Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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Vampyroteuthis infernalis 0.04 2.32 0.02 0.47 Madan and Wells (1996), Seibel et al. (1997) Nautilus pompilius 0.28 0.38 0.74 2.3 Brix et al. (1989), Wells et al. (1992), Eno (1994) Octopus vulgaris 0.35 0.45 0.77 2.45 Wells and Wells (1983), Bridges (1994), Eno (1994) Architeuthis monachis n.a. n.a. n.a. 1.65 Brix et al. (1989) Gnathophausia ingens 0.56 3.73 0.15 0.19 Belman and Childress (1976), Sanders and Childress (1990)

      n.a. = not available.

      a Normalized to 5 °C assuming Q10 = 2.

      b Measured at pH 7.4 near environmental temperature.

      The available evidence strongly suggests that most full‐ and part‐time residents of oxygen minima rely primarily on an aerobic strategy to make a living. The suite of adaptations exhibited by Gnathophausia ingens are likely found in whole or in part in all oxygen minimum layer residents.

      We noted in Chapter 1 that salinity in the open ocean does not vary enough to be a major determining factor in the distribution of oceanic species, in contrast to estuaries where large drops in salinity occur over the course of a few kilometers and species compositions change accordingly. In the open ocean, salinity varies little over large oceanic areas, with a low from about 33‰ (in near‐surface waters of the Northeastern Pacific for example) to about 38‰ in the Red Sea and Mediterranean.

      The second strategy, typified by the bony fishes, is one of hypo‐regulation. The prefix hypo is from the Greek meaning “under” or “below” (e.g. hypo‐dermic = under the skin). A hypo‐regulator keeps its internal salinity well below that of seawater. Bony, or teleost, fishes like tunas, sardines, and swordfishes have internal salinities 45–60% that of seawater. They maintain their low internal salinities using the suite of mechanisms briefly described below. However, it is important to remember that relative to the large difference between a fish’s internal salinity and that in the open ocean (e.g. 16 vs. 35‰), the small changes in oceanic salinity (typically <1‰) encountered by a pelagic fish during its lifetime are a trivial matter.

      From a physiological perspective, one of a marine fish’s greatest problems is the loss of its body water. The basic laws of diffusion are such that ions and water, when left to their own devices, will move across biological membranes until their concentrations are equal on both sides of the membrane. For our purposes here, we can think of a fish’s skin as a membrane or barrier that is fighting the natural tendency of the ions in seawater to move into the fish, and the water inside of the fish to move out. Unfortunately for the fish, no skin is perfectly impermeable. Further, in order to breathe, the fishes must extract oxygen from seawater using their gills. In order to function effectively, the physical barrier between blood and seawater at the gill must be very thin indeed. Penultimately, fishes need to open their mouths to feed, which provides yet another chink in the armor. The final insult is that fishes cannot produce a concentrated urine like a kangaroo rat, or for that matter even like a human. The best that they can produce is a urine that is about the same salinity as the blood, so they do not lose much ground with their excretory system, but they do not gain any either.

      The third osmoregulatory strategy exhibited by marine species is shown by the elasmobranchs: the sharks, skates, and rays. It is also employed by the ratfishes (chimaeras) and the ancient lobe‐finned fish, the coelacanth. Admittedly, only a very few representatives of those groups are the “small swimmers” that are the focus of this book, but their water‐balancing strategy is an important one in the marine system and is included here for completeness.

Schematic illustration of osmoregulation in teleosts. Schematic illustration of osmoregulation in elasmobranchs.

      By now you may be growing to appreciate the profound changes in the physical environment of the open ocean in the horizontal and vertical planes and their effects


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