Acclimation to fluctuating hypoxia alters activity and escape performance, but not metabolism, in guppies

Elise Doddema, Malin Fløysand, Andrea Campos-Candela, Beatriz Diaz Pauli, Rachael Morgan 

Organisms living in fluctuating environments must cope with constantly changing conditions. Here we investigated how acclimation to either fluctuating or constant oxygen affects behavioural and physiological responses to hypoxia in guppies (Poecilia reticulata). Guppies were acclimated to either fluctuating hypoxia (100% of air saturation during day to 40% at night) or constant normoxia (100% of air saturation). Their activity, escape response, and metabolic rates (standard and maximum) were quantified under three oxygen exposure scenarios: (i) normoxia: 100%, (ii) acute hypoxia: 40%, and (iii) reoxygenation: 100% after experiencing 40% of air saturation. We observed that activity decreased under acute hypoxia, but to a lesser extent in the fish acclimated to fluctuating hypoxia. Acclimation also influenced the responses after reoxygenation as normoxia-acclimated guppies fully recovered their activity levels after hypoxia, whereas the guppies acclimated to fluctuating hypoxia only partially recovered. Escape response also differed between acclimation groups, with normoxia-acclimated guppies showing decreased responsiveness during and after acute hypoxia, whereas fluctuating hypoxia-acclimated guppies were most responsive during acute hypoxia but showed similar decreased responsiveness after reoxygenation. Metabolic rates decreased under acute hypoxia, with sex-specific effects on aerobic scope, whereas acclimation had no significant effect on metabolism. Our results demonstrate that the effects of acclimation to fluctuating hypoxia on whole-organism performance in guppies are scenario- and trait-specific, but that overall, an acute exposure to hypoxia had stronger effects on performance than acclimation. Organism performance after exposure to acute hypoxia can also differ depending on the periodicity and recurrence of hypoxia. We highlight the reoxygenation period following acute hypoxia as a critical period that deserves more research to get a better understanding of the overall impact that fluctuating hypoxia has on organism performance.