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 degrees C began at approximately D2, peaking at similar to 75 beats/min on D5, before declining to similar to 55 beats/min at D30. Heart beat began similar to 36-48 h postfertilization and similar to 1 day before hatching. fH peaked between D3 and D10 at similar to 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 similar to 30 breaths/h at D15 and D30. Forced activity at all stages resulted in a rapid but brief increase in both fG and 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 fG in D2.5-D10 larvae, but decreased fG in older larvae (>= D15), possibly to prevent branchial O-2 loss into surrounding water. Temperature sensitivity of fG and fH measured at 20 degrees C, 25 degrees C, 28 degrees C and 38 degrees C was largely independent of development, with a Q10 between 20 degrees C and 38 degrees C of similar to 2.4 and similar to 1.5 for fG and 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.