The effect of different levels of dissolved oxygen (DO) and a decreased temperature combined with severe hypoxia were studied in the blue shrimp Litopenaeus stylirostris in relation to molt stage. In the first experiment, shrimps were submitted to DO concentrations of 5 to 1 mg l(-1). Osmoregulatory capacity (OC) was measured in shrimps at molt stage C after 6, 24, or 48 h of exposure. No mortality was recorded but a significant negative effect of DO concentration on hypo-OC was observed for DO concentrations below 3 mg l(-1). Osmoregulation improved after 48 h exposure, compared to 6 and 24 h exposure.
In a second experiment, temperature was decreased from 28 to 22 degreesC in 24 h, before a severe hypoxia of 0.5+/-0.1 mg O-2 l(-1) was applied. Mortality and OC were studied in relation to molt stages. Half of the shrimps died after respectively 100 min at 0.5 mg O-2 l(-1) at 28 degreesC and 153 min at 0.4 mg O-2 l(-1) at 22degreesC. Mortality was significantly higher in stages D-2 and B compared to stages C and Do. Control shrimps in stage D2 had a lower hypo-OC than shrimps in stages C and Do both at 28 and 22 degreesC. Temperature decrease had no effect on hypo-OC. Hypoxia reduced hypo-OC for all stages, whatever the temperature, but the effect was greater in stage C at 27 degreesC (68% decrease) compared to 22 degreesC (49%) and stage D-2 at both temperatures (respectively, 43% and 58% at 27 and 22 degreesC). Combination of temperature and hypoxia had a significant effect on hypo-OC. Low temperature reduced the effect of hypoxia presumably by slowing down the metabolism. Results are discussed in relation to pond observation. (C) 2004 Elsevier B.V. All rights reserved.
