Developmental cardiorespiratory physiology of the air-breathing tropical gar,Atractosteus tropicus

The physiological transition to aerial breathing in larval air-breathing fishes is poorly understood. We investigated gill ventilation frequency ( fG), heart rate ( fH), and air breathing frequency ( fAB) as a function of development, activity, hypoxia, and temperature in embryos/larvae from day (D) 2.5 to D30 posthatch of the tropical gar, Atractosteus tropicus, an obligate air breather. Gill ventilation at 28°C began at approximately D2, peaking at ∼75 beats/min on D5, before declining to ∼55 beats/min at D30. Heart beat began ∼36–48 h postfertilization and ∼1 day before hatching. fHpeaked between D3 and D10 at ∼140 beats/min, remaining at this level through D30. Air breathing started very early at D2.5 to D3.5 at 1–2 breaths/h, increasing to ∼30 breaths/h at D15 and D30. Forced activity at all stages resulted in a rapid but brief increase in both fGand fH, (but not fAB), indicating that even in these early larval stages, reflex control existed over both ventilation and circulation prior to its increasing importance in older fishes. Acute progressive hypoxia increased fGin D2.5–D10 larvae, but decreased fGin older larvae (≥D15), possibly to prevent branchial O2loss into surrounding water. Temperature sensitivity of fGand fHmeasured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q10between 20°C and 38°C of ∼2.4 and ∼1.5 for fGand fH, respectively. The rapid onset of air breathing, coupled with both respiratory and cardiovascular reflexes as early as D2.5, indicates that larval A. tropicus develops “in the fast lane.”