All vertebrates have to regulate their intracellular ionic compositions for the normal operation of cellular and biochemical reactions. Compared to terrestrial animals, aquatic vertebrates are faced with more challenging osmoregulatory environments with «uctuating ionic compositions and osmolarities,which directly affect the homeostasis of body «uids. Aquatic vertebrates have developed different strategies so that their body «uids can cope with the dramatic ionic and osmotic gradients found in aquatic environments. According to the fossil record, early vertebrates, hagŒshes, are believed to have originated in aseawater (SW) environment, and some agnathan Œsh groups, lampreys,were the Œrst invaders in freshwater (FW) (Halstead, 1985; Bartels and Potter, 2004; Evans and Claiborne, 2009). It was also proposed that lampreys and subsequent teleosts originally evolved in FW, and later some of them returned to SW (Halstead, 1985; Bartels and Potter, 2004; Evans and Claiborne, 2009). On the other hand, most extant chondrichthyan Œshes are marine with very few FW species; however, no fossil record is available to clarify the early evolution of these cartilaginousspecies in FW (Halstead, 1985; Evans and Claiborne, 2009). Table 6.1 shows the concentrations of major solutes and osmolarity in plasma of different Œshes in various aquatic environments. Following the early evolution of vertebrates, strategies dealing with body «uids also evolved from osmoconforming to osmo-and ionoregulating (Table 6.1). HagŒshes are osmoconformers with limited regulation of some divalent ions (Mg2+ and Ca2+). Lampreys are pioneers in developing osmoand ionoregulatory strategies, and teleosts are strict osmo-and ionoregulators. On the other hand, most marine chondrichthys are osmoconformers (slightly hyperosmotic to SW due to high plasma levels of urea and the counteracting solute, trimethylamine oxide) and ionoregulators, and the stenohalineFW stingrays (Potamotrygon sp.) are osmo-and ionoregulators with much reduced plasma levels of urea (Wood et al., 2002). Acid-base regulatory mechanisms achieved by apical Na+/H+ and
TABLE 6.1 Major Solutes (mM) and Osmolarity (mOsm) in Plasma of Different Fishes
Cl /HCO − −3 exchangers in the transporting epithelia evolved early in primitive vertebrates, such as hagŒshes. Lampreys were probably pioneers in evolving both ion absorption and secretory mechanisms other than the acid/base-linked Na+/Cl− uptake pathways that evolved in hagŒshes.