Enhanced tolerance to narcosis in starved Daphnia magna neonates

Guidelines for ecotoxicity testing with Daphnia magna specify particular feeding protocols during the exposure, yet standardization for preexposure feeding remains ambiguous despite its recognized significance in affecting organismal metabolic capacity and tolerance. This ambiguity may contribute to disparate responses and heightened uncertainty in determining the effect concentrations of test chemicals, particularly those inducing metabolic effects through narcosis. Here, we address this gap through a three-step dose-response experiment with neonates of D. magna subjected to two alternative feeding regimes in the preexposure phase: starved and moderately fed during the first 24 hr after birth. Following this treatment, the daphnids were exposed to narcosis-inducing substances (polycyclic aromatic hydrocarbons; PAHs) for 72 hr before being transferred to clean media with algal food ad libitum for a 48 hr recovery phase. Daphnid survivorship, individual protein content, and body size at the end of each experiment phase-pre-exposure, postexposure and postrecovery-were compared between the treatments. Significant treatment effects were observed, including lower and less variable protein content in the starved daphnids entering the PAH exposure phase, yet higher survivorship and greater recovery potential in these daphnids compared with the fed individuals. Our findings underscore the importance of early-life food access and advocate for mandatory reporting of pre-exposure feeding regimes, particularly when testing substances acting via nonpolar narcosis.