ABSTRACT
Introduction Hagsh are the living representative of the most basal craniate and share an ionic prole similar to that seen in marine invertebrates, representing the evolutionary transition between invertebrate and vertebrate organisms. It is these unique physiological and evolutionary positions that make hagsh an excellent model to study the evolution of acid/base (A/B) and ionic regulation. Hagshes diverged from the main vertebrate lineage more than 500 million years ago with fossil evidence supporting that their
Introduction .................................................................................................... 277 Acid/base (A/B) regulation ......................................................................... 278
Hagsh habitat and behavior: Naturally acidotic environments ....... 279 A/B regulatory organs ............................................................................. 279
Gills ......................................................................................................... 280 Cellular mechanisms for gill A/B regulation ................................... 281
Responses to metabolic acidosis ............................................................. 284 Responses to metabolic alkalosis ............................................................ 285
Nitrogen balance in the hagsh ................................................................... 286 Nitrogen handling organs ........................................................................ 287
Gills ......................................................................................................... 287 Cellular mechanisms for nitrogen excretion..................................... 287
Responses to nitrogen challenges ........................................................... 288 Ionoregulation ................................................................................................ 289 Extrabranchial contributions to acid/base and ionoregulation .............. 290 Concluding remarks ...................................................................................... 291 Acknowledgments ......................................................................................... 291 References ....................................................................................................... 292
morphology has remained unchanged for ~450 million years (Holland and Chen, 2001). Hagshes are exclusively osmoconforming marine animals with most living at considerable depths and are the only living chordate group to maintain their plasma [Na+] to [Cl−] almost iso-osmotic to that of seawater (Hardisty, 1979). Coupling the unique osmoregulatory characteristics of hagsh along with the fact that fossil evidence comes solely from marine-based sediments, it may be inferred that hagsh never entered a freshwater environment. Considering that their morphology has remained relatively unchanged, it can be hypothesized that their current physiology may reect the physiology of vertebrate ancestors prior to their invasion of estuaries, freshwater, and land (Hardisty, 1979; Holland and Chen, 2001).
