Developmental warming induces severe deformities and mortality in a thermally tolerant fish species.

Climate change poses an unprecedented global threat to wildlife, with projected environmental warming likely to push many species beyond optimum temperature ranges. Understanding thermal limits is therefore critical for risk predictions and conservation management, especially in the context of sensitive life stages such as early development. We investigated the effects of elevated developmental temperatures on the eastern mosquitofish, Gambusia holbrooki. This species is a highly successful invader across temperate to tropical regions, making it an ideal model for understanding potential outcomes for more tolerant species. Wild mosquitofish were collected from tropical waterways and bred under their thermal optimum of 27 °C. Offspring were reared from birth across three temperature treatments: 27 °C – control, 30 °C – a higher temperature within their thermal range, and 33 °C, a temperature designed to test their thermal boundaries. Alarmingly, 84% of the fish exposed to 33 °C developed severe spinal deformities and there was 100% mortality by 135 days old. While spinal deformation was less common at 30 °C, there was significantly reduced survival by 220 days. Our study demonstrates the importance of studying critical life stages, including early development, to understand the thermal sensitivity of wildlife, as even species that are known to have highly robust adults may face detrimental outcomes in climate change scenarios.