Water flux and osmoregulatory physiology of the West Indian Manatee (Trichechus manatus)

Abstract

The ability of West Indian manatees (T-richechus manatus latirostris and T. m. manatus) to inhabit both fresh water and marine habitats presents an interesting model to study the question of water flux and osmoregulation in sirenians. Water flux rates were estimated using a deuterium oxide dilution technique for five captive animals in fresh water and one in salt water. Plasma sodium, potassium, chloride, osmolarity, vasopressin, aldosterone, and renin activity were analyzed in captive animals in fresh and salt water, as well as in wild animals in fresh, brackish, and salt water to examine the mechanisms these animals employ for proper osmoregulation. Responses of these systems to changes in salinity were evaluated by switching four animals between fresh and salt water. Urine samples were obtained from these animals and electrolyte concentrations and osmolarities were measured. Data from the water turnover trials indicate that when in fresh water West Indian manatees actively consumed large quantities of water; however, there was no evidence of active salt water drinking, or mariposia. Captive animals held in salt water relied on preformed water in food to maintain appropriate water balance. Data for wild manatees living in salt water suggested that incidentally ingested or dietary salt loads may have been sufficient to alter plasma osmolarities and electrolytes. Captive and free-ranging animals exhibited endocrine adjustments in vasopressin and aldosterone which served to conserve water in marine habitats and to conserve sodium in fresh water. The apparent lack of dehydration in wild animals exposed to a hyperosmotic environment suggests these manatees can maintain proper water balance; however, the definitive work in this regard is yet to be done.