When warming hits harder: survival, cellular stress and thermal limits of Sparus aurata larvae under global change

Understanding physiological and molecular compensation mechanisms that shape thermotolerance is crucial for estimating the effects of ocean warming on fish stocks, especially during early life stages, whose tolerance determines recruitment success and population viability. The aims of this study were to assess the sensitivity of fish larvae toward ocean warming and heat wave events in the commercial species, Sparus aurata, whose habitat is likely to be affected by rising water temperatures. We (1) estimated its critical thermal maximum (CTmax) and relative mortality upon warming, (2) quantified stress biomarkers: heat shock protein 70 kDa, total ubiquitin, antioxidant enzymes (superoxide dismutase, catalase, glutathione-S-transferase), lipid peroxidation and protein carbonylation, and (3) analyzed histopathological changes as a result of thermal stress. Larvae showed increasing levels of lethargy with increasing temperature, attaining a cumulative CTmax value of 30 °C. Relative mortality increased upon warming, reaching 80 % at 30 °C. Oxidative damage was higher at moderate temperatures and decreased at 24 °C probably due to a significant increase in superoxide dismutase’s (SODs) activity. Hsp70 chaperone levels also increased at 26 °C, but unfolding persisted at higher temperatures as shown by the increase in total ubiquitin at 26 and 28 °C, indicating protein damage. Skeletal muscle showed disorganization of muscle fibers from 24 °C onwards. Overall, protein denaturation seems to be the major cause of larval mortality, potentially compromising recruitment’s success from 22 °C onwards, since larvae migrate into nursery grounds by spring and summer (i.e., high temperatures), thus hindering the viability of local fish stocks. These data demonstrate that the biochemical homeostasis of fish can be disturbed within an ecologically realistic thermal range and emphasize the risks of rising global temperatures for larval fishes.