Rapid and reversible modulation of blood haemoglobin content during diel cycles of hypoxia in killifish (Fundulus heteroclitus).

We investigated whether fish can make dynamic haematological adjustments to support aerobic metabolism during repeated cycles of hypoxia-reoxygenation. Killifish were acclimated to normoxia, constant hypoxia (2 kPa O), or intermittent cycles of nocturnal hypoxia (12 h of normoxia: 12 h of 2 kPa O hypoxia) for 28 days. Normoxia-acclimated fish were sampled in the daytime in normoxia and after exposure to a single bout of nocturnal hypoxia. Each hypoxia acclimation group were sampled at the PO experienced during acclimation during both the day and night. All acclimation groups had increased blood haemoglobin content and haematocrit and reduced spleen mass during nocturnal hypoxia compared to normoxic controls. Blood haemoglobin content was negatively correlated with spleen mass at both the individual and group level. Fish acclimated to intermittent hypoxia rapidly reversed these changes during diurnal reoxygenation. The concentrations of haemoglobin, ATP, and GTP within erythrocytes did not vary substantially between groups. We also measured resting O consumption rate (MO) and maximum MO (induced by an exhaustive chase) in hypoxia in each acclimation group. Fish acclimated to intermittent hypoxia maintained higher resting MO than other groups in hypoxia, comparable to the resting MO of normoxia-acclimated controls measured in normoxia. Differences in resting MO in hypoxia did not result from variation in O transport capacity, because maximal MO in hypoxia always exceeded resting MO. Therefore, reversible modulation of blood haemoglobin content along with metabolic adjustments help killifish cope with intermittent cycles of hypoxia in the estuarine environment.