Bimodal Breathing: Compromise Respiration
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The division of the Animal Kingdom into aquatic and terrestrial life is ancient: it is still relevant to contemporary life. This distinction is ascribed to the different structural and functional attributes which have been imposed by the physical characteristics of the two different fluid media. While looking at this broad picture, it is too often forgotten than a rich assortment of animals regularly commutes between water and air. While some largely subsist in water and extract their O2 needs from the air above, some start their development in water and end up on land at maturity. The mutual assemblage and the distribution of the animals that share this complex lifestyle demonstrates the highly pragmatic strategies which animals have adopted to overcome the ceaseless selective pressures that have beset them (Fig. 55). While hypoxic conditions are a rare occurrence in terrestrial environments except at high altitude and in deep, compact wet soil, many marine and freshwater habitats are characteristically hypoxic and/or hyperbaric (Carter and Beadle 1931; Carter 1935, 1955; Hora 1935; Saxena 1963; Rhoads and Morse 1971). Depending on nature and rate of organic putrefaction of plant and animal matter, such habitats may contain noxious gases such as H2S, a potent inhibitor of cytochrome c oxidase systems, and ammonia (e.g., Theede et al. 1969).