KUNKEL-PATTERSON, A.**; FIESS, J.C.; MATHIAS, L.; SAMEROTTE, A.; HIRANO, T.; YANCEY, P.H.; Whitman College, Walla Walla WA; Univ. of Hawai'i, Kaneohe; Univ. of Hawai'i, Kaneohe; Whitman College, Walla Walla WA; Univ. of Hawai'i, Kaneohe; Whitman College, Walla Walla WA: Effects of salinity and temperature on osmolyte composition in tilapia

Some tilapia species can survive in salinities from freshwater to three times that of seawater, and over a wide temperature range. As osmoregulators, their blood osmotic pressure changes less than that of the environment, but does increase at high salinities, potentially resulting in either cell shrinkage or osmolyte acummulation. Organic osmolytes increase in brain and kidney cells in dehydrated mammals, but have received little study in bony fish. We examined the effects of salinity, and also temperature, on the osmolyte composition of plasma and 4 organs of Oreochromis mossambicus tilapia. Fish were acclimated to 9 combinations of salinity and temperature: freshwater, seawater , and 2X seawater (2000 mOsm), and 20, 28, and 35C. Brain, liver, white muscle and kidney from 5-6 fish from each condition were analyzed for osmolytes. Plasma osmotic pressure was 320 mOsm in freshwater, and increased in 2X seawater (due to NaCl) to 370 at 20°C and 410 mOsm at 35°C. Increases in organ organic osmolytes were often intensified by higher temperatures, especially in brain and liver. In brain, myo-inositol increased from 1-3 in freshwater to 21-36 mmol/kg in 2X seawater. In kidney, myo-inositol increased from 5-13 in freshwater to 21-25 mmol/kg in 2X seawater. White muscle had only about 20% increases in the sum of taurine plus glycine between freshwater and seawater with no increases in 2x seawater. Liver similarly had only small increases in taurine, from 20-26 in freshwater to 25-35 mmol/kg in 2X seawater. Possibly muscle and liver are able to tolerate cell shrinkage better than brain or kidney, as previously found in mammals. Thus, organic osmolytes may help prevent cell shrinkage in some organs of tilapia in high salinities.