Effect of dissolved oxygen on the energy balance and survival of Penaeus setiferus juveniles

Penaeus setiferus, the white shrimp from the Gulf of Mexico, is an abundant species in the coastal lagoons and estuaries, where it can experience anox ic conditions. This study was designed with the purpose of measuring the ef fects of dissolved oxygen (DO) on the assimilation of ingested food (AS), b y measuring the respiratory rate (R) and the biomass production of P, setif erus during its growth (P) process (AS = R + P). Postlarvae of this species (PL27: 27 d after the last metamorphic molting) were exposed to 2, 3, 4 an d 5.8 mg l(-1) DO for 50 d. P was obtained from the transformation into ene rgy units (J g(-1) dw d(-1)) of the growth rate. The respiratory rate was d etermined by evaluating oxygen consumption of fasting shrimp and spontaneou s activity (routine metabolism: R-rout) and measuring apparent heat increas e (R-AHI). Throughout the experimental periods, survival was not affected b y DO levels and remained within 77 and 85%. P was constant between 5.4 and 4 mg l(-1) DO but decreased at lower DO levels (p < 0.05). R-rout was affec ted by DO, with the highest levels observed in shrimp exposed at 4 and 5.8 mg l(-1) DO and the lowest at 2 mg l(-1) DO (p < 0.05). The opposite effect was seen in R-AHI, where the highest levels were registered in shrimp kept at 2 mg l(-1) DO and the lowest at 5.8 mg l(-1) (p < 0.05). The reason beh ind a higher R-AHI in shrimp kept at a lower DO level was the result of mor e time being invested in the mechanical and biochemical transformation of t he food, which acts as a metabolical brake. AS was constant between 5.8 and 4 mg l(-1) DO, but decreased with respect to a DO reduction. Accordingly, a critical level was established for AS at 4 mg l(-1) DO, below which AS be comes dependent on DO. In contrast, the amount of assimilated energy direct ed to production (P/AS) increased with respect to DO reduction when the shr imp were exposed to DO levels below 4 mg l(-1). These results show that the side effects produced by low DO levels are generally compensated by an inc rease in production efficiency despite reduced respiratory efficiency.