Abstract
This chapter focuses on intestinal water absorption that is ultimately linked to salt absorption. The gastrointestinal tract was recognized early to play a vital role in marine teleost osmoregulation by absorbing water. The esophagus and the intestine of marine fish contribute in functionally distinct ways to marine teleost osmoregulation. Despite the obvious influence of feeding on intestinal transport processes associated with osmoregulation, only a minor disturbance of salt and water balance was evident from plasma osmolality and ionic composition. Water loss to the marine environment is inevitable through renal excretion and non-renal loss, most of which is likely to occur across the gill surface. The intestinal epithelium provides for osmoregulation by absorbing water. Urotensin II is secreted from the urophysis at rates responding to external salinities with circulating levels being higher in seawater-than in freshwater-adapted fish, suggesting a role in water retention.