LAUSR

The capacity of fishes to tolerate low oxygen (hypoxia) through behavioral and physiological adjustments varies among species in a fashion that correlates with oxygen availability in their natural habitats. Less is known about variation in hypoxia tolerance within a species, but it is expressly this interindividual variation that will determine which individuals will survive during severe hypoxia. Here, we measured aquatic surface respiration (ASR) and loss of equilibrium (LOE), two common indexes of hypoxia tolerance of fishes, in gulf killifish, Fundulus grandis, subjected to multiple trials of a highly reproducible hypoxia protocol over a period of 6–8 wk. The time and PO2 at the first occurrence of ASR and the time and PO2 at LOE differed significantly among individuals in a repeatable fashion. This interindividual variation in ASR and LOE was significantly correlated with general body and gill morphology. The time to ASR was shorter and the PO2 at ASR was higher for fish with greater mass. After correcting for mass, fish with longer or more numerous gill filaments had longer times to ASR or LOE, respectively. Fish in better condition factor (heavier for their length) had lower PO2 at LOE. Repeatable interindividual variation in hypoxia tolerance, if genetically based, could influence the capacity of species to adapt as their habitats become increasingly threatened by aquatic hypoxia.