ABSTRACT
All freshwater Œshes regulate osmotic pressure and ionic concentrations above their ambient levels. In contrast, all three possible strategies for maintaining salt and water balance are found among Œshes inhabiting marine environments. (1) Near osmoconformity/ionoconformity is found in the marine agnathan hagŒshes, which are restricted to marine environments and do not regulatemain electrolytes and osmotic pressure to any great extent (Morris, 1958). HagŒsh do exert limited control over plasma ions by reducing Ca2+ and Mg2+ concentrations to ∼50% of ambient with aresulting slight hyper-regulation of plasma Na+ (Sardella et al., 2009). However, this control of plasma ionic composition is very minor compared to that displayed by elasmobranchs and teleosts. (2) Osmoconformity with regulation of main ions is seen in marine and some euryhaline elasmobranchs (Evans and Claiborne, 2008; Hazon et al., 2003) and in the lobe-Œnned coelacanth
5.1 Introduction .......................................................................................................................... 175 5.2 Freshwater Fishes .................................................................................................................. 176
5.2.1Integrative Perspective .............................................................................................. 176 5.2.2Dietary Ion Absorption ............................................................................................. 176
5.3Marine Teleost Fishes ........................................................................................................... 177 5.3.1 Integrative Perspective .............................................................................................. 177 5.3.2 Drinking ................................................................................................................... 180 5.3.3Processing of Ingested Fluids ................................................................................... 180
5.3.3.1Na+, Cl−, and K+ Absorption ....................................................................... 181 5.3.3.2 Divalent Ions .............................................................................................. 186 5.3.3.3Water Absorption ....................................................................................... 187 5.3.3.4Reduction of Intestinal Fluid Osmotic Pressure ........................................ 187
5.3.4Impact of Feeding on Intestinal Salt and Water Transport....................................... 188 5.3.5 Neuroendocrine Control and Regulation of Intestinal Transport Processes ............ 189 5.3.6 Marine Teleost Acid-Base Balance Is Intimately Linked to Osmoregulation ......... 191
5.4Marine Elasmobranchs and Other Osmoconformers ........................................................... 192 5.4.1Integrative Perspective .............................................................................................. 192 5.4.2Drinking ................................................................................................................... 192 5.4.3 Processing of Ingested Fluids ................................................................................... 192 5.4.4 Impacts of Feeding on Intestinal Osmolyte and Water Absorption ......................... 193
5.5Interactions between Marine Fishes and the Environment .................................................. 193 5.5.1Impact of Environmental Factors on Intestinal Osmoregulatory Transport
Processes ................................................................................................................... 193 5.5.2 Impact of Intestinal Processes on the Environment ................................................. 195
5.6 Future Directions .................................................................................................................. 196 Acknowledgments .......................................................................................................................... 196 References ...................................................................................................................................... 196
(GrifŒth et al., 1974), which, in marine environments, maintain plasma osmolality slightly above that of the surrounding medium but NaCl concentrations at30%–35% of ambient levels. (3) The most widespread strategy is osmoregulation, found in all teleosts (Evans and Claiborne, 2008; Hwang et al., 2011; Marshall and Grosell, 2006) and lamprey (Evans and Claiborne, 2008; Marshall and Grosell, 2006; Morris, 1958), which regulate Na+ and Cl− concentrations and osmotic pressure at∼150 mM and ∼300 mOsm, respectively, regardless of ion concentrations in their surrounding environment.
