Hypoxia tolerance of intertidal triplefin fish is associated with low critical oxygen tension and high phosphorylating capacity in brain mitochondria.

At the terminus of the O2 cascade, mitochondria play an important role in O2 utilisation and energy conservation, with adaptive modifications occasionally shared among hypoxia-tolerant species. Here, we sought to determine whether mitochondrial adaptations in brain tissue explain the hypoxia tolerance of New Zealand triplefin fishes (Tripterygiidae). We compared two intertidal species (Bellapiscis medius and Forsterygion lapillum), both likely adapted to hypoxia-reoxygenation exposures, and two subtidal species (F. varium and F. malcomi), which inhabit normoxic waters. To assess hypoxia tolerance, we determined loss of equilibrium (LOE) during hypoxia exposure and measured the critical O2 tension (Pcrit). Intertidal species displayed superior hypoxia tolerance as assessed by LOE and also had lower Pcrit (LOE versus Pcrit R2 = 0.99). High-resolution respirometry was used to measure mitochondrial respiration in homogenate and permeabilised fragments of brain. While a weak relationship was apparent between mitochondrial O2 binding affinity (mP50) and LOE, maximum phosphorylating O2 consumption (OxPhos) and O2 catalytic rates were strongly correlated with hypoxia tolerance. Although cytochrome-c-oxidase activity was highest in the most hypoxia-tolerant species B. medius, it was only weakly correlated with hypoxia tolerance across species. Notably, the high OxPhos capacity of intertidal species was not associated with higher whole animal resting O2 consumption, suggesting intertidal species maintain high capacity for ATP production without incurring increased basal energetic costs. While somewhat paradoxical, the low Pcrit/high OxPhos strategy of intertidal species may provide flexibility in the dynamic intertidal environment where short, severe periods of hypoxia are interspersed with high energy demand environmental conditions, including acute warming.