).

      The benthic and benthopelagic fishes that have been studied are quite a bit larger than the pelagic species, generally at least 10 times larger and sometimes as much as 100–1000 times (Drazen and Seibel 2007). As adults at least, they are far more likely to be predators than to be prey. Reduced light levels at depths >500 m restrict visual ranges just as profoundly on the bottom as they do in the midwater, so active searching for prey is likely to be a high‐cost/low‐benefit activity even though hunting is essentially restricted to the horizontal plane. It is thus beneficial for bottom‐oriented fishes to cut daily maintenance costs just as the pelagic species do. The tradeoff is a slower journey to the occasional food‐fall, but obviously evolution has assured that it is fast enough.

Concluding Thoughts

      In most of the world ocean, profound changes in the physical environment occur over very short distances in the ocean’s vertical plane. Temperature, pressure, light levels, and sometimes oxygen concentrations vary drastically within a kilometer’s journey of the surface. To flourish, open‐ocean fauna must accommodate the challenges posed by the environment within their biological characteristics.

Because small swimming species that are the focus of this book are ectotherms, temperature is the environmental variable with the most potential to influence survival, zoogeographic distributions, and biological rate processes such as metabolism and growth. Upper and lower lethal limits of open‐ocean species, like those of all ectotherms, are dictated by their physiological and biochemical characteristics. Limited live animal experimentation on species from a variety of open‐ocean systems as well as more extensive work with enzymes and membranes strongly suggest that open‐ocean pelagic species show the basic responses to temperature described here in detail. That is, we see no exotic or unusual adaptations to temperature. Rate measurements show Q₁₀s in the range of 2–3 and temperature adaptation is observed when ecological analogues in polar and temperate systems are compared.

      The increased pressure associated with mesopelagic depths has the potential to influence biochemical and physiological processes ranging from the ion transport necessary for nerve and muscle function to enzyme function in the anaerobic and aerobic pathways of intermediary metabolism. Animals that live at modest pressures (<100 atm) are either insensitive to it, as are the vertically migrating euphausiids, or show a slight acceleratory response as in the deeper‐living mesopelagic migrators. In contrast, surface‐dwelling species exposed to pressures outside those of their normal environment show excitement at low (50 atm) pressures, moribundity at higher pressures (150 atm), and death due to tetany at high pressures (200 atm). Adaptations to pressure include increases in the fluidity of biological membranes as well as slight changes in the structure of enzymes to confer pressure insensitivity.

      Zones of minimum oxygen are present at intermediate depths throughout the world ocean, but in a few locations oxygen reaches levels low enough to limit animal life. Three such locations are coastal California, the eastern tropical Pacific, and the Arabian Sea. When there is oxygen present in sufficient quantities to enable extraction, such as off California, pelagic species have evolved mechanisms to live aerobically despite the vanishingly low oxygen. Such adaptations include a high gill surface area to allow for efficient extraction of oxygen, a well developed circulatory system, and an efficient means of ventilating the gills. Animals that migrate into regions of zero oxygen, such as in the Arabian Sea, use a strategy of minimizing accumulation of toxic end products by changing the end point of their anaerobic metabolism from lactate to ethanol.

      Depth itself exerts a profound influence on the metabolic characteristics of pelagic species. In swimming species