Physiological Plasticity and Tolerance of Fundulid Killifish Species to Aquatic Hypoxia

Aquatic hypoxia is a naturally occurring phenomenon in waters subject to high primary production, eutrophication, and salinity‐derived stratification. Fish utilize a suite of physiological responses in order to cope with aquatic hypoxia. Species that regularly experience repeated bouts of hypoxia may rely more heavily on compensatory responses in order to survive. We hypothesized that species from hypoxia‐prone environments such as estuaries are more tolerant to hypoxia than species living in stable environments. We also hypothesized that species from variable environments would show a greater scope for phenotypic plasticity in their physiological acclimation responses to hypoxia. Fundulid killifish species from different ecological niches (marine‐tolerant Lucania parva and Fundulus confluentus and freshwater‐tolerant L. goodei and F. rathbuni ) were acclimated for 21–28 d to either constant hypoxia or diurnal cycles of intermittent hypoxia (12 h normoxia: 12 h hypoxia). We then assessed the resting O 2 consumption rates and hypoxia tolerance (critical O 2 tension, O 2 tension at loss of equilibrium, and time to loss of equilibrium at 0.6 kPa O 2 ). Our results show that F. confluentus , an estuarine killifish, demonstrated much greater hypoxia tolerance than F. rathbuni, which inhabits freshwater rivers and creeks. Also, the sister taxa, L ucania parva and L. goodei demonstrated very similar baseline hypoxia tolerance. Acclimation to intermittent or constant hypoxia often (but not always) significantly improved hypoxia tolerance, but there were no clear patterns of variation in this plasticity between species from different environments. Our data suggest that plasticity can vary between even closely related species. Future work will compare the unique physiological strategies of each species for coping with hypoxia. Support or Funding Information We would like to thank the American Physiological Society's Undergraduate Summer Research Fellowship program, Louisiana Sea Grant, Louisiana State University, McMaster University, the Society of Experimental Biology, the National Science Foundation, and the Natural Sciences and Engineering Research Council of Canada for funding and support.