Water Breathing: the Inaugural Respiratory Process

Abstract

By way of the hydrologic cycle, water on Earth is believed to have remained unchanged in amount and character for about 3000 million of years (Leopold and Davis 1968). From the current concepts of paleobiology, it is popularly considered that life started in water (e.g., Thompson 1980; Selden and Edwards 1989). Currently, as many as 21000 species of fishes (e.g., Nelson 1976; Gilbert 1993), the largest extant vertebrate taxon, live in it. More than half of the living vertebrates have arisen from evolutionary lineages which still inhabit water (Pough et al. 1989). For the first 150 to 200 million years of life on Earth, owing to the harmful effects of the UV light, life was consigned to water. The earliest complete fish fossils, members of the long extinct group named ostracoderms, date to at least 425 million years ago (Repetski 1978). As an ecosystem, water presents greater microhabitat diversity than air and land. Some extreme aquatic habitats include the hot geothermal springs at the floor of the deep oceans 3km from the surface (e.g., Childress et al. 1989) and the volcanic, hot, alkaline lakes, e.g., Lake Magadi of the Kenyan Rift Valley where the osmolarity of the water is 600mOsml-1’, pH 9.6-10.5, 02 level 2.2mg1-1’ and temperature about 43°C (Reite et al. 1974; Johansen et al. 1975). During the millions of years that they have lived in water, fish have adapted very well. Presently, they have a cosmopolitan distribution, occupying diverse ecological niches.