Effect of growth hormone and g-aminobutyric acid on Brachionus plicatilis (Rotifera) reproduction at low food or high ammonia levels

21 Growth hormone (GH, 0.0025 and 0.025 I.U. ml ) and g-aminobutyric acid (GABA, 50 mg 21 ml ) enhance rotifer population growth in batch cultures. In order to further understand the mechanism of their actions, we conducted experiments culturing isolated females at low food and 6 high free ammonia levels. At an optimum food level of 7 3 10 Nannochloropsis oculata cells 21 21 ml or at low free ammonia level of 2.4 mg ml , the F offspring of rotifers treated with GH at 1 21 0.0025 I.U. ml had significantly higher population growth rate (r) and net reproduction rate 5 (Ro), and shorter generation time than untreated rotifers. At a lower food level of 7 3 10 21 21 cells ml or at high free ammonia level of 3.1 mg ml , rotifers treated with GABA at 50 mg 21 ml had significantly higher r and Ro, and shorter generation time. These results indicate that GABA is effective in enhancing rotifer reproduction when rotifers are cultured under stress whereas GH enhances rotifer reproduction when culture conditions are optimal. Significant effects 1 2 were also observed in F and F generations which were not treated with hormones. These data may be useful for treating rotifer mass cultures to mitigate the effects of stress caused by high population densities.  1999 Elsevier Science B.V. All rights reserved.