Physiological effects of seawater intake in adult harp seals during phase I of fasting

Previous studies have shown that harp seals may drink considerable amounts of seawater. The current study was undertaken to study the physiological responses to bolus administration of seawater. Adult harp seals ( Phoca groenlandica) were fasted without access to water for 48 h and then given 1000 or 1500 ml of seawater by a stomach tube. Changes in urine and plasma parameters were thereafter monitored for another 12–20 h. Urine production and urine excretion rate of Na + and Cl − increased soon after administration and reached a maximum 3–4 h later. Urine osmolality was kept rather stable and high (∼1500 mOsm kg −1) following seawater administration, due to a drop in urine concentration of urea that was proportional to the simultaneous increase in urine concentration of NaCl. Plasma osmolality remained at approximately 340 mOsm kg −1, while plasma concentration of urea decreased some 20–25% due to increased excretion of urea when seawater was ingested. Despite bolus administrations of seawater of up to ∼2% of body mass, homeostasis was maintained and no ill effects observed. It is concluded that the concentrating abilities of the kidneys of harp seals are sufficient to prevent net loss of body water following seawater ingestion. Seawater ingestion may, moreover, increase urinary osmotic space and thus serve as a mechanism to excrete additional urea produced during phase I of fasting.