Effects of temperature and salinity on the thermal death in postlarval brown shrimp, Penaeus aztecus.
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Laboratory studies were conducted on the effects of temperature and salinity on resistance of postlarval Penaeus aztecus to lethally high temperatures. Tests were conducted on postlarval brown shrimp acclimated at three temperatures (24, 29, and 34 C) at nine lethal temperatures for each acclimation temperature ( 36.0 to 37.6 C; 38.0 to 39.6 C; 38.6 to 40.2 C). The results indicated that the upper incipient lethal temperature for shrimp acclimated at 20.4 C is between 36.6 and 36.8 C. No evidence of the upper incipient lethal temperature for postlarvae acclimated at 29 and 34 C was found in the range of lethal temperatures used. Acclimatin to an increase in temperature by postlarval bronw shrimp was studied, using increases in resistance to a lethal temperature as an indication of increasing acclimation. Acclimation to 34 degrees C, after 8 days at 29 degrees C, took place at a fairly steady rate for the first 3 days after transfer. No further acclimation took place between 3 and 7 days. A 10% decrease in salinity, concurrent with a temperature increase (29 to 34 degrees C), cause a 24 hour increase in acclimation time to a total of 4 days. Postlarval brown shrimp acclimated at the nine possible combinations of three acclimation salinities (5 o/oo, 15 o/oo, and 25 o/oo) for each of two lethal high temperatures. Resistance time increase with increasing acclimation temperatures and decreased with increasing lethal temperatures. Longer resistance times occurred at the higher test salinity (25 o/oo) that at the other two salinities. The lower acclimation salinity (5 o/oo) proved to be a better preparation for resisting lethal high temperature, at all three test salinities, than either of the other two acclimation salinities. These results provide evidence of a new relationship between environmental salinity and the temperature tolerance of an estuarnine organism. A reduction in the amount of work necessary for osmoregulation at 25 o/oo, as compared to the two lower test salinities. Improved thermal resistence at all test salininites after acclimation at 5 o/oo is considered to be an adaptation allowing the postlarvae to resist high temperatures when they are in low salinity bays.