ABSTRACT
Adaptation to exposure to saltier water is affected by the steroid hormone cortisol, which is secreted by the kidney. Cortisol secretion peaks with exposure to hypertonic (saltier) water. Cortisol promotes: an increase in the number of chloride cells, increased activity of chloride cells, increased permeability of the urinary bladder to promote retention of water, and increased uptake of water in the gut due to drinking of seawater. In fishes that migrate to saltwater as part of their life cycle, ion regulatory ability is closely associated with life stage. For example, salmon undergo profound physiological changes to prepare for migration to sea.
