Oxygen limitation is not a major physiological mechanism restricting early life development in zebrafish

Early life stages are considered particularly vulnerable to warming because tissue oxygen supply is thought to become limiting, given their underdeveloped gill function and reliance on passive oxygen diffusion. Here, we tested whether oxygen availability constrains early development under warming in zebrafish (Danio rerio). We exposed embryos and early-stage larvae to a high-resolution factorial design spanning 50 combinations of temperature and oxygen levels, and quantified multiple developmental and physiological responses (including growth- and survival-related performance) as well as carry-over effects on juvenile warming tolerance. Across traits, embryonic and larval performance was less restricted by oxygen availability than expected. Moderate hypoxia did not impair performance across a wide thermal range, while hyperoxia did not rescue performance under warming, indicating that thermal failure was not alleviated by additional oxygen. Developmental failure occurred primarily under the combined effects of severe hypoxia and extreme warming. Severe hypoxia also induced developmental slowing and premature hatching, especially near thermal extremes. Juvenile warming tolerance was reduced by severe hypoxia and extreme developmental temperatures, but the small effect sizes indicate limited carryover effects of developmental plasticity. Together, these findings do not support oxygen limitation as a primary mechanism limiting early-life performance under warming, refining mechanistic expectations of how warming constrains fish performance.