Cellular stress response and acclimation capacity of the ditch shrimp Palaemon varians to extreme weather events - How plastic can a plastic species be?

Species from shallow marine environments are particularly vulnerable to extreme weather events (heatwaves and extreme rainfall) that can promote abrupt environmental shifts, namely in temperature and salinity (respectively). To assess how these shifts impact species' cellular stress responses (CSR), ditch shrimps Palaemon varians were exposed to a chronic (28 days) thermohaline stress experiment. Three levels of temperature (20, 23 and 26 °C) and two levels of salinity (20 and 40) were tested in a full factorial experiment, and shrimps sampled at the 7th, 14th, 21st and 28th day of exposure. Survival, wet weight, and cellular stress biomarkers associated with oxidative stress (LPO - Lipid Peroxidation, GST - Glutathione-S-Transferase, SOD - Superoxide Dismutase, TAC - Total Antioxidant Capacity and CAT - Catalase) and protein denaturation (UBI - Ubiquitin and HSP-70 - Heat Shock Protein 70 kDa) were analysed in shrimps' muscle at each sampling day. Temperature and time of exposure significantly affected biomarkers levels, with shrimps exposed to 20 and 26 °C revealing more pronounced differences. No interactions were detected between temperature and salinity, suggesting that these factors display additive effects on shrimps' CSR. Antioxidant agents (CAT and TAC) increased under elevated temperature, while protein denaturation markers (UBI and HSP-70) were mostly affected by time of exposure, decreasing at 28 days. Total protein reserves increased throughout time and no effects on growth were observed. A negative correlation between wet weight and HSP-70 was detected, suggesting that HSP-70 levels are dependent on organism size. Peak survival (~73 %) was found under 20 °C and salinity 40 and lower survival (~30-40 %) was associated with higher temperatures (23 and 26 °C) and lower salinity (20). We conclude that P. varians displays some level of acclimation capacity but differences in survival may indicate effects on osmoregulation processes and the need for longer timeframes to fully acclimate to heat and hyposaline stress.