Temperature-dependent critical oxygen pressures (P_crit) of 4 marine invertebrate species display thermal optima even as metabolic demands increase exponentially.

(A) New closed system respirometry experiments exhibit a minimum of P_crit (mean ± SE) at intermediate temperatures, indicating a thermal optimum in hypoxia tolerance. The species include an oligochaete worm (Tubifex tubifex), a sea urchin (Lytechinus pictus), an anemone (Nematostella vectensis), and a cephalopod (Doryteuthis opalescens). (B) Normalized mass-specific metabolic O₂ demand (mean ± SD) follows a simple Arrhenius relationship in all species and cannot explain the complex patterns observed in P_crit. Metabolic rate data are plotted relative to the rate at the coldest measured temperature within a species. See text for details. The data underlying this figure can be found in S1 Data. Untreated experimental data can be found in https://doi.org/10.6084/m9.figshare.24236257.v1.