Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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rel="nofollow" href="#ulink_eb9800de-ff9b-5134-b4e7-98bf34cf9c77">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.

Schematic illustration of 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.

      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.

      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.

      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.

      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


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