Critical windows in embryonic development: Shifting incubation temperatures alter heart rate and oxygen consumption of Lake Whitefish (Coregonus clupeaformis) embryos and hatchlings

Critical windows are periods of developmental susceptibility when the phenotype of an embryonic, juvenile or adult animal may be vulnerable to environmental fluctuations. Temperature has pervasive effects on poikilotherm physiology, and embryos are especially vulnerable to temperature shifts. To identify critical windows, we incubated whitefish embryos at control temperatures of 2 °C, 5 °C, or 8 °C, and shifted treatments among temperatures at the end of gastrulation or organogenesis. Heart rate (fH) and oxygen consumption (V˙O2) were measured across embryonic development, and V˙O2 was measured in 1-day old hatchlings. Thermal shifts, up or down, from initial incubation temperatures caused persistent changes in fH and V˙O2 compared to control embryos measured at the same temperature (2 °C, 5 °C, or 8 °C). Most prominently, when embryos were measured at organogenesis, shifting incubation temperature after gastrulation significantly lowered V˙O2 or fH. Incubation at 2 °C or 5 °C through gastrulation significantly lowered V˙O2 (42% decrease) and fH (20% decrease) at 8 °C, incubation at 2 °C significantly lowered V˙O2 (40% decrease) and fH (30% decrease) at 5 °C, and incubation at 5 °C and 8 °C significantly lowered V˙O2 at 2 °C (27% decrease). Through the latter half of development, V˙O2 and fH in embryos were not different from control values for thermally shifted treatments. However, in hatchlings measured at 2 °C, V˙O2 was higher in groups incubated at 5 °C or 8 °C through organogenesis, compared to 2 °C controls (43 or 65% increase, respectively). Collectively, these data suggest that embryonic development through organogenesis represents a critical window of embryonic and hatchling phenotypic plasticity. This study presents an experimental design that identified thermally sensitive periods for fish embryos.