Thermal and Halo Tolerance of a Brackish Cladoceran Daphniopsis australis (Sergeev & Williams)

The thermal and halo tolerances of Daphniopsis australis were investigated with individuals from a single clone under laboratory conditions. Measurements of acute tolerance were used to estimate the range of tolerance and indicate the optimum range of survival. Acute tolerance was compared between adult and juvenile over time. In both temperature and salinity trials, the upper limit of tolerance was successfully detected, but this limit was not significantly different between life stages or over exposure times. A minimum temperature increment of 6.4oC above the control temperature (22oC) was detrimental to survival. Similarly, a detrimental effect was observed when salinity increased by 11 PSU above the control salinity (22 PSU). In contrast, animals seem to be more resistant to lower temperature and lower salinity. While the lower limit was found at salinity of 0.2-0.3 PSU in juveniles, this study did not reach the lower limit of temperature and salinity in other studies. Likely, the thermal tolerance of this species is approximately from 4 to 28.4oC while the halo tolerance is estimated not more than 33 PSU. The optimal temperature for survival falls in the range of 4-28oC whereas the optimal salinity occurs from 5 to 30 PSU. In general, this study shows that D. australis is a eurythermal and euryhaline species, but a rapid increase of temperature could threaten its survival. This study contributes to the understanding of the impact of global warming and increasing salinity on the biodiversity of zooplankton fauna in arid regions.