Life in the Open Ocean. Joseph J. Torres

other swimming organisms. Marine Biology 78: 231–237.

      17 Torres, J.J., Grigsby, M.D., and Clarke, M.E. (2012). Aerobic and anaerobic metabolism in oxygen minimum layer fishes: the role of alcohol dehydrogenase. Journal of Experimental Biology 215: 1905–1914.

      18 UNESCO (1983). Algorithms for Computation of Fundamental Properties of Seawater. Paris: UNESCO.

      19 Vogel, S. (1981). Life in Moving Fluids. Princeton University Press: Princeton.

      20 Withers, P.C. (1992). Comparative Animal Physiology. Orlando: Saunders.

      21 Zhou, M., Paduan, J.D., and Niiler, P.P. (2000). Surface currents in the Canary Basin from drifter observations. Journal of Geophysical Research 105: 21893–21911.

2 Physiological Accommodation to Environmental Challenges

All open‐ocean species have a biogeographic range over which they are typically found. That is to say, there are boundaries or limits to a species range in the horizontal and vertical planes. For species inhabiting the epipelagic zone, those boundaries often coincide well with the patterns in the surface oceanic circulation discussed in Chapter 1 and are shown, with euphausiids (shrimp‐like zooplankton – Chapter 7) as an example, in Figure 2.1. That concurrence is not surprising since currents define the living space of open‐ocean species. Populations and communities in the open ocean are quite literally traveling together!

      It takes some mental adjustment to embrace the three dimensionality of the pelagic lifestyle. As Longhurst (1998) observed, for virtually every physical and biological characteristic in the ocean, there is a far more profound change in the vertical plane than in any horizontal excursion, even at oceanic fronts. A 500 m vertical transit from the surface in a tropical region yields a temperature change of >10 °C, a pressure change of 50 atm, and a reduction in light to <1% of that in surface waters with a concomitant change in wavelengths from the entire visible spectrum to entirely blue‐green (Chapter 1). If we compare that with a 500 m surface transit into the Gulf Stream from the Sargasso Sea, we will see temperature changes of <5 °C with little change in visible light and no change in pressure. Considering further, if we are not at an oceanographic boundary, a lateral movement of 500 m would not change anything very much, but a 500 m change in depth will always yield substantial changes in temperature at all but polar latitudes and will result in large changes in ambient pressure and light everywhere. Even the “shallow” deep‐sea is different enough from the surface environment that its characteristics must be accommodated within the physiology of the species that live there.

      Because they can directly limit survival, three physical factors loom large in determining a species’ range boundaries in the horizontal and vertical planes. Those factors are temperature, pressure, and oxygen. No less important from an organismal perspective are the sensory mechanisms that must be adjusted in order to survive vision, perception of sound and motion (mechanoreception), and perception of chemicals (olfaction and gustation). In this chapter, we will cover the basics of how animals respond to temperature, and how pressure and low oxygen may be accommodated in the biology of pelagic species. Sensory mechanisms are treated in later chapters dealing with specific taxa.

As observed in Chapter 1, salinity varies from place to place in the ocean but it does not vary enough to physiologically limit the distributions of oceanic fauna. Nonetheless, it is part of the external milieu that must be dealt with by the physiological systems of oceanic species. We will cover the basics of how oceanic salinity is accommodated by invertebrate and vertebrate fauna with any eye toward developing a basic understanding of osmotic and ionic regulation in both major groups.

Figure 2.1 Biogeographic ranges and distributions of four species of epipelagic euphausiids showing their correspondence to surface oceanic circulation patterns (currents and water masses).

      Source: Longhurst (1998), figure 1.2 (p. 13). Reproduced with the permission of Academic Press.

Temperature

      Active life exists from −20 °C in arctic insects to 113 °C in some of the Archaea found at hydrothermal vents (Hochachka and Somero 2002). In marine systems, the most challenging habitat is in the intertidal zone, where at low tide, winter cold and summer heat can act directly on an exposed fauna. For pelagic species, the window of life is more narrow and more comfortable, ranging from −1.86 °C in polar waters to about 40 °C or slightly more in the waters of the Red Sea. With the possible exception of some small species of copepods (e.g. Oithona similis), no species is found over the entire range of temperatures in the pelagic zone.

      Two basic effects of temperature are our main concern in this book: its effects on survival and its effects on vital rate processes such as metabolism. The literature on temperature and animal life is as fascinating as it is huge. We can only delve into it far enough to appreciate the basics. The best way to start will be with terminology.

      Terms

      Organisms with a wide range of temperature tolerance are termed eurythermal; “eury” is from the Greek for “wide” or “broad.” Eurythermal species have a broad geographic range or live in a habitat subject to wide swings in temperature, or both. Most intertidal species, particularly those that are sessile like barnacles and mussels, are quite eurythermal and tolerate temperature changes of >20 °C in a single tidal cycle. Geographic ranges for intertidal species can be quite broad. On the eastern seaboard of the United States, intertidal species often extend from Cape Cod in Massachusetts to Cape Hatteras in North Carolina and sometimes a great deal further north and south. Eurythermal pelagic species typically have a more modest temperature range. The widely distributed Pacific krill Euphausia pacifica has a temperature range of about 10 °C, inhabiting the northern half of the great Pacific Gyre from the Sea of Japan across the northern Pacific and down the US coast to southern California. Pelagic species that vertically migrate from depths >200 m to the surface and back each day can encounter large temperature swings, particularly in the tropics. Vertical migration is an interesting and widespread lifestyle that will be discussed at length later in the book.

      The opposite of eurythermal is stenothermal, “steno,” the Greek word for “narrow.” Stenothermal species have a narrow range of temperature tolerance and are typically found in habitats with small daily and annual temperature deviations. An example of such a habitat for pelagic species is the Antarctic, where the cryopelagic fish Pagothenia borchgrevinki has been documented to succumb to heat death at temperatures above 4 °C but can readily tolerate the low temperatures (−1.86 °C) found beneath Antarctic ice.

      The next two terms, briefly introduced in Chapter 1, are also opposites: ectotherm and endotherm. They describe organismal body temperature and what determines it. The body temperature of ectotherms is nearly identical to the ambient temperature. Once again, the term is derived from the Greek: “ecto” meaning “outer” or “outside.” In layman’s terms, the ectotherms are “cold‐blooded.” Other than mammals and birds and some highly adapted fishes like the tunas, all animals are ectotherms. The particular subjects of this book, the micronekton and macrozooplankton,