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 (f_G), heart rate (f_H), and air breathing frequency (f_AB) 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. f_H peaked 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 f_G and f_H, (but not f_AB), 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 f_G in D2.5-D10 larvae, but decreased f_G in older larvae (≥D15), possibly to prevent branchial O₂ loss into surrounding water. Temperature sensitivity of f_G and f_H measured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q₁₀ between 20°C and 38°C of ∼2.4 and ∼1.5 for f_G and f_H, 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."