Gill ionic transport, acid-base regulation, and nitrogen excretion

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 fluctuating ionic compositions and osmolarities, which directly affect the homeostasis of body fluids. Aquatic vertebrates have developed different strategies so that their body fluids can cope with the dramatic ionic and osmotic gradients found in aquatic environments. According to the fossil record, early vertebrates, hagfishes, are believed to have originated in a seawater (SW) environment, and some agnathan fish groups, lampreys, were the first 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 fishes are marine with very few FW species; however, no fossil record is available to clarify the early evolution of these cartilaginous species in FW (Halstead, 1985; Evans and Claiborne, 2009). Table 6.1 shows the concentrations of major solutes and osmolarity in plasma of different fishes in various aquatic environments. Following the early evolution of vertebrates, strategies dealing with body fluids also evolved from osmoconforming to osmo- and ionoregulating (Table 6.1). Hagfishes 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 stenohaline FW 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 Cl- /HCO- 3 exchangers in the transporting epithelia evolved early in primitive vertebrates, such as hagfishes. 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 hagfishes.