Ammonia efflux rates and free amino acid levels in Litopenaeus vannamei postlarvae during sudden salinity changes

A laboratory simulation of a salinity test on Litopenaeus vannamei postlarvae was performed [a 30-min exposure to bottled freshwater (1.5 ppt) followed by an abrupt return to seawater (37 ppt) for another 30 min] in order to evaluate physiological responses such as ammonia efflux rates and free amino acid levels. Ammonia efflux rate in the control group (37 ppt; 28 degreesC) was 4.55 +/- 0.25 mumol g(-1) h(-1). Postlarvae exposed to freshwater media had a significantly higher ammonia efflux rate during the first 5 min post-transfer which reduced after 30 min, but was still higher than in the control group. A return to seawater (37 ppt) induced the highest ammonia efflux rate of 82.04 +/- 7.79 mumol g(-1) h(-1) and higher rates persisted throughout the exposure period. Total free amino acids (TFAAs) in whole PL were 2.96 mumol gfw(-1) (gfw(-1) = gram fresh weight), but increased after exposure to both salinity changes, with methionine, serine, valine, phenylalanine, isoleucine, glutamic, lysine, and arginine being several-fold higher after the sudden return to seawater. In the control group, glycine and alanine were the most abundant free amino acids (FAA) with the essential amino acid (EAA) arginine also present in significant amounts. Postlarvae of the white shrimp L. vannamei may be able to resist sudden salinity changes by manipulation of nitrogen metabolism and rapidly increase ammonia efflux rates and increase organic osmolytes such as certain amino acids for osmotic regulation. (C) 2004 Elsevier B.V. All rights reserved.