Temperature and the progression of developmental milestones in embryogenesis of a marine ectotherm.

Embryos are among the most temperature-sensitive life stages. To survive and produce juvenile stages, embryos must be robust to changes in temperature that also change development time profoundly. Yet, how robustness is achieved during embryogenesis, and which developmental events are most prone to perturbation by temperature, is only known for a handful of species. Such insights are especially lacking for marine ectotherms, which often develop in direct contact with the external environment. We address these gaps using the tubeworm, Galeolaria caespitosa, a typical marine ectotherm with external development. We fluorescently labelled F-actin and nuclear DNA in embryos sampled hourly throughout embryogenesis at the minimum temperature of the coldest month (11°C), annual mean temperature (17°C) and maximum temperature of the warmest month (22°C) in nature. Based on confocal imaging, we identified key developmental stages (milestones) in embryogenesis and compared their progression across temperatures. We found that developmental progression is similar across temperatures when normalized to development time, but earlier milestones are less robust to warming than later ones. Our results suggest that embryos achieve robustness by tightly coordinating the relative timing of embryonic events, offering clues to how embryos may withstand contemporary climate change in marine systems.