Heatwaves during embryonic development reveal duration-dependent effects in zebrafish

Heatwaves, defined as periods of unusually high temperatures are increasingly frequent due to climate change. It is well known that ectotherms are particularly vulnerable to heat stress, especially during sensitive life stages such as embryonic development. However, the consequences of short, ecologically relevant heat stress, remain less investigated. Here, we investigate the effects of short-term heat stress during embryonic development in zebrafish (Danio rerio). For this, 1753 embryos were exposed to simulated heatwaves of different durations (6, 24, or 48 h) and frequencies (one or two events) during development, with a temperature of 35 °C (+7 °C above the standard temperature of 28 °C). Their pre- and post-natal survival, hatching time, larval body size, and locomotor activity once hatched were evaluated. Additionally, the larval locomotor activity was assessed at control and heat-stress temperatures to determine if embryonic heat stress would result in larvae coping better with high temperatures. Our results showed that exposure to high temperatures (35 °C) increased pre-natal mortality, and further effects varied depending on the duration of the heatwave. Longer heatwaves (48 h) accelerated the rate of development but resulted in smaller larvae that swam more under heat stress, while short heatwaves (24 h and two repeated periods of 6 h) did not have any effect. Our results indicate that the embryonic stage is highly sensitive to even brief periods of heat stress, such as those experienced during a heatwave, and that the duration of exposure leads to different responses. These findings offer crucial insights into the extreme sensitivity of embryos to heatwaves and highlight wider consequences for individuals and populations, which are crucial for predicting how species will respond to ongoing climate change.