![]() ![]() Gene expression of gill and gut urea transporters were transiently reduced as were gill rhesus glycoprotein Rhbg and Rhcg-2. Additionally, fish relied on carbohydrates with lowered muscle glycogen levels, but there were no signs indicating use of lipid stores. Their nitrogen use switched to endogenous sources as liver and muscle protein levels decreased after a 5-day fast, indicating proteolysis. Urea-N excretion remained stable despite the lack of their N-rich food source. Surprisingly, during a 5-day fasting period, routine metabolic rates increased and swimming performance (critical swimming speed) was not affected. Therefore, we hypothesized that during food deprivation, Magadi tilapia would economize their energy expenditure and reduce metabolic rate, aerobic performance and urea-N excretion. Their exceptionally high aerobic metabolic rate, together with their emaciated appearance, suggests that they are energy-limited. These Lake Magadi tilapia largely depend on nitrogen-rich cyanobacteria for food and are 100% ureotelic. Recently, increased water demand and siltation have threatened the viable hot springs near the margins of the lake where Alcolapia grahami, the only fish surviving in the lake, live. Lake Magadi, Kenya, is one of the most extreme aquatic environments on Earth (pH~10, anoxic to hyperoxic, high temperatures). Urea thereby played a small but significant role (up to 8% of internal osmolality) in osmoregulation. Urea levels accounted for only 2≣% of internal osmolality in 100% lake water but responded to a greater extent than plasma ions during exposure to 10 and 200% lake water, decreasing by 28≤2% in the former and increasing by over 500% in the latter relative to simultaneous-control values. However, the fish survived well during gradual adaptation to 200% lake water, although plasma cortisol remained chronically elevated. Significant mortality and elevated cortisol occurred after acute transfer to 200% lake water. Curiously, drinking continued at the same rate in fish adapted to 1% lake water, but chloride cells were now exclusively of the freshwater type. Plasma osmolytes were here maintained at levels typical of freshwater teleosts. All fish succumbed upon acute transfer to 1% lake water, but tolerated acute transfer to 10% lake water well, and gradual long-term acclimation to both 10 and 1% lake water without change in plasma cortisol. Whole-body Na+ and Cl concentrations were relatively high and exhibited larger relative changes in response to salinity transfers than did plasma ions. Gill chloride cells were predominantly of the sea water type (recessed, with apical pits) but a few freshwater-type chloride cells (surficial, with flat apical exposure) were also present. In 100% lake water, the Magadi tilapia maintained plasma Na+, Cl, and osmolality at levels typical of marine teleosts and drank the medium at 8.01 ± 1.29 mL The unique ureotelic tilapia Alcolapia grahami lives in the highly alkaline and saline waters of Lake Magadi, Kenya (pH ~10.0, alkalinity ~380 mmol ![]()
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