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Penaeus setiferus, the white shrimp from the Gulf of Mexico, is an abundant species in the coastal lagoons and estuaries, where it can experience anoxic conditions. This study was designed with the purpose of measuring the effects of dissolved oxygen (DO] on the assimilation of ingested food (AS), by measuring the respiratory rate (R) and the biomass production of P. setiferus during its growth (P) process (AS = R + P). Postlarvae of this species (PLz7: 27 d after the last metamorphic molting) were exposed to 2, 3, 4 and 5.8 mg 1-L DO for 50 d. P was obtained from the transformation into energy units (J g-' dw d-l) of the growth rate. The respiratory rate was determined by evaluating oxygen consumption of fasting shrimp and spontaneous activity (routine metabolism: R,,,,) and measuring apparent heat increase (RAHl). Throughout the experimental periods, survival was not affected by DO levels and remained within 77 and 85%. Pwas constant between 5.4 and 4 mg 1-l DO but decreased at lower DO levels (p < 0.05). R,,,, was affected by DO, with the highest levels observed in shrimp exposed at 4 and 5.8 mg 1-' DO and the lowest at 2 mg 1-' DO (p < 0.05). The opposite effect was seen in RAHI, where the highest levels were registered in shrimp kept at 2 mg 1-' DO and the lowest at 5 8 mg 1-' (p < 0.05). The reason behind a higher RA,, in shrimp kept at a lower DO level was the result of more time being invested in the mechanical and biochemical transformation of the food, which acts as a metabolical brake. AS was constant between 5.8 and 4 mg 1-' DO, but decreased with respect to a DO reduction. Accordingly, a critical level was established for ASat 4 mg 1-' DO, below which Asbecomes dependent on DO. In contrast, the amount of assimilated energy directed to production (PIAS) increased with respect to DO reduction when the shrimp were exposed to DO levels below 4 mg 1-l. These results show that the side effects produced by low DO levels are generally compensated by an increase in production efficiency despite reduced respiratory efficiency.

This study was undertaken to determine the effect of salinity and pH on tolerance to low dissolved oxygen concentration in postlarval (PL 12; 18 mg wet weight) and early juvenile (720 mg wet weight) Peneaus setiferus. Lethal levels of dissolved oxygen were determined in animals at four combinations of salinity and pH (36 and 15 ppt salinity and pH 6 and 8). For postlarvae, the 48-h LC50 for dissolved oxygen was 1.27 mgO& at 15 ppt and pHS, and was significantly lower than that obtained in the other combinations of salinity and pH (P < 0.05). For juveniles, the 72-h LC50 was 1.16 mg/L at 15 ppt and pH 8, and was significantly lower than that obtained in the other combinations of salinity and pH (P < 0.05). A significant interaction between salinity and pH was observed. These results suggest that the intensive culture of €? serifems may be conducted at moderate salinity, where shrimp appear to be more tolerant of low concentrations of oxygen than at higher salinities.

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