THE ADAPTATIONS OF FISH TO EXTREMELY ALKALINE ENVIRONMENTS

The Lake Magadi Tilapia (MT; Oreochromis alcalicus grahami, the Lahont an cutthroat trout (LCT; Oncorhynchus clarki henshawi) and the tarek ( Ct; Chacalburnus tarichi) have evolved unique strategies that allow th em to overcome problems associated with ammonia excretion (J(Amm)) and acid-base regulation in their alkaline environments. In Lake Magadi, Kenya (pH 10), the MT circumvents problems associated with J(Amm) by e xcreting virtually all (>90%) of its waste-nitrogen as urea. Base excr etion appears to be facilitated by modified seawater-type gill chlorid e cells, through apical Cl-/HCO3- exchangers and an outwardly directed OH-/HCO3-/CO3= excretion system. The LCT avoids potentially toxic inc reases in internal ammonia by permanently lowering ammonia production rates following transfer into alkaline (pH 9.4) Pyramid Lake, Nevada, from its juvenile freshwater (pH 8.4) environment. Greater apical expo sure of LCT gill chloride cells, presumably the freshwater variety, pr obably facilitates base excretion by elevating Cl-/HCO3- exchange capa city. In Lake Van, Turkey (pH 9.8) high ammonia tolerance enables C. t arichi to withstand the high internal ammonia concentrations that it a pparently requires for the facilitation of J(Amm) It also excretes unu sually high amounts of urea. We conclude that adjustments to nitrogeno us waste metabolism and excretion patterns, as well as modifications t o gill functional morphology, are necessary adaptations that permit th ese animals to thrive in environments considered unsuitable for most f ishes.