Running on empty: locomotor compensation preserves fish schooling under hypoxia and informs principles for...

Environmental stressors such as hypoxia challenge the balance between individual physiological performance and the coordination required for collective behaviors like schooling. Here, we investigate how glass catfish (Kryptopterus vitreolus) modulate locomotor and group-level behavior across a gradient of oxygen saturation (95%-20%) while swimming steadily at a constant cruising speed. We found that tailbeat frequency decreased significantly with declining oxygen (p < 0.0001), alongside reductions in wave speed (p = 0.007). Tailbeat amplitude, by contrast, increased significantly under hypoxia (p < 0.0001), and posterior segment angles showed a slight, non-significant increase, consistent with modestly greater tail bending. Despite these changes, the Strouhal number remained fairly constant, and waveform topology was conserved. School structure, including nearest-neighbor distance and distance to the center of the school, remained stable across oxygen treatments, but with significant variation across individual schools. A clear behavioral threshold was observed below 25% oxygen saturation, beyond which coordinated schooling deteriorated. These findings demonstrate that glass catfish employ internally coordinated, energetically economical kinematic adjustments to preserve group cohesion under metabolic constraint. This strategy highlights a decentralized mechanism for sustaining collective behavior near physiological limits and offers biologically-grounded insights relevant to energy-aware coordination in bioinspired swarms.