Abstract
Bottlenose dolphins (Tursiops truncatus) have a worldwide distribution in temperate and tropical waters and often inhabit estuarine environments, indicating their ability to maintain homeostasis in low salinity for limited periods of time. Epidermal and biochemical changes associated with low salinity exposure have been documented in stranded bottlenose dolphins; however, these animals are often found severely debilitated or deceased and in poor condition. Dolphins in the U.S. Navy Marine Mammal Program travel globally, navigating varied environments comparable to those in which free-ranging dolphins are observed. A retrospective analysis was performed of medical records from 46 Navy dolphins and blood samples from 43 Navy dolphins exposed to a variety of salinity levels for different durations over 43 years (from 1967–2010). Blood values from samples collected during low salinity environmental exposure (salinity ranging from 0–30 parts per thousand (ppt) were compared to samples collected while those same animals were in a seawater environment (31–35 ppt). Epidermal changes associated with low salinity exposure were also assessed. Significant decreases in serum sodium, chloride, and calculated serum osmolality and significant increases in blood urea nitrogen and aldosterone were observed in blood samples collected during low salinity exposure. Epidermal changes were observed in 35% of the animals that spent time in low salinity waters. The prevalence of epidermal changes was inversely proportional to the level of salinity to which the animals were exposed. Future work is necessary to fully comprehend the impacts of low salinity exposure in bottlenose dolphins, but the physiological changes observed in this study will help improve our understanding of the upper limit of duration and the lower limit of salinity in which a bottlenose dolphin can maintain homeostasis.
Highlights
Bottlenose dolphins (Tursiops truncatus) are observed worldwide in temperate and tropical waters, and certain populations are known to inhabit bays or estuaries of brackish waters with salinities ranging from 15–25 ppt [1,2,3]
Our analyses suggested that lower salinity exposure had the most influence on serum sodium, chloride, blood urea nitrogen (BUN), aldosterone, and calculated serum osmolality values, but had little effect on serum potassium, cortisol, glucose, phosphorus, or creatinine (Table 1)
We do not suspect that water temperature had an effect on serum sodium, chloride, BUN, and calculated serum osmolality values; water temperature may have affected the changes in serum aldosterone that were observed
Summary
Bottlenose dolphins (Tursiops truncatus) are observed worldwide in temperate and tropical waters, and certain populations are known to inhabit bays or estuaries of brackish waters with salinities ranging from 15–25 ppt [1,2,3]. Dolphins inhabiting Barataria Bay in the northern Gulf of Mexico have been found to inhabit environments with salinities ranging between 1.6–32.08 ppt, spending between 1–12 consecutive days at salinities below 7.89 ppt [2,5] It is likely these animals have homeostatic mechanisms that allow them to be in freshwater for periods of time despite their evolutionary adaptation toward conservation of freshwater [6,7,8]. Like West Indian manatees (Trichechus manatus), do not voluntarily drink seawater Manatees, which inhabit both freshwater and marine habitats, will drink fresh water when it is available to maintain water balance [9,10]. The reniculate kidney of the cetacean excretes more water and salts than is typical of most terrestrial mammals [9,10,12,13]
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