Increased heart rate and blood oxygen extraction are key to the ability of schoolmaster snapper ( <i>Lutjanus apodus</i> ) to swim maximally at high temperatures

Consistent with data on other tropical fish species: we previously showed that schoolmaster snapper (Lutjanus apodus) exhibited a maximum metabolic rate (MMR) during a critical thermal maximum while swimming (CTSMax) test that was 60% higher than during a standard critical thermal maximum (CTMax) test; and based on this data suggested that this elevated MMR contributed to swimming fish fatiguing at a temperature only 1.4oC lower than their CTMax (37.5 vs. 38.9 °C, respectively). However, the mechanism(s) underlying the greater MMR and realistic aerobic scope (ASR) in fish given a CTSMax test remained unclear. To answer this question, CTMax, Ucrit (critical swimming speed) and CTSMax tests were performed on snapper surgically implanted with Transonic® blood flow probes, and we measured heart rate (fH), stroke volume (SV), cardiac output (Q˙), oxygen consumption (ṀO₂) and blood oxygen extraction (EO₂; ṀO₂ / Q˙). The ∼1.75-fold higher MMR observed in CTSMax- vs. CTMax-tested snapper was not explained by a greater contribution of SV to Q˙. Instead, the higher MMR was primarily driven by a∼40 beats min⁻¹ (25%) higher maximum fH and a ∼1.55-fold greater increase in EO₂. This elevated EO₂ may reflect enhanced oxygen uptake at the gills (i.e., higher arterial oxygen content) due to the fish engaging in ram ventilation while swimming at elevated temperatures and/or improved tissue oxygen extraction. These findings highlight the critical role of EO₂ in setting tolerance limits under combined thermal and activity-related stressors, and caution against relying solely on fH variables (i.e., fHMax) as an indicator of thermal tolerance.