ABSTRACT
The great atmospheric oxygenation event occurred some 2.3 billion years ago. Since then the atmospheric oxygen began to dissolve in water and the normoxic waters contain 6-8 mg O2/l (see Diaz and Breitburg, 2009). Hypoxia is defi ned as Dissolved Oxygen (DO) of less than 2.8 mg O2/l (≈2 ml O2/lor 91.4 MM) (Diaz and Rosenberg, 1995) and anoxia means no oxygen. The amount of oxygen contained in a unit of water is only 1/30th of the amount present in the same of air. Further, the rate of diffusion of oxygen in water is 10,000 times slower than that in air. As temperature or concentration of salt increases, the amount of oxygen that can be dissolved in water is decreased. Understandably, the terrestrial mammals can tolerate only a narrow range of oxygen levels but fi shes may survive wider and rapid fl uctuations in DO (see Nikinmaa and Rees, 2005). For example, (i) air-breathing fi shes may switch to acquire atmospheric oxygen (e.g., Channa striatus, Pandian and Vivekanandan, 1976), (ii) many gill-breathing fi shes undertake aquatic surface respiration (Kramer and McClure, 1982), a behavior, in which fi shes respire water in the upper well-oxygenated layer of the water column (e.g., Pseudocrenilabrus multicolor victoriae, Chi-Corrie et al., 2008; Reardon and Chapman, 2010; see also Pandian, 2010, pp. 27-28) or (iii) hold air bubbles in the buccal cavity to aerate water passing through the gills (Val, 1995) or hold the developing embryos in a bubble nest in land, as in Hepsetus odoe (see Kramer, 1978) or in surface water, as in many Anabantids (e.g., Betta splendens, Kirankumar and Pandian, 2002). Others like the hypoxic-tolerant cyprinids are periodic breathers in normoxic water and have low mean arterial Po2 values (e.g., 3kPa in resting goldfi sh), in
comparison to salmonids, which continuously ventilate their gills until their arterial Po2 approaches that of surrounding water (15 kPa) (see Nikinmaa and Rees, 2005). Understandably, the cyprinids are amenable to crowding in aquaculture farms and others like the goldfi sh survive in indoor aquaria even when water is not changed for long periods. However, chronic exposure to hypoxia impairs reproductive performance of the cyprinids. Obligatory coral-inhabitant fi shes also display remarkable tolerance to diurnal changes in DO levels. During nights, the DO levels outside and in between corals falls to 50 and 20% of air-saturation, respectively (Fig. 5.1). Interestingly, obligate coral fi shes like Gobiodon, Paragobiodon and Caracanthus withstand the nocturnal hypoxia down to 8% saturation in their microhabitat (Nilsson and Ostlund-Nilsson, 2008).
