Intertidal triplefin fishes have a lower critical oxygen tension (P crit ), higher maximal aerobic capacity, and higher tissue glycogen stores than their subtidal counterparts.

Decreased oxygen (O ₂ ) availability (hypoxia) is common in rock pools and challenges the aerobic metabolism of fishes living in these habitats. In this study, the critical O ₂ tension (P _crit ), a whole animal measure of the aerobic contribution to hypoxia tolerance, was compared between four New Zealand triplefin fishes including an intertidal specialist (Bellapiscis medius), an occasional intertidal inhabitant (Forsterygion lapillum) and two exclusively subtidal species (F. varium and F. malcolmi). The intertidal species had lower P _crit values than the subtidal species indicating traits to meet resting O ₂ demands at lower O ₂ tensions. While resting O ₂ demand (standard metabolic rate; SMR) did not show a major difference between species, the intertidal species had higher maximal rates of O ₂ consumption ([Formula: see text]) and higher aerobic metabolic scope (MS). The high O ₂ extractive capacity of the intertidal species was associated with increased blood O ₂ carrying capacity (i.e., higher Hb concentration), in addition to higher mass-specific gill surface area and thinner gill secondary lamellae that collectively conveyed a higher capacity for O ₂ flux across the gills. The specialist intertidal species B. medius also had higher glycogen stores in both white muscle and brain tissues, suggesting a greater potential to generate ATP anaerobically and survive in rock pools with O ₂ tensions less than P _crit . Overall, this study shows that the superior P _crit of intertidal triplefin species is not linked to a minimisation of SMR, but is instead associated with an increased O ₂ extractive capacity of the cardiorespiratory system (i.e., [Formula: see text], MS, Hb and gill O ₂ flux).