Effects of temperature and salinity on thermal death in postlarval brown shrimp, Penaeus aztecus.

Postlarval brown shrimp acclimated at three temperatures (24.0, 29.0, and 34.0 C) were exposed to nine lethal temperatures for each acclimation temperature (36.0-37.6 C, 38.0-39.6 C, 38.6-40.2 C). Postlarval brown shrimp acclimated at the nine possible combinations of three acclimation temperatures (24.0, 29.0 , and 34.0 C) and three acclimation salinities (%5, 15%, and 25%) were tested for thermal resistance time at three test salinities ( 5%, 15%, and 25%) for each of two lethal high temperatures. Resistance time increased with increasing acclimation temperatures and decreased with increasing lethal temperature. Longer resistance times occurred at the higher test salinity (25%) than at the other two salinities. The lower acclimation salinity (5%) proved to be a better preparation for resisting lethally high temperatures, 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 estuarine organism. A reduction in the amount of work necessary for osmoregulation at 25% as compared with the two lower test salinities is thought to explain the higher thermal resistance at higher test salinities. Improved thermal resistance at all test salinities after acclimation at 5% is considered to be an adaption allowing the postlarvae to resist high temperatures when they are in low-salinity bays.