Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 37

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.