Alterations in energy metabolism associated with the transition from water to air breathing in fish

Hoplias malaharicus and Hoplerythrinus unitaeniatus are two closely related species which live in poorly oxygenated water. The former is an obligate water breather, whereas the latter has the additional capacity to respire atmospheric oxygen through a highly vascularized swim bladder. Hoplerythrinus has greater lipid stores than Hoplias and probably places a greater dependence upon this metabolic fuel as an energy source than Hoplias. In contrast with this, Hoplias has greater heart and liver glycogen reserves than Hoplerythrinus. Hoplias hearts perfused in vitro with buffer containing cyanide continued to beat eight times longer than Hoplerythrinus hearts. The performance of both types of hearts was markedly increased when glucose was added to the perfusate; however, the Hoplias hearts still continued to function better than the Hoplerythrinus hearts. It is concluded that the Hoplias heart is better adapted to function anaerobically than the Hoplerythrinus heart.