AbstractEnvironmental cues terminating embryonic diapause and a potential regulatory mechanism involving depressed pH were investigated in the encysted embroyos of the brine shrimp, Artemia. Two species, Artemia franciscana and Artemia monica, were used for this comparative study. To determine if alteration of intracellular pH (pHi) can terminate diapause, the pHi of diapause cysts was manipulated by exposure to NH3 and CO2. Alkalinization did not result in activation of cysts from either species but acidification activated A. franciscana cysts. A. monica did not respond to acidification. The pHi of aerobic diapausing embryos was determined using 31P nuclear magnetic resonance spectroscopy. During diapause, embryos from both species had pHi's similar to those reported for activated (nondiapause) embryos, indicating that diapause is not imposed by depressed pHi, as is the case during suppression of metabolism under anaerobic conditions. Diapause embryos were exposed to varying conditions of temperature, salinity, and oxygen to determine what conditions are necessary to break diapause. A. franciscana cysts were activated by cold temperature or salinites above 2.0 M NaCl, while A. monica cysts only responded to cold temperature. The availability of oxygen did not influence either population's ability to terminate diapause. The results are discussed with respect to each population's environment and potential mechanisms of diapause regulation.