Book Description

ABSTRACT: The Atlantic stingray is unique because it is one of the few elasmobranchs to establish populations in both fresh and seawater environments. Before this dissertation, the mechanisms that allow elasmobranchs to live in such contrasting environments were unknown. The goals of this dissertation were to describe the general osmoregulation of the Atlantic stingray, and to establish the extra-renal mechanisms this species uses for osmotic and acid/base balance in fresh and seawater environments. First, I described the osmoregulatory strategy of freshwater Atlantic stingrays. I established that the plasma of freshwater stingrays had relatively low urea and NaCl concentrations, and a small salt-secreting rectal gland, compared to marine Atlantic stingrays. When freshwater stingrays were acclimated to seawater, plasma urea and NaCl concentrations increased to typical seawater Atlantic stingray levels, which suggested that the stingrays were not physiologically restricted to freshwater environments. Second, I investigated the influence of salinity on Na+, K+-ATPase expression in gills and rectal glands of Atlantic stingrays. In gills, Na+, K+-ATPase expression (activity, immunoreactivity, and number of Na+, K+-ATPase-rich cells) was highest in freshwater stingrays, compared to seawater individuals. In rectal glands, Na+, K+-ATPase activity and immunoreactivity were higher in seawater stingrays, compared to freshwater individuals. These results suggested that the gills are important for active ion uptake in fresh water, while the rectal gland is important for active ion secretion in seawater. Third, I focused on expression of vacuolar-proton-ATPase (V-H-ATPase) in stingray gills. The V-H-ATPase immunoreactivity was higher in gills from freshwater stingrays, compared to seawater individuals. Localization of V-H-ATPase was basolateral in relatively large cells of the gill epithelium that were not Na+, K+-ATPase-rich. I proposed that V-H-ATPase-rich cells were sites of Cl-/HCO3- exchange and that Na+, K+-ATPase-rich cells were sites of Na+/H+ exchange. Last, I described the expression of a pendrin-like transporter in the Atlantic stingray gill. Pendrin is a Cl-/HCO3- exchanger that plays an important role in HCO3- excretion in the mammalian kidney. Pendrin immunoreactivity was highest in gills of freshwater stingrays and occurred in the apical region of V-H-ATPase-rich cells. This suggested that V-H-ATPase-rich cells are sites of Cl- uptake and HCO3- excretion in stingray gills.