Projected Warming of the Southern Ocean Disrupts Embryonic Development and Hatch Timing in Antarctic Fish

ABSTRACT Rising ocean temperatures pose significant threats to marine ectotherms. Sensitivity to temperature varies across life stages, with embryos often being less tolerant to thermal perturbation than adults. Antarctic notothenioid fishes evolved to occupy a narrow, cold thermal regime (−2 to +2°C) as the high‐latitude Southern Ocean (SO) cooled to its present icy temperatures, and they are particularly vulnerable to small temperature changes, which makes them sentinel species for assessing climate change impacts. Here, we detail how predicted warming of the SO may affect embryonic development in the Antarctic bullhead notothen, Notothenia coriiceps . Experimental embryos were incubated at +4°C, a temperature projected for the SO within the next 100–200 years under high‐emission climate models, whereas control embryos were incubated at present‐day ambient temperature, ~0°C. Elevated temperature caused a high incidence of embryonic morphological abnormalities, including body‐axis curvature and reduced length. Experimental embryos also developed more rapidly, hatching 68 days earlier than controls (87 vs. 155 days post‐fertilization). Accelerated development disrupted the timing of seasonal hatching, shifting larval emergence into the polar winter when food is scarce. Transcriptomic analyses revealed molecular signatures of hypoxia and disrupted protein folding in near‐hatching embryos, indicating severe cellular stress. Predictive modeling suggested that temperature‐induced developmental disruptions would narrow seasonal reproductive windows, thereby threatening population viability under future climate scenarios. Our findings underscore the vulnerability of Antarctic fish embryos to higher water temperature and highlight the urgent need to understand the consequences of disruption of this important trophic component on ecosystem stability in the SO.