Thermal resistance and acclimation rate in young white and brown shrimp, Penaeus setiferus Linn, and Penaeus aztecus Ives.

Penaeus aztecus postlarvae acclimated at three temperatures (24, 29, and 34 C) were tested for thermal resistance at five lethal temperatures for each acclimation temperature (34-38 C; 35-39 C; and 36-40 C). Penaeus setiferus postlarvae acclimated at two temperatures (29 and 34 C) were tested for thermal resistance at six lethal temperatures for each acclimation temperature (35-40 C; 36-41 C). The temperature which caused 50% mortality at 10,000 minutes was between 35 and 36 C for postlarvae acclimated at 24 C, and between 36 and 37 for those acclimated at 29 and 34 C. Twenty-four hour LC50's were 36.3, 37.5, and 38.3 C for Penaeus aztecus postlarvae acclimated at 24, 29, and 34 C, respectively, and 38.3 and 38.9 C for Penaeus setiferus postlarvae acclimated at 29 and 34 C, respectively. Postlarval Penaeus aztecus were more resistant at most lethal temperatures than were 30 mm Penaeus aztecus. Postlarval and 30 mm Penaeus setiferus had similar resistance times, which were greater than those of 50 mm Penaeus setiferus. Most of the acclimation of Penaeus aztecus postlarvae to a 5 C increase (25-30 C) and a 9 C increase (25-34 C) in temperature was completed in 3-4 days. Acclimation to the 5 C increase in temperature continued for 14 to 22 days after the transfer to 30 C. Postlarvae subjected to the 9 C increase tended to overshoot the new acclimation level for 4-6 days after transfer to 34 C. Penaeus aztecus postlarvae completed most of their acclimation to a 5C decrease (29 to 24 C) in temperature in 2 days, but complete acclimation required over 21 days. Penaeus setiferus postlarvae completed acclimation to a 5 C increase (29 to 34 C) in temperature in 1 day, whereas acclimation to an 8 C increase (27 to 35 C) required 2 days. Penaeus setiferus postlarvae completed most of their acclimation to a 5 C decrease (29 to 24 C) in temperature in 3 days, but acclimation was not complete even after 22 days. Penaeus aztecus postlarvae, which enter the bays in the spring, when temperatures are mild, have less thermal resistence and a slower rate of acclimation to a temperature increase than do Penaeus setiferus postlarvae which enter in the summer. Separate groups of Penaeus setiferus and Penaeus aztecus postlarvae were acclimated at the six combinations of two temperatures (29 and 34 C) and three acclimation salinities (25, 35, and 45 ppt), and tested for thermal resistance at each of the six combinations of three test salinities (25, 35, and 45 ppt), and two lethal temperatures. Thermal resistance was greatest at a test salinity of 25 ppt, but an acclimation salinity fo 45 ppt proved to be the best preparation for thermal resistance at all test salinities. Penaeus setiferus postlarvae were acclimated at the six combinations of two temperatures (29 and 34 C) and three salinities (5, 15, and 25 ppt) and tested at the six combinations of three test salinities (5, 15, and 25 ppt) and two lethal temperatures. Thermal resistance was greatest at a test salinity of 25 ppt, but an acclimation salinity of 5 ppt was the best preparation for thermal resistance at all test salinities. A test salinity of 25 ppt was the most favorable for resisting lethal temperatures in all experiments. However, acclimation to a higher or lower salinity gave maximum protection against heat death at that salinity and at all salinities closer to 25 ppt. In both species, 25 ppt is the test salinity closest to the isosmotic salinity (28-30 ppt) reported for larger shrimp (>100 mm). Therefore, thermal resistance appears to be greatest at salinities near the isosmotic salinity of both species. Good thermal resistance at all test salinities after acclimation to hyper- or hypo-osmotic levels may be an adaptation allowing postlarvae of both species to resist high temperatures in hyper- or hyposaline bays.