THE HOURS THAT MATTER: UNMASKING HIDDEN VULNERABILITY WINDOWS IN ZEBRAFISH (DANIO RERIO) EMBRYOS

Sammanfattning

Extreme heat events are increasing in frequency, intensity, and duration, with likely severe consequences for the survival of fish populations under climate change. However, climate vulnerability assessments often treat life stages as uniform units, overlooking variation in heat tolerance both among and within life stages. This is concerning because embryonic development involves major physiological and morphological changes that are likely to affect survival under heat stress. Thus, ignoring this variation may mask important windows of vulnerability to environmental change. In this study I aimed to measure acute heat tolerance in zebrafish (Danio rerio) embryos across six distinct developmental stages (2, 6, 25, 30, 49, and 54 hours post-fertilization, hpf). I found non-monotonic changes in heat tolerance throughout embryonic development, characterised by reduced heat tolerance at early cleavage (2 hpf), a subsequent peak in heat tolerance during mid-embryogenesis (25–30 hpf), and a secondary decline in resilience during gastrulation (6 hpf) and prior to hatching (49–54 hpf). However, the rate of which thermal injury accumulates with temperature was constant across all developmental stages. In other words, life stages differed primarily in the thermal load they can accumulate before reaching lethal thresholds, rather than the dynamics of thermal injury accumulation. These results suggest that some cellular and physiological mechanisms leading to the accumulation of heat injury may be consistent across life stages, although the exact mechanism triggering heat failure may vary through embryogenesis. Finally, this study shows that coarse life-stage categorisations used in climate vulnerability assessments can sometimes overestimate species resilience or entirely miss highly sensitive windows of increased sensitivity. High-resolution knowledge of heat tolerance both within and across life stages is thus needed for developing biologically realistic models capable of predicting the resilience of fish populations to extreme heat events.