An ancient oxidase lost in vertebrates promotes extreme stress tolerance in an emerging cnidarian model for ecology, evolution and biomedicine.

Aerobic respiration underpins animal performance, yet the mitochondrial electron transport system is often treated as a single, vertebrate-centric blueprint. This view overlooks alternative oxidase (AOX), an enzyme that allows electrons to bypass complexes III and IV, partially uncoupling oxidative phosphorylation. Although AOX is well studied in plants for its role in stress tolerance, its presence in animals was recognized only recently, leaving its contribution to metabolic flexibility underappreciated. Here, we used an emerging ecological-evolutionary-developmental biology (eco-evo-devo) and biomedical model, Nematostella vectensis, to test the hypothesis that AOX supports stress tolerance by bypassing complex IV (cytochrome c oxidase; COX) during hydrogen sulphide (H2S) exposure and by mitigating oxidative damage under hypoxia and heat stress via reduced reactive oxygen species production. We found that anemones upregulated AOX protein expression after H2S exposure and exhibited cyanide-resistant respiration, consistent with continued electron flow despite COX inhibition. Behavioural assays showed that AOX inhibition increased sensitivity to H2S, declining oxygen and heat, whereas biochemical assays revealed that AOX inhibition led to elevated lipid peroxidation and protein carbonylation with hypoxia and heat exposure. Together, these results establish AOX as a critical yet overlooked mechanism of metabolic flexibility that buffers aerobic metabolism against multiple stressors, challenging textbook portrayals of conserved mitochondrial function and offering new perspectives on how animals persist in a rapidly changing world.