Temperature affects all living organisms on earth – a suitable temperature allows them to grow and thrive, a lack of it can slow-down their development or even cause them to die. Especially embryos of ectotherm animals are susceptible to changes in temperature, not only because ectotherms regulate their body temperature by exchanging heat with the environment but also because embryos, being inherently passive, have little potential to cope with environmental challenges. While adult stages of ectotherms can use different physiological and behavioral strategies to maintain optimal temperatures, embryos cannot making them particularly vulnerable to environmental stress. Furthermore, any disturbance during the embryonic phase can cascade into later developmental stages.

Consequently, the way embryos cope with ambient temperature critically determines the persistence of populations or species. As climate change dramatically affects temperature patterns around the globe, investigating the most sensitive phase of animal development is crucial for predicting how species will respond to and cope with future climate change scenarios. This is especially important for freshwater animals and their ecosystems which, despite their high ecological, economic, and cultural importance, face an unprecedented biodiversity crisis that is expected to escalate even further with the planet’s rising temperatures.