American Journal of Physiology-Regulatory, Integrative and Comparative Physiology · 2016 · 29 citations · 55 references
The physiological transition to aerial breathing in larval air-breathing fishes is poorly understood. We investigated gill ventilation frequency (f<sub>G</sub>), heart rate (f<sub>H</sub>), and air breathing frequency (f<sub>AB</sub>) 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<sub>H</sub> 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<sub>G</sub> and f<sub>H</sub>, (but not f<sub>AB</sub>), 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<sub>G</sub> in D2.5-D10 larvae, but decreased f<sub>G</sub> in older larvae (≥D15), possibly to prevent branchial O<sub>2</sub> loss into surrounding water. Temperature sensitivity of f<sub>G</sub> and f<sub>H</sub> measured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q<sub>10</sub> between 20°C and 38°C of ∼2.4 and ∼1.5 for f<sub>G</sub> and f<sub>H</sub>, 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."
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Air-Breathing Fishes: Evolution, Diversity, and Adaptation
Karen L. M. Martin, Jeffrey B. Graham · Copeia · 1998 · 525 citations