Respiratory anatomy, physiology, and central CO₂ chemosensitivity of the Arctic air-breathing fish Dallia pectoralis

Abstract

"Aerial respiration using a ancestral lung, central respiratory rhythm generation, and central CO₂ chemosensitivity arose early in vertebrate evolution prior to the divergence of sarcopterygian and actinopterygian fish. All vertebrate air breathing, however, is not homologous as this trait evolved independently several times among teleost fishes. Two long-standing questions in respiratory physiology are whether air breathing in fish is controlled by a central rhythm generator and whether air breathing and central CO₂ chemosensitivity co-evolved. One means to address these questions is to investigate control of breathing in the brainstem; therefore, we established an isolated brainstem preparation from the Alaska blackfish, Dallia pectoralis, a rare example of an arctic air-breathing fish. In blackfish, air breathing consists of gulping and swallowing an air bubble into the esophagus and holding it in place by closing off the esophagus from the buccal cavity with a sphincter. Gulping the air bubble is accomplished by the same opercular and mandibular muscles that draw water into the buccal cavity during gill ventilation. Activation of the opercular and mandibular muscles for ventilation is effected by a central rhythm generator in the brainstem that is spontaneously active in the absence of peripheral input. This central rhythm generator, however, is not modulated by central CO₂ chemosensitivity. Unless central CO₂ chemosensitivity was lost in blackfish, we might conclude that centrally controlled vertebrate air breathing can evolve independent of central CO₂ chemosensitivity"--Leaf iii