Salinity responses of the juvenile penaeid shrimp Penaeus japonicus: II. Free amino acids

The free amino acids (FAA) of juvenile Penaeus japonicus were measured after 48 h of exposure of the shrimps to hypo- and hyperosmotic salinity stress. The concentrations of FAA decreased linearly with decreasing salinity from 50‰ to 10‰. The highest amounts of individual FAA in seawater-adapted P. japonicus are exhibited by glycine (51%), followed by taurine (14%), arginine (10%), proline (9%), and alanine (6%). The main osmoeffectors are glycine, proline, and alanine, which together lower the FAA pool by 80% after a hypoosmotic shock from 40 to 10‰. The concentrations of taurine, arginine, ornithine, and glutamate are not affected by external salinity change. Under hypoosmotic conditions taurine, glutamate, and arginine decrease by only 13–15%, and asparate, ornithine, and lysine by 28–34%. All other amino acids in the FAA pool are reduced by 50 to 88% — the greatest loss is shown by proline (by 96%). Below 25‰ intracellular volume regulation is maintained entirely by proline and glycine, whereas the other FAA concentrations remain more or less constant. After hyperosmotic shock, the concentrations of nearly all FAA increase. This is also true for the essential FAA (+ 86%) — indicating protein hydrolysis. A salinity range between 25 and 40‰ is recommended for the extensive aquaculture of P. japonicus.